Glossary of cellular and molecular biology (0–L)

{{Short description|none}} This '''glossary of cellular and molecular biology''' is a list of definitions of terms and concepts commonly used in the study of cell biology, molecular biology, and related disciplines, including genetics, biochemistry, and microbiology.<ref>{{#invoke:cite|web|url=https://www.genome.gov/genetics-glossary|title=Talking Glossary of Genomic and Genetic Terms|date=8 October 2017|publisher=genome.gov|access-date=8 October 2017}}</ref> It is split across two articles: *This page, '''Glossary of cellular and molecular biology (0–L)''', lists terms beginning with numbers and with the letters A through L. *Glossary of cellular and molecular biology (M–Z) lists terms beginning with the letters M through Z.

This glossary is intended as introductory material for novices (for more specific and technical detail, see the article corresponding to each term). It has been designed as a companion to Glossary of genetics and evolutionary biology, which contains many overlapping and related terms; other related glossaries include Glossary of virology and Glossary of chemistry.

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==0–9== {{glossary}} {{term|3' untranslated region (3'-UTR)}} {{ghat|Also '''three-prime untranslated region''', '''3' non-translated region (3'-NTR)''', and '''trailer sequence'''.}} <dd></dd>

{{term|3'-end}}{{anchor|3' end|3'}} {{ghat|Also '''three-prime end'''.}} <dd>One of two ends of a single linear strand of {{gli|DNA}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, specifically the end at which the chain of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} terminates at the third carbon atom in the furanose ring of {{gli|deoxyribose}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} (i.e. the terminus at which the 3' carbon is not attached to another nucleotide via a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester bond}}; ''{{gli|in vivo}}'', the 3' carbon is often still bonded to a hydroxyl group). By convention, sequences and structures positioned nearer to the 3'-end relative to others are referred to as {{gli|downstream}}. Contrast ''{{gli|5'-end}}''.</dd> [[Image:Nukleotid num.svg|300px|thumb|right|A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} ring with the carbon atoms numbered 1' through 5' according to chemical convention. The '''{{gli|5'-end|5' carbon}}''' is said to be ''upstream''; the '''{{gli|3'-end|3'{{nbsp}}carbon}}''' is said to be ''downstream''. Bonds to a generic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nitrogenous base|base}} and a phosphate group are also shown.]]

{{term|5' cap}} {{ghat|Also '''five-prime cap'''.}} <dd>A specially altered {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide}} attached to the {{gli|5'-end}} of some {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primary transcript|primary RNA transcripts}} as part of the set of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-transcriptional modifications}} which convert raw transcripts into mature RNA products. The precise structure of the 5' cap varies widely by organism; in eukaryotes, the most basic cap consists of a {{gli|DNA methylation|methylated}} {{gli|guanine}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside}} bonded to the triphosphate group that terminates the 5'-end of an RNA sequence. Among other functions, capping helps to regulate the export of mature RNAs from the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}}, prevent their degradation by {{gli|exonucleases}}, and promote {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}} in the cytoplasm. Mature {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA|mRNAs}} can also be decapped.</dd>

{{term|5' untranslated region (5'-UTR)}} {{ghat|Also '''five-prime untranslated region''', '''5' non-translated region (5'-NTR)''', and '''leader sequence'''.}} <dd></dd>

{{term|5-bromodeoxyuridine}} <dd>See ''{{gli|BUDR|bromodeoxyuridine}}''.</dd>

{{term|5'-end}}{{anchor|5' end|5'}} {{ghat|Also '''five-prime end'''.}} <dd>One of two ends of a single linear strand of {{gli|DNA}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, specifically the end at which the chain of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} terminates at the fifth carbon atom in the furanose ring of {{gli|deoxyribose}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} (i.e. the terminus at which the 5' carbon is not attached to another nucleotide via a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester bond}}; ''{{gli|in vivo}}'', the 5' carbon is often still bonded to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate}} group). By convention, sequences and structures positioned nearer to the 5'-end relative to others are referred to as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|upstream}}. Contrast ''{{gli|3'-end}}''.</dd>

{{term|5-methyluracil}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine}}''.</dd>

==A== {{glossary}} {{term|acentric}} <dd>(of a linear {{gli|chromosome}} or chromosome fragment) Having no {{gli|centromere}}.<ref name="CoG">{{#invoke:cite|book|last1=Klug |first1=William S. |last2=Cummings |first2=Michael R. |title=Concepts of Genetics |date=1986 |publisher=Scott, Foresman and Company |location=Glenview, Ill. |isbn=0-673-18680-6 |edition=2nd}}</ref></dd>

{{term|acetyl coenzyme A (acetyl-CoA)}}{{anchor|acetyl coenzyme A|acetyl-CoA}} <dd>A biochemical compound consisting of a {{gli|coenzyme A}} molecule to which an acetyl group ({{chem|–COCH|3}}) is attached via a high-energy thioester bond. {{gli|acetylation|Acetylation}} of coenzyme A occurs as part of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, {{gli|carbohydrates}} ({{gli|glycolysis}}), and {{gli|fatty acids}} ({{gli|beta oxidation}}), after which it participates as an energy carrier in several important {{gli|biochemical pathways}}, notably the {{gli|citric acid cycle}}, in which hydrolysis of the acetyl group releases energy which is ultimately captured in 11 {{gli|ATP}} and one {{gli|GTP}}.</dd> [[File:Acetyl-CoA-2D colored.svg|thumb|right|400px|The chemical structure of '''{{gli|acetyl-CoA}}''', with the acetyl group highlighted in blue]]

{{term|acetylation}}{{anchor|acetylate|acetylates|acetylating|acetylated}} <dd>The covalent attachment of an acetyl group ({{chem|–COCH|3}}) to a chemical compound, protein, or other biomolecule via an esterification reaction with acetic acid, either spontaneously or by {{gli|enzymatic}} catalysis. Acetylation plays important roles in several {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathways}} and in {{gli|histone modification}}. Contrast ''{{gli|deacetylation}}''.</dd>

{{term|acetyltransferase}}{{anchor|acetyltransferases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transferase}} {{gli|enzymes}} which catalyze the covalent bonding of an acetyl group ({{chem|–COCH|3}}) to another compound, protein, or biomolecule, a process known as {{gli|acetylation}}.</dd>

{{term|acrocentric}} <dd>(of a linear {{gli|chromosome}} or chromosome fragment) Having a {{gli|centromere}} positioned very close to one end of the chromosome, as opposed to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|telocentric|at the end}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metacentric|in the middle}}.<ref name="CoG"/></dd>

{{term|action potential}} <dd>The local change in voltage that occurs when the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane potential}} of a specific location along the {{gli|cell membrane|membrane}} of a {{gli|cell}} rapidly depolarizes, such as when a nerve impulse is transmitted between {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|neurons}}.</dd>

{{term|activation}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|upregulation}}''.</dd>

{{term|activator}}{{anchor|activators}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factor}} that increases the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} of a {{gli|gene}} or set of genes. Most activators work by binding to a specific {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence}} located within or near an {{gli|enhancer}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoter}} and facilitating the binding of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA polymerase}} and other transcription machinery in the same region. See also ''{{gli|coactivator}}''; contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repressor}}''.</dd>

{{term|active site}}{{anchor|active sites}} {{ghat|Also '''{{gli|binding site}}''' and '''catalytic site'''.}} <dd>The region of an {{gli|enzyme}} to which one or more {{gli|ligand|substrate molecules}} bind, causing the substrate or another molecule to undergo a chemical reaction. This region usually consists of one or more {{gli|amino acid}} residues (commonly three or four) which, when the enzyme is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein folding|folded}} properly, are able to form temporary chemical bonds with the atoms of the substrate molecule; it may also include one or more additional residues which, by interacting with the substrate, are able to catalyze a specific reaction involving the substrate. Though the active site constitutes only a small fraction of all the residues comprising the enzyme, its specificity for particular substrates and reactions is responsible for the enzyme's biological function.</dd>

{{term|active transport}} <dd>Transport of a substance (such as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} or {{gli|drug}}) across a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane}} against a concentration gradient. Unlike {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|passive transport}}, active transport requires an expenditure of energy.</dd>

{{term|acylation}} <dd>The covalent attachment of any acyl group (e.g. acetyl or benzoyl) to a chemical compound, protein, or other biomolecule via the substitution of the acyl group for a hydrogen atom, either spontaneously or by enzymatic catalysis.<ref name="Oxford B&MB"/> {{gli|acetylation|Acetylation}} is a type of acylation.</dd>

{{term|term=adenine|content=adenine ({{font|A|font=courier|size=large}})}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|purine}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase}} used as one of the four standard nucleobases in both {{gli|DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules. Adenine forms a {{gli|base pair}} with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine}} in DNA and with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|uracil}} in RNA.</dd>

{{term|term=adenosine|content=adenosine ({{font|A|font=courier|size=large}})}} <dd>One of the four standard {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosides}} used in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules, consisting of an {{gli|adenine}} {{gli|base}} with its N9 nitrogen {{gli|glycosidic bond|bonded}} to the C1 carbon of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} sugar. Adenine bonded to {{gli|deoxyribose}} is known as {{gli|deoxyadenosine}}, which is the version used in {{gli|DNA}}.</dd>

{{term|adenosine diphosphate (ADP)}}{{anchor|adenosine diphosphate|ADP}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside}} diphosphate consisting of {{gli|adenosine}} attached to two consecutive {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate}} groups via high-energy ester bonds. ADP can be {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphorylated}} to produce {{gli|ATP}} and thus is a precursor for its synthesis; it can also be dephosphorylated into {{gli|AMP}}.</dd>

{{term|adenosine monophosphate (AMP)}}{{anchor|adenosine monophosphate|AMP}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside}} consisting of {{gli|adenosine}} attached to a single {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate}} group via a high-energy ester bond. Additional phosphate groups can be added to AMP to produce {{gli|ADP}} and {{gli|ATP}}; the {{gli|cyclic AMP|cyclic ester of AMP}} serves as a second messenger in some signaling pathways.</dd>

{{term|adenosine triphosphate (ATP)}}{{anchor|adenosine triphosphate|ATP}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside triphosphate}} consisting of {{gli|adenosine}} attached to three consecutive {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate}} groups via high-energy ester bonds. The conversion of ATP into {{gli|ADP}} or {{gli|AMP}} via hydrolysis of these phosphates releases energy which is used to drive the majority of energy-consuming chemical reactions in all living cells, and hence ATP functions as a universal and ubiquitous energy carrier which is often referred to as the "molecular currency" of intracellular {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism}}. It is continuously regenerated via {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphorylation}} of ADP and AMP by enzymes such as ATP synthase. Like other nucleoside triphosphates, it also serves as a precursor for {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} synthesis.</dd> thumb|right|350px|'''{{gli|adenosine triphosphate|Adenosine triphosphate}} (ATP)''' is continuously decomposed into {{gli|adenosine diphosphate}} (ADP) and regenerated by the loss and gain, respectively, of one or more phosphate groups.

{{term|adipocyte}}{{anchor|adipocytes}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mesenchymal}} cell found in {{gli|adipose|fat tissue}}, containing large {{gli|lipid}}-filled {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicles}}.<ref name="Lackie"/></dd>

{{term|A-DNA}}{{anchor|A-DNA}} <dd>One of three main biologically active {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid tertiary structure|structural conformations}} of the {{gli|DNA}} {{gli|double helix}}, along with {{gli|B-DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Z-DNA}}. The A-form helix has a right-handed twist with 11 {{gli|base pairs}} per full turn, only slightly more compact than B-DNA, but its bases are sharply tilted with respect to the helical axis. It is often favored in dehydrated conditions and within sequences of consecutive {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|purine}} nucleotides (e.g. {{font|GAAGGGGA|font=courier|size=big}}); it is also the primary conformation adopted by {{gli|dsRNA|double-stranded RNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA-DNA hybrids}}.<ref name="Ussery">Ussery, David W. "DNA Structure: A-, B-, and Z-DNA Helix Families". Lyngby, Denmark: Danish Technical University.</ref></dd>

{{term|aerobic}} <dd>1. Describing conditions in which gaseous or dissolved diatomic oxygen is present. Aerobic environments are said to be oxygenated.<ref name="Oxford B&MB"/></dd> <dd>2. Describing an organism, pathway, or process that requires or makes use of diatomic oxygen; e.g. {{gli|aerobic respiration}}.<ref name="Oxford B&MB"/> Contrast ''{{gli|anaerobic}}''.</dd>

{{term|affected relative pair}} <dd>Any pair of organisms which are related genetically and both affected by the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|trait}}. For example, two cousins who both have blue eyes are an affected relative pair since they are both affected by the {{gli|allele}} that codes for blue eyes.</dd>

{{term|alkaline lysis}} <dd>A laboratory method used in molecular biology to extract and isolate {{gli|extrachromosomal DNA}} such as the DNA of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmids}} (as opposed to {{gli|gDNA|genomic or chromosomal DNA}}) from certain cell types, commonly {{gli|cell culture|cultured}} bacterial cells.</dd>

{{term|allele}}{{anchor|alleles|allelic}} <dd>One of multiple alternative versions of an individual {{gli|gene}}, each of which is a viable {{gli|DNA}} sequence occupying a given position, or {{gli|locus}}, on a {{gli|chromosome}}. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the same gene produces brown eyes.</dd>

{{term|allosome}}{{anchor|sex chromosome|sex chromosomes|allosomes}} {{ghat|Also '''sex chromosome''', '''heterochromosome''', or '''idiochromosome'''.}} <dd>Any {{gli|chromosome}} that differs from an ordinary {{gli|autosome}} in size, form, or behavior and which is responsible for determining the sex of an organism. In humans, the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|X chromosome}} and the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Y chromosome}} are sex chromosomes.</dd>

{{term|alpha helix (α-helix)}} <dd>A common structural {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motif}} in the secondary structures of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} consisting of a right-handed helix conformation resulting from hydrogen bonding between {{gli|amino acid}} residues which are not immediately adjacent to each other.</dd>

{{term|alternative splicing}} {{ghat|Also '''differential splicing''' or simply '''splicing'''.}} <dd>A regulated phenomenon of eukaryotic {{gli|gene expression}} in which specific {{gli|exons}} or parts of exons from the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primary transcript}} are variably included within or removed from the final, mature {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA|messenger RNA}} transcript. A class of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-transcriptional modification}}, alternative splicing allows a single {{gli|gene}} to code for multiple protein {{gli|isoforms}} and greatly increases the diversity of proteins that can be produced by an individual {{gli|genome}}. See also ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA splicing}}''.</dd>

{{term|amber}} <dd>One of three {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stop codons}} used in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|standard genetic code}}; in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, it is specified by the nucleotide triplet {{font|UAG|font=courier|size=big}}. The other two stop codons are named {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ochre}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|opal}}.</dd>

{{term|amino acid}}{{anchor|amino acids}} <dd>Any of a class of organic compounds whose basic structural formula includes a central carbon atom bonded to amine and carboxyl functional groups and to a variable side chain. Out of nearly 500 known amino acids, a set of 20 are coded for by the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|standard genetic code}} and incorporated into long polymeric chains as the building blocks of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptides}} and hence of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptides}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}. The specific sequences of amino acids in the polypeptide chains that form a protein are ultimately responsible for determining the protein's structure and function.</dd> [[File:Amino acid generic structure.png|500px|thumb|right|Every '''{{gli|amino acid}}''' has the same basic structural formula, consisting of a central carbon atom (α) bonded to three major substituents: one amino group (blue), one carboxyl group (red), and one variable side chain (green). The side chain, which can range from a simple methyl group (alanine) to more complex functional groups such as a double-ringed indole (tryptophan), gives each particular amino acid its unique identity. During {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}, amino acids are joined into a linear chain by condensation reactions which create {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptide bonds}} between the carboxyl group of one amino acid and the amino group of an adjacent amino acid. The first and last amino acids in the chain are said to be {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|N-terminal}} and {{gli|C-terminal}}, respectively, in reference to the unbonded amino group of the first amino acid and the unbonded carboxyl group of the last.]]

{{term|amino terminus}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|N-terminus}}''.</dd>

{{term|aminoacyl-tRNA synthetase}} {{ghat|Also '''tRNA-ligase'''.}} <dd>Any of a set of enzymes which catalyze the transesterification reaction that results in the attachment of a specific {{gli|amino acid}} (or a precursor) to one of its cognate {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tRNA|transfer RNA}} molecules, forming an {{gli|aa-tRNA|aminoacyl-tRNA}}. Each of the 20 different amino acids used in the {{gli|genetic code}} is recognized and attached by its own specific synthetase enzyme, and most synthetases are cognate to several different tRNAs according to their specific {{gli|anticodons}}.</dd>

{{term|aminoacyl-tRNA (aa-tRNA)}} {{ghat|Also '''aminoacylated tRNA''' and '''charged tRNA'''.}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transfer RNA}} to which a cognate {{gli|amino acid}} is chemically bonded; i.e. the product of a transesterification reaction catalyzed by an {{gli|aminoacyl-tRNA synthetase}}. Aminoacyl-tRNAs bind to the {{gli|A site|aminoacyl site}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosome}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}.</dd>

{{term|amplicon}} <dd>Any DNA or RNA sequence or fragment that is the source and/or product of an {{gli|amplification}} reaction. The term is most frequently used to describe the numerous copied fragments that are the products of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymerase chain reaction}} or {{gli|ligase chain reaction}}, though it may also refer to sequences that are amplified naturally within a genome, e.g. by {{gli|gene duplication}}.</dd>

{{term|amplification}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication}} of a biomolecule, in particular the production of one or more copies of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence}}, known as an {{gli|amplicon}}, either naturally (e.g. by spontaneous {{gli|duplications}}) or artificially (e.g. by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|PCR}}), and especially implying many repeated replication events resulting in thousands, millions, or billions of copies of the target sequence, which is then said to be ''amplified''.</dd>

{{term|anabolism}}{{anchor|anabolic}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic}} reaction or process in which energy is expended in order to build complex substances such as macromolecules from simpler compounds, including aspects of growth and biosynthesis. Anabolic processes and {{gli|biochemical pathway|pathways}} tend to involve reductive steps that create high-enthalpy, low-entropy compounds such as proteins and nucleic acid polymers.<ref name="Oxford B&MB"/> Contrast ''{{gli|catabolism}}''.</dd>

{{term|anaerobic}} <dd>1. Describing conditions in which diatomic oxygen is entirely absent, as opposed to {{gli|aerobic}} conditions.<ref name="Oxford B&MB"/></dd> <dd>2. Describing an organism that is able to survive and grow in the absence of diatomic oxygen, or a pathway or process characterized by the absence of diatomic oxygen; e.g. {{gli|anaerobic respiration}}.<ref name="Oxford B&MB"/></dd>

{{term|anaphase}} <dd>The stage of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}} that occurs after {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metaphase}} and before {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|telophase}}, when the replicated chromosomes are segregated and each of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sister chromatids}} are moved to opposite sides of the {{gli|cell}}.</dd>

{{term|anaphase lag}} <dd>The failure of one or more pairs of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sister chromatids}} or {{gli|homologous chromosomes}} to properly migrate to opposite sides of the cell during {{gli|anaphase}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}} due to a defective {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spindle apparatus}}. Consequently, both daughter cells are {{gli|aneuploid}}: one is missing one or more chromosomes (creating a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monosomy}}) while the other has one or more extra copies of the same chromosomes (creating a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polysomy}}).</dd>

{{term|aneucentric}} <dd>(of a linear {{gli|chromosome}} or chromosome fragment) Having an abnormal number of {{gli|centromeres}}, i.e. more than one.<ref name="DoG7"/></dd>

{{term|aneuploidy}}{{anchor|aneuploid}} <dd>The condition of a cell or organism having an abnormal number of one or more particular {{gli|chromosomes}} (but excluding abnormal numbers of complete sets of chromosomes, which instead is known as {{gli|euploidy}}).</dd>

{{term|annealing}}{{anchor|anneal|anneals|annealed}} <dd>The {{gli|hybridization}} of two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecules containing {{gli|complementary}} sequences, creating a {{gli|double-stranded}} molecule with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|paired}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}}. The term is used in particular to describe steps in laboratory techniques such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymerase chain reaction}}, where double-stranded DNA molecules are repeatedly {{gli|denatured}} into single strands by heating and then exposed to cooler temperatures, causing the strands to reassociate with each other or with complementary {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primers}}. The exact {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reassociation kinetics|temperature at which annealing occurs}} is strongly influenced by the length and specific sequence of the individual strands.</dd>

{{term|antibiotic resistance gene}}{{anchor|antibiotic resistance genes}} <dd>A gene that confers resistance to one or more specific antibiotic compounds. In molecular cloning, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmid}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vectors}} are often designed to carry antibiotic resistance genes as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|selectable markers}} alongside other genes of interest, because it permits the artificial selection of successfully {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transformed}} cell populations when the cells are cultured in the presence of the antibiotic.</dd>

{{term|antibody}}{{anchor|antibodies}} <dd>Any of a diverse family of {{gli|glycoproteins}} known as {{gli|immunoglobulins}} capable of binding specifically but reversibly via non-covalent interactions to a particular {{gli|antigen}} or {{gli|immunogen}}. Antibodies are generated as part of an organism's immune response to the introduction of a specific antigen into a host organism, and their binding of the antigen frequently (though not always) counteracts or inhibits any biological activity the antigen may have.<ref name="Oxford B&MB"/> Antibodies have a characteristic Y-shaped structure consisting of a {{gli|heavy chain}} and {{gli|light chain}} held together by disulfide bonds.</dd>

{{term|anticodon}}{{anchor|anticodons}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|triplet|series of three}} consecutive {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} within a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tRNA|transfer RNA}} which {{gli|complement}} the three nucleotides of a {{gli|codon}} within an {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA}} transcript. During {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}, each tRNA recruited to the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosome}} contains a single anticodon triplet that pairs with its complementary codon from the mRNA sequence, allowing each codon to specify a particular {{gli|amino acid}} to be added to the growing peptide chain. Anticodons containing {{gli|inosine}} in the first position are capable of pairing with more than one codon due to a phenomenon known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|wobble base pairing}}.</dd>

{{term|antigen}}{{anchor|antigens|antigenic}} <dd>Any {{gli|exogenous}} agent that, upon introduction into an immunocompetent organism, stimulates a response from the organism's immune system that results in the production of one or more {{gli|antibodies}} which can bind to it specifically; in this sense the term is synonymous with {{gli|immunogen}}. Antigens may be pure substances, mixtures of substances, or particulate matter such as cells or cell fragments. Broader definitions may include substances that can bind to a specific antibody but are not themselves immunogenic, i.e. those which are only able to stimulate antibody production when combined with a {{gli|carrier protein|carrier}}.<ref name="Oxford B&MB"/></dd>

{{term|antimetabolite}}{{anchor|antimetabolites}} <dd>Any molecule that functions as an antagonist to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic}} process, limiting or inhibiting normal cellular metabolism; i.e. a metabolic poison.<ref name="DoG7"/></dd>

{{term|antimitotic}}{{anchor|antimetabolites}} <dd>Any compound that suppresses normal {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} in a cell or population of cells.<ref name="DoG7"/></dd>

{{term|antioncogene}} <dd>A {{gli|gene}} which helps to regulate cell growth and suppress tumors when functioning correctly, such that its absence or malfunction can result in uncontrolled cell growth and possibly cancer.<ref name="NCI-G">{{#invoke:cite|web|title=NCI Dictionary of Genetics Terms |url=https://www.cancer.gov/publications/dictionaries/genetics-dictionary |website=National Cancer Institute |publisher=PDQ® Cancer Genetics Editorial Board}}</ref> Compare ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oncogene}}''.</dd>

{{term|antiparallel}} <dd>The contrasting orientations of the two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}} of a {{gli|double-stranded}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} (and more generally any pair of biopolymers) which are parallel to each other but with opposite {{gli|directionality}}. For example, the two {{gli|complementary}} strands of a {{gli|DNA}} molecule run side-by-side but in opposite directions with respect to chemical numbering conventions, with one strand oriented {{gli|5'}}-to-{{gli|3'}} and the other 3'-to-5'.</dd>

{{term|antiporter}}{{anchor|antiporters}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transport protein}} which works by exchanging two different ions or small molecules across a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane}} in opposite directions, either at the same time or consecutively.<ref name="Alberts et al."/></dd>

{{term|antisense}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|template strand}}''.</dd>

{{term|antisense RNA (asRNA)}}{{anchor|asRNA|antisense RNA}} {{ghat|Also '''antisense transcript''' and '''antisense oligonucleotide (ASO)'''.}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|non-coding RNA}} molecule containing an {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sense|antisense}} sequence that is {{gli|complementary}} to a sense strand, such as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|messenger RNA}}, with which it readily {{gli|hybridizes}}, thereby inhibiting the sense strand's further activity (e.g. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}} into protein). Many different classes of naturally occurring RNA such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|siRNA}} function by this principle, making them potent gene {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|silencing|silencers}} in various {{gli|gene regulation}} mechanisms. Synthetic antisense RNA has also found widespread use in gene {{gli|knockdown}} studies, and in practical applications such as antisense therapy.</dd>

{{term|anucleate}} {{ghat|Also '''anuclear'''.}} <dd>(of a cell or organism) Lacking a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}}, i.e. a discrete, membrane-bound organelle enclosing the cell's {{gli|genomic DNA}}, used especially of cells which normally have a nucleus but from which the nucleus {{gli|enucleate|has been removed}} (e.g. in artificial {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear transfer}}), and also of specialized cell types that develop without nuclei despite that the cells of other tissues comprising the same organism ordinarily do have nuclei (e.g. mammalian erythrocytes).</dd>

{{term|apical constriction}} <dd>The process by which contraction of the {{gli|apical}} side of a cell (and often a corresponding expansion of the opposing {{gli|basal}} side) causes the cell to assume a wedge-shaped morphology. The process is common during early development, where it is often coordinated across many adjacent cells of an {{gli|epithelial}} layer simultaneously in order to generate bends or folds in developing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissues}}.</dd> thumb|right|350px|The '''{{gli|apical constriction}}''' of specific groups of cells during developmental morphogenesis allows bends and turns to form in higher-order tissues.

{{term|apoptosis}} <dd>A highly regulated form of {{gli|programmed cell death}} that occurs in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|multicellular}} organisms.</dd>

{{term|aptamer}}{{anchor|aptamers}} <dd>Any artificial {{gli|DNA}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|XNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligonucleotide}} molecule, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} or {{gli|double-stranded}}, which functions as a {{gli|ligand}} by binding selectively to one or more specific target molecules, usually other nucleic acids or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, and often a family of such molecules. The term is used in particular to describe short nucleic acid fragments which have been randomly generated and then artificially selected ''{{gli|in vitro}}'' by procedures such as SELEX. Aptamers are useful in the laboratory as {{gli|antibody mimetics}}, particularly in applications where conventional protein {{gli|antibodies}} are not appropriate.</dd>

{{term|artificial gene synthesis}} <dd>A set of laboratory methods used in the {{gli|de novo synthesis|''de novo'' synthesis}} of a {{gli|gene}} (or any other {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence}}) from free {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}}, i.e. without relying on an existing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|template strand}}.</dd>

{{term|assimilatory process}} <dd>Any process by which chemical compounds containing biologically relevant elements (e.g. carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, selenium, iron, cobalt, nickel, copper, zinc, molybdenum, etc.) are uptaken by microorganisms and incorporated into complex {{gli|biomolecules}} in order to synthesize various cellular components. In contrast, a {{gli|dissimilatory process}} uses the energy released by decomposing exogenous molecules to power the cell's {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism}} and {{gli|excretes}} {{gli|metabolic waste|residual or toxic compounds}} out of the cell, instead of reusing them to build new molecules.</dd>

{{term|aster}} <dd>In animal cells, a star-shaped system of non-{{gli|kinetochore}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microtubules}} that radiates from a {{gli|centrosome}} or from either of the poles of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spindle apparatus|mitotic spindle}} during the early stages of cell division.<ref name="Alberts et al."/></dd>

{{term|asynapsis}} <dd>The failure of {{gli|homologous chromosomes}} to properly pair with each other during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}.<ref name="DoG7"/> Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|synapsis}}'' and ''{{gli|desynapsis}}''.</dd>

{{term|attached X}} {{ghat|Also '''compound X'''.}} <dd>A single {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monocentric}} {{gli|chromosome}} containing two or more physically attached copies of the normal {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|X chromosome}} as a result of either a natural internal {{gli|chromosomal duplication|duplication}} or any of a variety of {{gli|genetic engineering}} methods. The resulting compound chromosome effectively carries two or more doses of all genes and sequences included on the X, yet functions in all other respects as a single chromosome, meaning that haploid 'XX' {{gli|gametes}} (rather than the ordinary 'X' gametes) will be produced by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}} and inherited by progeny. In mechanisms such as {{gli|genic balance}} in which the sex of an organism is determined by the total dosage of X-linked genes, an abnormal 'XXY' {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygote}}, fertilized by one XX gamete and one Y gamete, will develop into a female.</dd>

{{term|autolysis}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|lysis}} or digestion of a {{gli|cell}} by its own {{gli|enzymes}}; or of a particular enzyme by another instance of the same enzyme. See also ''{{gli|autophagy}}''.</dd>

{{term|autophagy}} {{ghat|Also '''autophagocytosis'''.}} <dd>The orderly {{gli|autolysis|autolytic}} degradation and recycling of dysfunctional or unnecessary cellular components by the cell's own enzymes as part of a carefully regulated, {{gli|lysosome}}-dependent pathway. Autophagic programs play important roles in nutrient-deprived and {{gli|cellular senescence|senescent cells}} but also help maintain {{gli|homeostasis}} in healthy cells.</dd>

{{term|autosome}}{{anchor|autosomes|autosomal}} <dd>Any {{gli|chromosome}} that is not an {{gli|allosome}} and hence is not involved in the determination of the sex of an organism. Unlike the sex chromosomes, the autosomes in a {{gli|diploid}} cell exist in pairs, with the members of each pair having the same structure, morphology, and genetic {{gli|loci}}.</dd>

{{term|autozygote}} <dd>A cell or organism that is {{gli|homozygous}} for a {{gli|locus}} at which the two homologous {{gli|alleles}} are identical by descent, both having been derived from a single gene in a common ancestor.<ref name="DoG7"/> Contrast ''{{gli|allozygote}}''.</dd>

{{term|auxesis}} <dd>The growth of a multicellular organism due to an increase in the size of its cells rather than an increase in the number of cells.</dd>

{{term|axenic}} <dd>Describing a {{gli|cell culture}} in which only a single species, variety, or strain is present, and which is therefore entirely free of contaminating organisms including symbiotes and parasites.</dd>

==B== {{glossary}} {{term|B chromosome}} <dd>Any supernumerary {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear DNA}} molecule which is not a duplicate of nor {{gli|homologous chromosomes|homologous}} to any of the standard complement of normal "A" chromosomes comprising a genome. Typically very small and devoid of structural genes, B chromosomes are by definition not necessary for life. Though they occur naturally in many eukaryotic species, they are {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|non-Mendelian inheritance|not stably inherited}} and thus {{gli|copy-number variation|vary widely in copy number}} even between closely related individuals.<ref name="DoG7"/></dd>

{{term|back mutation}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} that reverses the effect of a previous {{gli|forward mutation}} which had inactivated a gene, thus restoring {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|wild-type}} function.<ref name="Lewin">{{cite book|title=Genes VIII|last1=Lewin|first1=Benjamin|year=2003|publisher=Pearson Prentice Hall|location=Upper Saddle River, NJ|isbn=0-13-143981-2}}</ref> See also ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reverse mutation}}''.</dd>

{{term|bacterial artificial chromosome (BAC)}} <dd></dd>

{{term|base}} <dd>An abbreviation of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nitrogenous base}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase}}.</dd>

{{term|base pair (bp)}}{{anchor|base pairs|base pairing|base-pair|pairing|paired}} <dd>A pair of two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}} on {{gli|complementary}} {{gli|DNA}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} strands which are loosely attracted to each other via hydrogen bonding, a type of non-covalent electrostatic interaction between individual atoms in the purine or pyrimidine rings of the complementing bases. This phenomenon, known as ''base pairing'', is the mechanism underlying the {{gli|hybridization}} that commonly occurs between nucleic acid polymers, allowing two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} molecules to combine into a more energetically stable {{gli|double-stranded}} molecule, as well as enabling certain individual strands to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|self-complementary|complement themselves}}. The ability of consecutive base pairs to stack one upon another contributes to the long-chain {{gli|double helix}} structures observed in both {{gli|dsDNA|double-stranded DNA}} and {{gli|dsRNA|double-stranded RNA}} molecules.</dd>

{{term|baseline}} <dd>A measure of the {{gli|gene expression}} level of a {{gli|gene}} or genes prior to a perturbation in an experiment, as in a negative control. ''Baseline expression'' may also refer to the expected or historical measure of expression for a gene.</dd>

{{term|basic local alignment search tool (BLAST)}} <dd>A computer algorithm widely used in {{gli|bioinformatics}} for {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence alignment|aligning}} and comparing primary biological sequence information such as the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide sequences}} of DNA or RNA or the {{gli|amino acid sequences}} of proteins. BLAST programs enable scientists to quickly check for homology between two or more sequences by directly comparing the nucleotides or amino acids present at each position within each sequence; a common use is to search for matches between a specific query sequence and a digital sequence database such as a {{gli|library|genome library}}, with the program returning a list of sequences from the database which resemble the query sequence above a specified threshold of similarity. Such comparisons can permit the identification of an organism from an unknown sample or the inference of evolutionary relationships between genes, proteins, or species.</dd>

{{term|B-DNA}}{{anchor|B-DNA}} <dd>The "standard" or classical {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid tertiary structure|structural conformation}} of the {{gli|DNA}} {{gli|double helix}} ''{{gli|in vivo}}'', thought to represent an average of the various distinct conformations assumed by very long DNA molecules under physiological conditions.<ref name="Ussery"/> The B-form double helix has a right-handed twist with a diameter of 23.7 ångströms and a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pitch}} of 35.7 ångströms or about 10.5 {{gli|base pairs}} per full turn, such that each nucleotide pair is rotated 36° around the helical axis with respect to its neighboring pairs. See also ''{{gli|A-DNA}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Z-DNA}}''.</dd> 350px|thumb|right|The three principal biologically active conformations of {{gli|DNA}} molecules: '''{{gli|A-DNA}}''', '''{{gli|B-DNA}}''', and '''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Z-DNA}}''' (left to right), as viewed from the side and down the axis of the {{gli|double helix}}.

{{term|beta oxidation}} {{ghat|Also '''β-oxidation'''.}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathway}} by which {{gli|fatty acid}} molecules are {{gli|catabolism|broken down}} into simpler molecules, generating {{gli|glossary=Glossary of cellular and molecular biology (0–L)|acetyl-CoA}} in the process. This occurs via a series of enzyme-catalyzed reactions which oxidize the beta carbon of the fatty acid chain and ultimately convert it into a carbonyl group, which is then susceptible to nucleophilic attack by another molecule of {{gli|coenzyme A}}, causing thiolysis of the bond between the alpha and beta carbons; this process can be repeated to sequentially digest long chains of hydrocarbons into shorter chains, generating an additional molecule of acetyl-CoA with every cycle. In prokaryotes, beta oxidation occurs in the cytosol, while in eukaryotes it primarily takes place in the inner {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondrial}} membrane or in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peroxisomes}}.</dd>

{{term|bidirectional replication}} <dd>A common mechanism of {{gli|DNA replication}} in which two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication forks}} move in opposite directions away from the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|origin of replication|origin}}; this results in a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication bubble|bubble-like region}} where the {{gli|duplex}} molecule is locally separated into two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssDNA|single strands}}.<ref name="DoG7"/></dd>

{{term|binary fission}} <dd>The separation of a single entity (e.g. a {{gli|cell}}) into exactly two discrete entities closely resembling the original. The term refers in particular to a type of {{gli|cell division}} used by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prokaryotes}} such as bacteria, whereby a single {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|parent cell}} divides evenly into two {{gli|daughter cells}} which are genetically identical to each other and to the parent. Binary fission is preceded by {{gli|DNA replication|replication}} of the parent cell's DNA, rapid growth of the {{gli|cell wall}}, and various other processes which ensure even distribution of the cell's contents between the two progeny, but is generally a quicker and simpler process than the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} and {{gli|cytokinesis}} that occur in {{gli|eukaryotes}}.</dd>

{{term|binding site}}{{anchor|binding sites}} <dd>A region of a macromolecule such as a nucleic acid or a protein that directly participates in chemical interactions with another molecule. A wide variety of chemical interactions of varying strength and specificity can be described as "binding"; they may be long-term or transient, reversible or irreversible, and may rely upon relatively weak intermolecular forces or much stronger covalent bonds. Binding sites are defined by the spatial proximity of one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|residues}} having functional groups with particular chemical properties. For example, the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein folding|folding}} of polypeptides in such a way that particular amino acids are positioned near each other in the protein's {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|quaternary structure}} may confer chemical properties that permit the interaction of those residues with a particular {{gli|ligand}}. Similarly, a specific sequence of nucleobases in a DNA molecule may function as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recognition site}} for a {{gli|DNA-binding protein}}. Whether and how the binding site functions depends on the precise spatial arrangement of the interacting residues and their physical accessibility to potential binding partners; thus mutations or changes in the chemical environment such as {{gli|conformational changes}} can dramatically alter functionality. See also ''{{gli|active site}}''.</dd>

{{term|bioassay}}{{anchor|bioassays}} <dd>Any analytical method that measures or qualifies the presence, effect, or potency of a substance within or upon a biological system, either directly or indirectly, e.g. by quantifying the concentration of a particular chemical compound within a sample obtained from living organisms, cells, or tissues, and ideally under controlled conditions that compare a sample subjected to an experimental treatment with an unmanipulated sample, so as to permit inferences about the effect of the treatment upon some measured variable.<ref name="MacLean"/></dd>

{{term|biochemistry}} <dd>A subdiscipline of both biology and chemistry which studies the chemical basis of biological phenomena, focusing on understanding the chemical reactions and interactions that occur between {{gli|biomolecules}} and give rise to the processes that define and characterize living systems. It is closely related to and largely overlaps with {{gli|molecular biology}}.</dd>

{{term|bioenergetics}} <dd>The branch of {{gli|biochemistry}} and {{gli|cell biology}} that studies the flow of energy through living systems, in particular how organisms acquire, produce, transform, and utilize energy in order to perform biochemical work such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism|metabolic reactions}}.</dd>

{{term|biofilm}}{{anchor|biofilms}} <dd>A community of symbiotic microorganisms, especially bacteria, where cells produce and embed themselves within a slimy, sticky {{gli|extracellular matrix}} composed of various high-molecular weight biopolymers, {{gli|cell adhesion|adhering}} to each other and sometimes also to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substratum}}, which may be a biotic or abiotic surface.<ref>{{#invoke:cite|journal| vauthors = López D, Vlamakis H, Kolter R | title = Biofilms | journal = Cold Spring Harbor Perspectives in Biology | volume = 2 | issue = 7 | article-number = a000398 | date = July 2010 | pmid = 20519345 | pmc = 2890205 | doi = 10.1101/cshperspect.a000398 }}</ref> Many bacteria can exist either as independent single cells or switch to a physiologically distinct biofilm phenotype; those that create biofilms often do so in order to shelter themselves from harmful environments. Cells residing within biofilms can easily share nutrients and {{gli|cell communication|communicate}}, and subpopulations of cells may {{gli|differentiate}} to perform specialized functions supporting the whole biofilm.<ref>{{#invoke:cite|journal| vauthors = Momeni B | title = Division of Labor: How Microbes Split Their Responsibility | journal = Current Biology | volume = 28 | issue = 12 | pages = R697–R699 | date = June 2018 | pmid = 29920261 | doi = 10.1016/j.cub.2018.05.024 | s2cid = 49315067 | doi-access = free | bibcode = 2018CBio...28.R697M }}</ref></dd>

{{term|biomarker}}{{anchor|biomarkers}} <dd>A measurable indicator of some biological state, especially a compound or {{gli|biomolecule}} whose presence or absence in a biological system is a reliable sign of a normal or abnormal process, condition, or disease.<ref name="NCI-C">{{#invoke:cite|web|title=NCI Dictionary of Cancer Terms |url=https://www.cancer.gov/publications/dictionaries/cancer-terms |website=www.cancer.gov |publisher=National Cancer Institute |date=2 February 2011}}</ref> Things that may serve as biomarkers include direct measurements of the concentration of a particular compound or molecule in a tissue or fluid sample, or any other characteristic physiological, histological, or radiographic signal (e.g. a change in heart rate, or a distinct {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|morphology}} under a microscope). They are regularly used as predictive or diagnostic tools in clinical medicine and laboratory research.</dd>

{{term|biometal}}{{anchor|biometals}} <dd>Any metallic element found naturally in small but measurable amounts in biological contexts. Metal ions play important roles in many biochemical processes and some are essential for normal function in living organisms, especially iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), magnesium (Mg), potassium (K), sodium (Na), and calcium (Ca).</dd>

{{term|biomolecular gradient}} <dd>Any difference in the concentration of {{gli|biomolecules}} between two spaces within a biological system, whether {{gli|intracellular}}, {{gli|extracellular}}, across a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane}}, or between different cells or different parts of a tissue or organ system. Gradients of one kind or another drive virtually all biochemical processes occurring within and between cells, as natural systems tend to move toward a thermodynamic equilibrium where concentrations are uniformly distributed in all spaces and no gradients exist. Gradients thus cause chemical reactions to occur in particular directions, which can be used by cells to accomplish essential biological functions, including {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism|metabolic energy transfer}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|signal transduction}}, and movement of ions and solutes into and out of cells and organelles. It is often necessary for cells to continuously regenerate gradients such as {{gli|membrane potentials}} in order to permit these processes to continue.</dd>

{{term|biomolecule}}{{anchor|biomolecules|biomolecular}} {{ghat|Also '''biological molecule'''.}} <dd>Any molecule or chemical compound involved in or essential to one or more biological processes within a biological system, especially large {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|macromolecules}} such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}}, {{gli|lipids}}, and {{gli|carbohydrates}}, but also broadly inclusive of smaller molecules such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vitamins}}, {{gli|hormones}}, and {{gli|biometals}} which are consumed or produced by biochemical reactions, often as part of {{gli|biochemical pathways}}. Most biomolecules are organic compounds; some are produced naturally within {{gli|cells}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissues}} ({{gli|endogenous}} compounds), while others can only be obtained from the organism's environment ({{gli|exogenous}} compounds).</dd>

{{term|bivalent}} <dd></dd>

{{term|blast cell}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|precursor cell}}''.</dd>

{{term|blot}}{{anchor|blots|blotting|blotted}} <dd>Any of a variety of molecular biology methods by which {{gli|electrophoresis|electrophoretically}} or chromatographically separated {{gli|DNA}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} samples are transferred from a support medium such as a polyacrylamide or agarose gel onto an immobilizing carrier such as a nitrocellulose or PVDF membrane. Some methods involve the transfer of molecules by capillary action (e.g. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Southern blot|Southern}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|northern blotting}}), while others rely on the transport of charged molecules by electrophoresis (e.g. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|western blotting}}). The transferred molecules are then visualized by colorant staining, by autoradiography, or by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|probing}} for specific sequences or {{gli|epitopes}} with {{gli|hybridization probes}} or {{gli|antibodies}} {{gli|conjugated}} to chemiluminescent reporters.<ref name="DoG7"/></dd>

{{term|blunt end}}{{anchor|blunt ends}} <dd>A term used to describe the end of a {{gli|double-stranded DNA}} molecule where the terminal nucleobases on each {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} are {{gli|base pairing|base-paired}} with each other, such that neither strand has a single-stranded "overhang" of unpaired bases. This is in contrast to a so-called "{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sticky end}}", where an overhang is created by one strand being one or more bases longer than the other. Blunt ends and sticky ends are relevant when {{gli|ligating}} multiple DNA molecules, e.g. in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|restriction cloning}}, because sticky-ended molecules will not readily {{gli|anneal}} to each other unless they have matching overhangs; blunt-ended molecules do not anneal in this way, so special procedures must be used to ensure that fragments with blunt ends are joined in the correct places.</dd>

{{term|bromodeoxyuridine (BUDR, BrdU)}} {{ghat|Also '''5-bromodeoxyuridine'''.}} <dd>A synthetic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside analogue}} with a chemical structure similar to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymidine}}, the only difference being the substitution of a bromine atom for the methyl group of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase}}.</dd>

==C== {{glossary}} {{term|cadastral gene}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|regulatory}} {{gli|gene}} that restricts the {{gli|expression}} of other genes to specific tissues or body parts in an organism, typically by producing {{gli|gene products}} which variably inhibit or permit {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} of the other genes in different {{gli|cell types}}.<ref name="DoG7"/> The term is used most commonly in plant genetics.</dd>

{{term|cadherin}}{{anchor|cadherins}} <dd>Any of a class of {{gli|integral membrane protein|transmembrane proteins}} which are dependent on calcium ions (Ca2+) and whose extracellular {{gli|domains}} function as mediators of cell–cell adhesion at {{gli|adherens junctions}} in eukaryotic tissues.</dd>

{{term|callus}} <dd>An unorganized mass of parenchymal cells that forms naturally at the site of wounds in plant tissues, and which is commonly artificially induced to form in plant {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissue culture}} as a means of initiating somatic embryogenesis.<ref name="MacLean"/></dd>

{{term|candidate gene}}{{anchor|candidate genes}} <dd>A {{gli|gene}} whose location on a chromosome is {{gli|genetic association|associated}} with a particular {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotype}} (often a disease-related phenotype), and which is therefore suspected of causing or contributing to the phenotype. Candidate genes are often selected for study based on ''a priori'' knowledge or speculation about their functional relevance to the trait or disease being researched.</dd>

{{term|canonical sequence}} <dd>See ''{{gli|consensus sequence}}''.</dd>

{{term|carbohydrate}}{{anchor|carbohydrates}} <dd>Any of a class of organic compounds having the generic chemical formula {{chem|(|C|H|2|O|)|n}}, and one of several major classes of {{gli|biomolecules}} found universally in biological systems. Carbohydrates include individual {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monosaccharides}} as well as larger {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymeric}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligosaccharides}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polysaccharides}}, in which multiple monosaccharide monomers are joined by {{gli|glycosidic bonds}}.<ref name="MacLean"/> Abundant and ubiquitous, these compounds are involved in numerous essential biochemical processes and {{gli|biochemical pathway|pathways}}; they are widely used as an energy source for cellular {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism}}, as a form of energy storage, as {{gli|cell signaling|signaling}} molecules, and as {{gli|biomarkers}} to {{gli|glycosylation|label or modify}} the activity of other molecules. Carbohydrates are often colloquially described as "sugars"; the prefix ''glyco-'' indicates a compound or process containing or involving carbohydrates, and the suffix ''-ose'' usually signifies that a compound is a carbohydrate or a derivative.</dd>

{{term|carboxyl terminus}} <dd>See ''{{gli|C-terminus}}''.</dd>

{{term|carrier protein}}{{anchor|carrier proteins}} <dd>1.  A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane protein}} that functions as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transporter}}, binding to a solute and facilitating its movement across the membrane by undergoing a series of {{gli|conformational changes}}.<ref name="Alberts et al.">{{#invoke:cite|book|last1=Alberts |first1=Bruce |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |title=Molecular Biology of the Cell |chapter=Glossary |date=2002 |publisher=Garland Science |location=New York |edition=4th |url=https://www.ncbi.nlm.nih.gov/books/NBK21052/ |format=Available from the National Center for Biotechnology Information}}</ref></dd> <dd>2.  A protein to which a specific {{gli|ligand}} or {{gli|hapten}} has been conjugated and which thereby carries an {{gli|antigen}} capable of eliciting an {{gli|antibody}} response.<ref name="Lackie"/></dd> <dd>3.  A protein which is included in an {{gli|assay}} at high concentrations in order to prevent non-specific interactions of the assay's reagents with vessel surfaces, sample components, or other reagents.<ref name="Lackie"/> For example, in many {{gli|blotting}} techniques, albumin is intentionally allowed to bind non-specifically to the blotted membrane prior to {{gli|fluorescent labelling}}, so as to "block" potential off-target binding of the {{gli|fluorophore}} to the membrane, which might otherwise cause background fluorescence that obscures genuine signal from the target.</dd>

{{term|caspase}} <dd></dd>

{{term|cassette}}{{anchor|cassettes|gene cassette|gene cassettes}} <dd>A pre-existing nucleic acid sequence or construct, especially a DNA {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vector}} with an annotated sequence and precisely positioned {{gli|gene regulation|regulatory elements}}, into which one or more {{gli|fragments}} can be readily {{gli|insert|inserted}} or recombined by various {{gli|genetic engineering}} methods. Recombinant {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmid}} vectors containing reliable {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoters}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|origins of replication}}, and antibiotic resistance genes are commercially manufactured as cassettes to allow scientists to easily swap {{gli|genes of interest}} into and out of an active "slot" or locus within the plasmid. See also ''{{gli|multiple cloning site}}''.</dd>

{{term|catabolism}}{{anchor|catabolic}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic}} reaction or process involving the decomposition of large or complex substances into smaller, simpler compounds, especially the breakdown of organic compounds in order to liberate energy.<ref name="Oxford B&MB"/> Catabolic processes and {{gli|biochemical pathway|pathways}} are usually exergonic and tend to involve oxidative steps that break chemical bonds, generating low-enthalpy, high-entropy products.<ref name="Lackie"/> Contrast ''{{gli|anabolism}}''.</dd>

{{term|catabolite}}{{anchor|catabolites}} <dd>Any product of {{gli|catabolism}} or of a catabolic pathway.<ref name="Oxford B&MB"/> See also ''{{gli|metabolite}}''.</dd>

{{term|catalysis}}{{anchor|catalyze|catalyzes|catalyzing|catalyzed}} <dd>An increase in the reaction rate of a chemical reaction due to the presence of a {{gli|catalyst}}.<ref name="Oxford B&MB"/> A reaction whose rate is increased in this manner is said to be ''catalyzed''. {{gli|enzyme|Enzyme}}-directed catalysis is the primary means by which many otherwise energetically unfavorable biochemical reactions occur.</dd>

{{term|catalyst}}{{anchor|catalysts}} <dd>Any chemical species or substance whose presence {{gli|catalysis|increases the reaction rate}} of one or more particular chemical reactions but which is itself unchanged by the reaction, being neither a reactant nor a product of the reaction. Catalysts often need only be present in very low concentrations relative to the reactants in order for catalysis to occur.<ref name="Oxford B&MB"/> They may be simple molecules which catalyze reactions spontaneously, though most biochemical reactions are catalyzed by specifically evolved {{gli|enzymes}} which allow them to proceed at rates millions or billions of times faster than they otherwise would.</dd>

{{term|CCAAT box}} {{ghat|Also '''CAAT box''' or '''CAT box'''.}} <dd>A highly {{gli|conserved sequence|conserved}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|regulatory}} DNA sequence located approximately 75 base pairs {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|upstream}} (i.e. -75) of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription start site}} for many eukaryotic genes.<ref name="CoG"/></dd>

{{term|cDNA}} <dd>See ''{{gli|cDNA|complementary DNA}}''.</dd>

{{term|cell}}{{anchor|cells}} <dd>The basic structural and functional unit of which all living organisms are composed, essentially a self-replicating ball of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protoplasm}} surrounded by a {{gli|cell membrane|surface membrane}} which separates the interior from the external environment, thus providing a protected space in which the carefully controlled chemical reactions necessary to sustain biological processes can be carried out unperturbed. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|unicellular|Unicellular}} organisms are composed of a single autonomous cell, whereas {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|multicellular}} organisms consist of numerous cells cooperating together, with individual cells more or less specialized or {{gli|differentiated}} to serve particular functions.<ref name="Lackie"/> Cells vary widely in size, shape, and substructure, particularly between {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prokaryotes}} and {{gli|eukaryotes}}. The typical cell is microscopic, averaging 1 to 20 micrometres (μm) in diameter, though they may range in size from 0.1 μm to more than 20 centimetres in diameter for the eggs laid by some birds and reptiles, which are highly specialized single-celled ova.<ref name="MacLean"/></dd>

{{term|cell biology}} {{ghat|Also '''cellular biology'''.}} <dd>The branch of biology that studies the structures, functions, processes, and properties of biological {{gli|cells}}, the self-contained units of life common to all living organisms.</dd>

{{term|cell compartmentalization}}{{anchor|cell compartmentalization|cell compartment|cell compartments|compartment|compartments|compartmentalization}} <dd>The subdivision of the interior of a {{gli|cell}} into distinct, usually {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane|membrane-bound}} compartments, including the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelles}} ({{gli|endoplasmic reticulum}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondria}}, {{gli|chloroplasts}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicle|intracellular vesicles}}, etc.), a defining feature of the Eukarya.<ref name="MacLean"/></dd>

{{term|cell cortex}} <dd>A specialized layer of {{gli|cytoplasmic}} proteins lining the inner face of the {{gli|cell membrane}} in most eukaryotic cells, composed primarily of {{gli|actin}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microfilaments}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|myosin}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motor proteins}} and usually 100–1000 nanometres thick, which functions as a modulator of membrane behavior and cell surface properties.</dd>

{{term|cell counting}} <dd>The process of determining the number of {{gli|cells}} within a biological sample or {{gli|cell culture|culture}} by any of a variety of methods. Counting cells is an important aspect of {{gli|cytometry}} used widely in research and clinical medicine. It is generally achieved by using a manual or digital {{gli|cytometer}} to count the number of cells present in small fractions of a sample, and then extrapolating to estimate the total number present in the entire sample. The resulting quantification is typically expressed as a density or concentration, i.e. the number of cells per unit area or volume.</dd>

{{term|cell culture}} <dd>The process by which living cells are grown and maintained, or "cultured", under carefully controlled conditions, generally outside of their natural environment. Optimal growth conditions vary widely for different cell types but usually consist of a suitable vessel (e.g. a {{gli|culture tube}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Petri dish}}) containing a specifically formulated {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substrate}} or {{gli|growth medium}} that supplies all of the nutrients essential for life ({{gli|amino acids}}, {{gli|carbohydrates}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vitamins}}, minerals, etc.) plus any desirable growth factors and {{gli|hormones}}, permits gas exchange (if necessary), and regulates the environment by maintaining consistent physico-chemical properties (temperature, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pH}}, osmotic pressure, etc.). Some cell types require a solid surface to which they can {{gli|adherent culture|adhere}} in order to reproduce, whereas others can be grown while floating freely in a liquid or gelatinous {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|suspension culture|suspension}}. Most cells have a genetically determined reproduction limit, but {{gli|immortalized}} cells will divide indefinitely if provided with optimal conditions.</dd>

{{term|cell cycle}} <dd></dd>

{{term|cell division}}{{anchor|cell divisions}} <dd>The separation of an individual {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|parent cell}} into two {{gli|daughter cells}} by any process. Cell division generally occurs by a complex, carefully structured sequence of events involving the reorganization of the parent cell's internal contents, the physical {{gli|cleavage}} of the {{gli|cytoplasm}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}}, and the even distribution of contents between the two resulting cells, so that each ultimately contains approximately half of the original cell's starting material. It usually implies reproduction via the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication}} of the parent cell's genetic material prior to division, though cells may also divide without replicating their DNA. In prokaryotic cells, {{gli|binary fission}} is the primary form of cell division. In eukaryotic cells, asexual division occurs by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} and {{gli|cytokinesis}}, while specific cell types reserved for sexual reproduction can additionally divide by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}.<ref name="Alberts et al."/></dd>

{{term|cell fusion}} <dd>The merging or coalescence of two or more cells into a single cell, as occurs in the fusion of {{gli|gametes}} to form a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygote}}. Generally this occurs by the destabilization of each cell's {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} and the formation of {{gli|cytoplasmic}} bridges between them which then expand until the two cytoplasms are completely mixed; intercellular structures or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelles}} such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclei}} may or may not fuse as well. Some cells can be artificially induced to fuse with each other by treating them with a fusogen such as polyethylene glycol or by passing an electric current through them.<ref name="MacLean"/></dd>

{{term|cell line}}{{anchor|cell lines}} <dd>A population of {{gli|cells}} {{gli|cell culture|cultured}} ''{{gli|in vitro}}'' that is descended from a single primary culture through one or more generations or subcultures. All of the cells of an established cell line are (hypothetically) genetically identical both within and across generations, and tend to share the same patterns of {{gli|gene expression}} when cultured in similar conditions. Established lines that are also {{gli|immortalized}} can be propagated indefinitely with little or no {{gli|cellular senescence}}.<ref name="Oxford B&MB"/></dd>

{{term|cell membrane}}{{anchor|cell membranes|plasma membrane|plasmalemma}} {{ghat|Also '''plasma membrane''', '''cytoplasmic membrane''', and '''plasmalemma'''.}} <dd>The selectively permeable {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane}} surrounding all prokaryotic and eukaryotic cells, defining the outermost boundary of the cell and physically separating the {{gli|cytoplasm}} from the {{gli|extracellular}} environment.<ref name="SwissBioPics">{{#invoke:cite|web|author1=SwissBioPics |title=Animal cell |url=https://www.swissbiopics.org/name/Animal_cell |website=www.swissbiopics.org |publisher=Swiss Institute of Bioinformatics (SIB)}}</ref> Like all membranes, the cell membrane is a flexible, fluid, sheet-like {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipid bilayer}} with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane proteins}}, {{gli|carbohydrates}}, and numerous other molecules embedded within or interacting with it from both sides. Embedded molecules often have {{gli|fluid mosaic model|freedom to move laterally}} alongside the membrane's lipids. Though the cell membrane can be freely crossed by many ions, small organic molecules, and water, most other substances require {{gli|active transport}} through special pores or {{gli|channel proteins|channels}} or by {{gli|endocytosis}} or {{gli|exocytosis}} in order to enter or exit the cell, especially very large or electrically charged molecules such as proteins and nucleic acids. Besides regulating the transport of substances into and out of the cell, the cell membrane creates an organized interior space in which to perform life-sustaining activities and plays fundamental roles in all of the cell's interactions with its environment, making it important in {{gli|cell signaling}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motility}}, defense, and {{gli|cell division|division}}, among numerous other processes.</dd> thumb|right|400px|Cross-sectional diagram of a typical eukaryotic '''{{gli|cell membrane}}'''

{{term|cell physiology}} <dd>The study of the various biological activities and biochemical processes which sustain life inside {{gli|cells}}, particularly (but not necessarily limited to) those related to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism}} and energy transfer, growth and {{gli|cell division|reproduction}}, and the ordinary processes of the {{gli|cell cycle}}.</dd>

{{term|cell polarity}} <dd>The spatial variation within a {{gli|cell}}, i.e. the existence of differences in shape, structure, or function between different parts of the same cell. Almost all {{gli|cell types}} exhibit some form of polarity, often along an invisible axis which defines opposing sides or poles where the variation is most extreme. Having internal polarity permits cells to accomplish specialized functions such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|signal transduction}} or to serve as {{gli|epithelial}} cells which must perform different tasks on different sides, or facilitates {{gli|cell migration}} or {{gli|cell division|division}}.</dd>

{{term|cell signaling}}{{anchor|signaling|cell communication}} {{ghat|Also '''cell communication'''.}} <dd>The diverse set of processes by which cells transmit information to and receive information from themselves, from other cells, or from their surroundings. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|signal transduction|Signal transduction}} occurs in all cell types, prokaryotic and eukaryotic, and is of critical importance to the cell's ability to navigate and survive its physical environment. Countless mechanisms of signaling have evolved in different organisms and are often categorized according to the proximity between sender and recipient ({{gli|autocrine}}, {{gli|intracrine}}, {{gli|juxtacrine}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|paracrine}}, or {{gli|endocrine}}).</dd>

{{term|cell surface receptor}}{{anchor|cell surface receptors}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|receptor}} proteins embedded within or attached to the external surface of the {{gli|cell membrane}}, with one or more {{gli|binding sites}} facing the {{gli|extracellular}} environment and one or more {{gli|effector|effector sites}} that couple the binding of a particular {{gli|ligand}} to an {{gli|intracellular}} event or process. Cell surface receptors are a primary means by which environmental signals are received by the cell and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|signal transduction|transmitted}} across the membrane into the cell interior. Some may also bind exogenous ligands and transport them into the cell in a process known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|receptor-mediated endocytosis}}.<ref name="Rieger"/></dd>

{{term|cell wall}}{{anchor|cell walls}} <dd>A tough, variously flexible or rigid layer of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polysaccharide}} or {{gli|glycoprotein}} polymers surrounding some cell types immediately outside of the {{gli|cell membrane}}, including plant cells and most {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prokaryotes}}, which functions as an additional protective and selective barrier and gives the cell a definite shape and structural support. The chemical composition of the cell wall varies widely between taxonomic groups, and even between different stages of the {{gli|cell cycle}}: in land plants it consists primarily of cellulose, hemicellulose, and pectin, while algae make use of carrageenan and agar, fungi use chitin, and bacterial cell walls contain {{gli|peptidoglycan}}.</dd>

{{term|cell-free DNA (cfDNA)}} <dd>Any {{gli|DNA}} molecule that exists outside of a {{gli|cell}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}}, freely floating in an {{gli|extracellular fluid}} such as blood plasma.</dd>

{{term|cellular}} <dd>Of, relating to, consisting of, produced by, or resembling a {{gli|cell}} or cells.</dd>

{{term|cellular differentiation}} <dd>See ''{{gli|differentiation}}''.</dd>

{{term|cellular immunity}} {{ghat|Also '''cell-mediated immunity'''.}} <dd>A class of immune response that does not rely on the production of {{gli|antibodies}} but rather the activation of specific {{gli|cell types}} such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phagocytes}} or cytotoxic T-lymphocytes, or the secretion of various {{gli|cytokines}} from cells, in response to an {{gli|antigen}}.</dd>

{{term|cellular noise}} <dd>Any apparently random variability observed in quantities measured in cell biology, particularly those pertaining to {{gli|gene expression}} levels.<ref>{{#invoke:cite|journal| journal=Annu. Rev. Biophys. Biomol. Struct. | year=2007 | volume=36 | pages=413–434 | title=Living with noisy genes: how cells function reliably with inherent variability in gene expression | vauthors=Maheshri N, O'Shea EK | doi=10.1146/annurev.biophys.36.040306.132705 | pmid=17477840 }}</ref></dd>

{{term|cellular reprogramming}} <dd>The conversion of a terminally {{gli|differentiated}} {{gli|cell}} from one {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissue}}-specific cell type to another. This involves {{gli|dedifferentiation}} to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pluripotent}} state; an example is the conversion of mouse {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|somatic cells}} to an undifferentiated embryonic state, which relies on the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}} ''Oct4'', ''Sox2'', ''Myc'', and ''Klf4''.<ref>{{#invoke:cite|journal|last1=Nishikawa |first1=S. |title=Reprogramming by the numbers |journal=Nature Biotechnology |date=2007 |volume=25 |issue=8 |pages=877–878|doi=10.1038/nbt0807-877 |pmid=17687365 |s2cid=39773318 }}</ref></dd>

{{term|cellular senescence}} <dd></dd>

{{term|centimorgan (cM)}} {{ghat|Also '''map unit (m.u.)'''.}} <dd>A unit for measuring {{gli|linkage|genetic linkage}} defined as the distance between chromosomal {{gli|loci}} for which the expected average number of intervening {{gli|chromosomal crossovers}} in a single generation is 0.01. Though not an actual measure of physical distance, it is used to infer the actual distance between two loci based on the apparent likelihood of a crossover occurring between them in any given {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiotic}} division.</dd>

{{term|central dogma of molecular biology}} <dd>A generalized framework for understanding the flow of genetic information between macromolecules within biological systems. The central dogma outlines the fundamental principle that the sequence information encoded in the three major classes of biopolymer—{{gli|DNA}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}}—can only be transferred between these three classes in certain ways, and not in others: specifically, information transfer between the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}} and from nucleic acid to protein is possible, but transfer from protein to protein, or from protein back to either type of nucleic acid, is impossible and does not occur naturally.</dd> thumb|350px|right|Possible types of information transfer according to the '''{{gli|central dogma of molecular biology}}'''. Three general transfers, in red, occur routinely in all living cells: DNA-to-DNA ({{gli|DNA replication}}), DNA-to-RNA ({{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}), and RNA-to-protein ({{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}). Three special transfers, in blue, are known to occur only in viruses or in the laboratory: RNA-to-RNA ({{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA replication}}), RNA-to-DNA ({{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reverse transcription}}), and DNA-to-protein (direct translation without an mRNA intermediate). An additional three transfers are believed not to be possible at all: protein-to-protein, protein-to-RNA, and protein-to-DNA—though it has been argued that there are exceptions by which all three can occur.

{{term|centriole}}{{anchor|centrioles}} <dd>A cylindrical {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelle}} composed of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microtubules}}, present only in certain eukaryotes. A pair of centrioles migrate to and define the two opposite poles of a {{gli|cell division|dividing cell}} where, as part of a {{gli|centrosome}}, they initiate the growth of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spindle apparatus}}.</dd>

{{term|centromere}}{{anchor|centromeres}} <dd>A specialized DNA sequence within a {{gli|chromosome}} that links a pair of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sister chromatids}}. The primary function of the centromere is to act as the site of assembly for {{gli|kinetochores}}, protein complexes which direct the attachment of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spindle apparatus|spindle fibers}} to the centromere and facilitate {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|segregation}} of the chromatids during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}.</dd>

{{term|centromeric index}} <dd>The proportion of the total length of a {{gli|chromosome}} encompassed by its {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|short arm}}, typically expressed as a percentage; e.g. a chromosome with a centromeric index of 15 is {{gli|acrocentric}}, with a short arm comprising only 15% of its overall length.<ref name="DoG7"/></dd>

{{term|centrosome}}{{anchor|centrosomes}} <dd></dd>

{{term|cfDNA}} <dd>See ''{{gli|cell-free DNA}}''.</dd>

{{term|channel protein}}{{anchor|channel proteins}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transmembrane protein}} whose shape forms an aqueous pore in a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane}}, permitting the passage of specific solutes, often small ions, across the membrane in either or both directions.<ref name="Alberts et al."/></dd>

{{term|chaperone}}{{anchor|chaperones}} <dd></dd>

{{term|Chargaff's rules}} <dd>A set of axioms which state that, in the {{gli|DNA}} of any chromosome, species, or organism, the total number of {{gli|adenine}} ({{font|A|font=courier|size=big}}) {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|residues}} will be approximately equal to the total number of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine}} ({{font|T|font=courier|size=big}}) residues, and the number of {{gli|guanine}} ({{font|G|font=courier|size=big}}) residues will be equal to the number of {{gli|cytosine}} ({{font|C|font=courier|size=big}}) residues; accordingly, the total number of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|purines}} ({{font|A|font=courier|size=big}} + {{font|G|font=courier|size=big}}) will equal the total number of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyrimidines}} ({{font|T|font=courier|size=big}} + {{font|C|font=courier|size=big}}). These observations illustrate the highly specific nature of the {{gli|complementary}} {{gli|base pair|base-pairing}} that occurs in all {{gli|duplex}} DNA molecules: even though non-standard pairings are technically possible, they are exceptionally rare because the standard ones are strongly favored in most conditions. Still, the 1:1 equivalence is seldom exact, since at any given time nucleobase ratios are inevitably distorted to some small degree by {{gli|DNA repair|unrepaired}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mismatch|mismatches}}, missing bases, and non-canonical bases. The presence of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssDNA|single-stranded DNA}} polymers also alters the proportions, as an individual {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} may contain any number of any of the bases.</dd>

{{term|charged tRNA}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tRNA|transfer RNA}} to which an {{gli|amino acid}} has been attached; i.e. an {{gli|aa-tRNA|aminoacylated tRNA}}. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|uncharged tRNA|Uncharged tRNAs}} lack amino acids.<ref name="DoG7"/></dd>

{{term|chDNA}} <dd>See ''{{gli|chloroplast DNA}}''.</dd>

{{term|checkpoint}}{{anchor|checkpoints}} <dd></dd>

{{term|chemiosmosis}}{{anchor|chemiosmotic}} <dd></dd>

{{term|chemokinesis}} <dd>A non-directional, random change in the movement of a molecule, cell, or organism in response to a chemical stimulus, e.g. a change in speed resulting from exposure to a particular chemical compound.</dd>

{{term|chemotaxis}} <dd>A directed, non-random change in the movement of a molecule, cell, or organism in response to a chemical stimulus, e.g. towards or away from an area with a high concentration of a particular chemical compound.<ref name="Alberts et al."/></dd>

{{term|chiasma}}{{anchor|chiasmata}} {{ghat|(pl.) '''chiasmata'''}} <dd>A cross-shaped junction that forms the physical point of contact between two non-sister {{gli|chromatids}} belonging to {{gli|homologous chromosomes}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|synapsis}}. As well as ensuring proper {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|segregation}} of the chromosomes, these junctions are also the {{gli|breakpoints}} at which {{gli|chromosomal crossover}} may occur during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}, which results in the reciprocal exchange of DNA between the synapsed chromatids.</dd>

{{term|chimerism}}{{anchor|chimera}} <dd>The presence of two or more populations of cells with distinct {{gli|genotypes}} in an individual organism, known as a ''chimera'', which has developed from the fusion of cells originating from separate {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygotes}}; each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical tissues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. by allogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor); in plants, it can result from grafting or errors in cell division. It is similar to but distinct from {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mosaicism}}.</dd>

{{term|chloroplast}}{{anchor|chloroplasts}} <dd>A type of small, lens-shaped {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plastid}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelle}} found in the cells of green algae and plants which contains light-sensitive photosynthetic pigments and in which the series of biochemical reactions that comprise photosynthesis takes place. Like {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondria}}, chloroplasts are bound by a double membrane, contain their own {{gli|cpDNA|internal circular DNA molecules}} from which they direct transcription of a unique set of genes, and replicate independently of the nuclear genome.<ref name="MacLean"/><ref name="Lackie"/></dd>

{{term|chloroplast DNA (cpDNA, chDNA, ctDNA)}} <dd>The set of {{gli|DNA}} molecules contained within {{gli|chloroplasts}}, a type of photosynthetic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plastid}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelle}} located within the cells of some eukaryotes such as plants and algae, representing a semi-autonomous {{gli|genome}} separate from that within the cell's nucleus. Like other types of plastid DNA, cpDNA usually exists in the form of small circular {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmids}}.</dd>

{{term|cholesterol}} <dd>A chemical compound that is the principal sterol biosynthesized by animal cells and an essential component of {{gli|cell membranes}}, in which it serves to buffer the membrane's {{gli|fluid mosaic model|fluidity}} and plays roles in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|signal transduction}}. It is produced primarily in tissues of the liver and the nervous system, and is transported in an esterified form by {{gli|lipoproteins}} in the blood plasma.<ref name="Lackie"/></dd>

{{term|chondriome}} <dd>The complete set of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondria}} or of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondrial DNA}} within a cell, tissue, organism, or species.</dd>

{{term|chromatid}}{{anchor|chromatids}} <dd>One copy of a newly copied {{gli|chromosome}}, which is joined to the original chromosome by a {{gli|centromere}}. Paired copies of the same individual chromosome are known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sister chromatids}}.</dd>

{{term|chromatin}} <dd>A complex of {{gli|DNA}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} found in eukaryotic cells that is the primary substance comprising {{gli|chromosomes}}. Chromatin functions as a means of {{gli|DNA condensation|packaging}} very long DNA molecules into highly organized and densely compacted shapes, which prevents the strands from becoming tangled, reinforces the DNA during {{gli|cell division}}, helps to prevent DNA damage, and plays an important role in regulating {{gli|gene expression}} and {{gli|DNA replication}}.</dd>

{{term|chromatin immunoprecipitation (ChIP)}} <dd></dd>

{{term|chromocenter}} <dd>A central amorphous mass of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polytene chromosomes}} found in the nuclei of cells of the salivary glands in ''Drosophila'' larvae and resulting from the fusion of {{gli|heterochromatic}} regions surrounding the {{gli|centromeres}} of the somatically paired chromosomes, with the distal {{gli|euchromatic}} arms radiating outward.<ref name="DoG7"/></dd>

{{term|chromomere}} {{ghat|Also '''idiomere'''.}} <dd>A region of a {{gli|chromosome}} that has been locally compacted or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|supercoiling|coiled}} into {{gli|chromatin}}, conspicuous under a microscope as a "bead", node, or dark-staining band, especially when contrasted with nearby uncompacted strings of DNA.</dd>

{{term|chromosomal crossover}}{{anchor|chromosomal crossovers|crossover|crossovers}} {{ghat|Also '''crossing over'''.}} <dd></dd>

{{term|chromosomal DNA}} <dd>{{gli|DNA}} contained in {{gli|chromosomes}}, as opposed to {{gli|extrachromosomal DNA}}. The term is generally used synonymously with {{gli|genomic DNA}}.</dd>

{{term|chromosomal duplication}} <dd>The {{gli|duplication}} of an entire {{gli|chromosome}}, as opposed to a segment of a chromosome or an {{gli|gene duplication|individual gene}}.</dd>

{{term|chromosomal instability}} <dd></dd>

{{term|chromosome}}{{anchor|chromosomes|chromosomal}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear}} {{gli|DNA}} molecule containing part or all of the genetic material of an organism. Chromosomes may be considered a sort of molecular "package" for carrying DNA within the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} of cells and, in most eukaryotes, are composed of long strands of DNA coiled with {{gli|histone|packaging proteins}} which bind to and {{gli|DNA condensation|condense}} the strands to prevent them from becoming an unmanageable tangle. Chromosomes are most easily distinguished and studied in their completely condensed forms, which only occur during {{gli|cell division}}. Some simple organisms have only one chromosome made of circular DNA, while most eukaryotes have multiple chromosomes made of linear DNA.</dd>

{{term|chromosome condensation}} <dd>The process by which eukaryotic chromosomes become shorter, thicker, denser, and more conspicuous under a microscope during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prophase}} due to systemic coiling and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|supercoiling}} of {{gli|chromatin|chromatic}} strands of DNA in preparation for {{gli|cell division}}.</dd>

{{term|chromosome segregation}} {{anchor|segregation|segregate|segregates|segregating|segregated}} <dd>The process by which {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sister chromatids}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|synapsis|paired}} {{gli|homologous chromosomes}} separate from each other and migrate to opposite sides of the dividing cell during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}.</dd>

{{term|chromosome walking}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primer walking}}''.</dd>

{{term|cilium}}{{anchor|cilia}} {{ghat|(pl.) '''cilia'''}} <dd>A slender, thread-like, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane-bound organelle|membrane-bound}} projection extending from the surface of a eukaryotic cell, longer than a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microvillus}} but shorter than a {{gli|flagellum}}. Most eukaryotic cells have at least one ''primary cilium'' serving sensory or signaling functions; some cells employ thousands of ''motile cilia'' covering their entire surface in order to achieve locomotion or to move extracellular material past the cell.</dd>

{{term|circular DNA}} <dd>Any {{gli|DNA}} molecule, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssDNA|single-stranded}} or {{gli|dsDNA|double-stranded}}, which forms a continuous closed loop without ends; e.g. bacterial chromosomes, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mtDNA|mitochondrial}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plastid|plastid DNA}}, as well as many other varieties of {{gli|extrachromosomal DNA}}, including {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmids}} and some viral DNA. Contrast ''{{gli|linear DNA}}''.</dd>

{{term|circulating tumor DNA (ctDNA)}} <dd>Any {{gli|cfDNA|extracellular DNA}} fragments derived from tumor cells which are circulating freely in the bloodstream.</dd>

{{term|''cis''}} <dd>On the same side; adjacent to; {{gli|cis-acting|acting}} from the same molecule. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|trans}}''.</dd>

{{term|''cis''-acting}} <dd>Affecting a {{gli|gene}} or sequence on the same nucleic acid molecule. A {{gli|locus}} or sequence within a particular DNA molecule such as a {{gli|chromosome}} is said to be ''cis''-acting if it influences or acts upon other sequences located within short distances (i.e. physically nearby, usually but not necessarily {{gli|downstream}}) on the same molecule or chromosome; or, in the broadest sense, if it influences or acts upon other sequences located anywhere (not necessarily within a short distance) on the same chromosome of a {{gli|homologous chromosomes|homologous pair}}. ''Cis''-acting factors are often involved in the {{gli|gene regulation|regulation}} of {{gli|gene expression}} by acting to inhibit or to facilitate {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|trans-acting}}''.</dd>

{{term|''cis''-dominant mutation}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} occurring within a {{gli|cis-regulatory element|''cis''-regulatory element}} (such as an {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|operator}}) which alters the functioning of a nearby {{gli|gene}} or genes on the same {{gli|chromosome}}. ''Cis''-dominant mutations affect the {{gli|expression}} of genes because they occur at sites that control transcription rather than within the genes themselves.</dd>

{{term|cisgenesis}} <dd></dd>

{{term|''cis''-regulatory element (CRE)}} {{ghat|Also '''''cis''-regulatory module (CRM)'''.}} <dd>Any sequence or region of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ncDNA|non-coding DNA}} which {{gli|regulates}} the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} of nearby {{gli|genes}} (e.g. a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoter}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|operator}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|silencer}}, or {{gli|enhancer}}), typically by serving as a binding site for one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|trans-regulatory}}''.</dd>

{{term|cisterna}}{{anchor|cisternae}} {{ghat|(pl.) '''cisternae'''}} <dd>Any of a class of flattened, membrane-bound {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicles}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|saccules}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|smooth ER|smooth}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|rough ER|rough}} {{gli|endoplasmic reticulum}} and the {{gli|Golgi apparatus}}. By traveling through one or more cisternae, each of which contains a distinct set of enzymes, newly created proteins and polysaccharides undergo chemical modifications such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphorylation}} and {{gli|glycosylation}}, which are used as packaging signals to direct their transport to specific destinations within the cell.<ref>{{#invoke:cite|journal|last1=Connerly |first1=P. L. |title=How Do Proteins Move Through the Golgi Apparatus? |journal=Nature Education |date=2010 |volume=3 |issue=9 |page=60 |url=https://www.nature.com/scitable/topicpage/how-do-proteins-move-through-the-golgi-14397318/#:~:text=The%20Golgi%20processes%20proteins%20made,the%20cell%20(trans%20side).}}</ref></dd>

{{term|citric acid cycle}} <dd></dd>

{{term|classical genetics}} <dd>The branch of {{gli|genetics}} based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular biology}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular genetics}}''.</dd>

{{term|cleavage}}{{anchor|cleave|cleaves|cleaved|cleaving}} <dd>1. The physical separation of a {{gli|cell division|dividing}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|parent cell}} into multiple individual {{gli|daughter cells}}.</dd> <dd>2. In {{gli|embryology}}, the series of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitotic}} divisions by which a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygote|fertilized ovum}} is divided, without an accompanying overall change in size, into a ball of smaller cells constituting the early {{gli|embryo}}.<ref name="Oxford B&MB"/></dd>

{{term|cleavage furrow}} <dd>A trough-like indentation in the surface of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|parent cell}}, often conspicuous when viewed through a microscope, that initiates the {{gli|cleavage}} of the cytoplasm ({{gli|cytokinesis}}) as the {{gli|contractile ring}} begins to narrow during {{gli|cell division}}.</dd>

{{term|clonal selection}} <dd></dd>

{{term|cloning}}{{anchor|cloned}} <dd>The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms of asexual reproduction, and cells that undergo {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular cloning}}, copies of DNA fragments or other molecules.</dd>

{{term|closed chromatin}} <dd>See ''{{gli|heterochromatin}}''.</dd>

{{term|coactivator}}{{anchor|coactivators}} <dd>A type of {{gli|coregulator}} that increases the {{gli|expression}} of one or more genes by binding to an {{gli|activator}}.</dd>

{{term|coding strand}} {{ghat|Also '''sense strand''', '''positive (+) sense strand''', and '''nontemplate strand'''.}} <dd>The strand of a double-stranded DNA molecule whose nucleotide sequence corresponds directly to that of the RNA transcript produced during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} (except that {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine}} bases are substituted with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|uracil}} bases in the RNA molecule). Though it is not itself transcribed, the coding strand is by convention the strand used when displaying a DNA sequence because of the direct analogy between its sequence and the {{gli|codons}} of the RNA product. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|template strand}}''; see also ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sense}}''.</dd>

{{term|codon}}{{anchor|codons}} <dd>A series of three consecutive {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} in a coding region of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} sequence. Each of these triplets codes for a particular {{gli|amino acid}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stop codon|stop signal}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation|protein synthesis}}. {{gli|DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules are each written in a language using four "letters" (four different {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}}), but the language used to construct proteins includes 20 "letters" (20 different amino acids). Codons provide the key that allows these two languages to be {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translated}} into each other. In general, each codon corresponds to a single amino acid (or stop signal). The full set of codons is called the {{gli|genetic code}}.</dd>

{{term|codon usage bias}} <dd>The preferential use of a particular {{gli|codon}} to code for a particular {{gli|amino acid}} rather than alternative codons that are synonymous for the same amino acid, as evidenced by differences between organisms in the frequencies of the synonymous codons occurring in their coding DNA. Because the {{gli|genetic code}} is {{gli|degeneracy|degenerate}}, most amino acids can be specified by multiple codons. Nevertheless, certain codons tend to be overrepresented (and others underrepresented) in different species.</dd>

{{term|coenocyte}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|multinucleate}} mass of {{gli|cytoplasm}} bounded by a {{gli|cell wall}} and resulting from continuous cytoplasmic growth and repeated nuclear division without {{gli|cytokinesis}}, found in some species of algae and fungi, e.g. ''Vaucheria'' and ''Physarum''.<ref name="MacLean">{{#invoke:cite|book|last1=MacLean |first1=Norman |title=Dictionary of Genetics & Cell Biology |date=1987 |publisher=New York University Press |location=New York |isbn=0-8147-5438-4 |url=https://openlibrary.org/works/OL4375066W/Dictionary_of_genetics_cell_biology?edition=key%3A/books/OL2398237M}}</ref></dd>

{{term|coenzyme}}{{anchor|coenzymes}} <dd>A relatively small, independent {{gli|cofactor}} which associates with a specific {{gli|enzyme}} and participates in the reaction(s) catalyzed by the enzyme, often by forming a covalent bond with the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substrate}}. Examples include biotin, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nicotinamide adenine dinucleotide|NAD+}}, and {{gli|coenzyme A}}.<ref name="Alberts et al."/></dd>

{{term|coenzyme A (CoA)}}{{anchor|coenzyme A|CoA}} {{ghat|Also abbreviated '''SHCoA''' and '''CoASH'''.}} <dd>A {{gli|coenzyme}} derived from pantothenic acid that functions as a substrate for a wide range of {{gli|enzymes}} in all living organisms and is notable for its role as a carrier of acyl groups (e.g. acetyl) which attach to its terminal sulphydryl (SH) group via a thioester bond.<ref name="MacLean"/> Its {{gli|acetylated}} form, known as {{gli|acetyl-CoA}}, is particularly important to numerous metabolic pathways, both {{gli|anabolic}} and {{gli|catabolic}}, including the synthesis and oxidation of {{gli|fatty acids}} and the oxidation of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyruvate}} as part of the {{gli|citric acid cycle}}.</dd>

{{term|cofactor}}{{anchor|cofactors}} <dd>Any non-protein organic compound capable of binding to or interacting with an {{gli|enzyme}}. Cofactors are required for the initiation of {{gli|catalysis}}.</dd>

{{term|comparative genomic hybridization (CGH)}}{{anchor|comparative genomic hybridization}} <dd></dd>

{{term|competence}}{{anchor|competent|competent cell|competent cells}} <dd></dd>

{{term|complementarity}}{{anchor|complementary|complement|complements}} <dd>A property of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} biopolymers whereby two polymeric chains or "{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}}" aligned {{gli|antiparallel}} to each other will tend to form {{gli|base pairs}} consisting of hydrogen bonds between the individual {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}} comprising each chain, with each type of nucleobase pairing almost exclusively with one other type of nucleobase; e.g. in {{gli|dsDNA|double-stranded}} {{gli|DNA}} molecules, {{font|{{gli|adenine|A}}|font=courier|size=big}} pairs only with {{font|{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine|T}}|font=courier|size=big}} and {{font|{{gli|cytosine|C}}|font=courier|size=big}} pairs only with {{font|{{gli|guanine|G}}|font=courier|size=big}}. Strands that are paired in such a way, and the bases themselves, are said to be ''complementary''. The degree of complementarity between two strands strongly influences the stability of the {{gli|duplex}} molecule; certain sequences may also be internally complementary, which can result in a single strand {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid secondary structure|binding to itself}}. Complementarity is fundamental to the mechanisms governing {{gli|DNA replication}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}, and {{gli|DNA repair}}.</dd>

{{term|complementary DNA (cDNA)}} {{ghat|Also '''copy DNA'''.}} <dd>{{gli|DNA}} that is synthesized from a single-stranded {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} template (typically {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|miRNA}}) in a reaction catalyzed by the enzyme {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reverse transcriptase}}. cDNA is produced both naturally by retroviruses and artificially in certain laboratory techniques, particularly {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular cloning}}. In {{gli|bioinformatics}}, the term may also be used to refer to the sequence of an mRNA transcript expressed as its DNA {{gli|coding strand}} counterpart (i.e. with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine}} replacing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|uracil}}).</dd>

{{term|compound X}} <dd>See ''{{gli|attached X}}''.</dd>

{{term|conditional expression}} <dd>The controlled, inducible {{gli|expression}} of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transgene}}, either ''{{gli|in vitro}}'' or ''{{gli|in vivo}}''.</dd>

{{term|confluence}}{{anchor|confluent}} {{ghat|Also '''confluency'''.}} <dd>In {{gli|cell culture}}, a measure of the proportion of the surface area of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Petri dish|culture vessel}} that is covered by {{gli|adherent culture|adherent cells}}, commonly expressed as a percentage. A culture in which the entire surface is completely covered by a continuous {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monolayer}}, such that all cells are immediately adjacent to and in direct physical contact with other cells, with no gaps or voids, is said to be 100-percent confluent. Different {{gli|cell lines}} may exhibit differences in morphology, growth rate, or {{gli|gene expression}} depending on the degree of confluence. Because of {{gli|contact inhibition}}, most show a significant reduction in the rate of {{gli|cell division}} as they approach complete confluence, though some {{gli|immortalized cells}} may continue to divide, expanding vertically rather than horizontally by stacking themselves on top of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|parent cells}}, until all available nutrients are depleted.<ref name="MacLean"/><ref name="Lackie"/></dd>

{{term|conformation}}{{anchor|conformations}} <dd>The three-dimensional spatial configuration of the atoms comprising a molecule or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|macromolecular}} structure.<ref name="MacLean"/> The conformation of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} is the physical shape into which its {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}} chains arrange themselves during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein folding}}, which is not necessarily rigid and may {{gli|conformational change|change}} with the protein's particular chemical environment.</dd>

{{term|conformational change}}{{anchor|conformational changes}} <dd>A change in the spatial {{gli|conformation}} or physical shape of a molecule or macromolecule such as a protein or nucleic acid, rarely spontaneously but more commonly as a result of some alteration in the molecule's chemical environment (e.g. temperature, pH, salt concentration, etc.) or an interaction with another molecule. Changes in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tertiary structures}} of proteins can affect whether or how strongly they bind {{gli|ligands}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substrates}}; inducing these changes is a common means (both naturally and artificially) of activating, inactivating, or otherwise controlling the function of many enzymes and receptor proteins.<ref name="Lackie"/></dd>

{{term|congression}} <dd>The movement of chromosomes to the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spindle equator}} during the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prometaphase}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metaphase}} stages of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}}.<ref name="DoG7"/></dd>

{{term|consensus sequence}} {{ghat|Also '''canonical sequence'''.}} <dd>A calculated order of the most frequent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|residues}} (of either {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} or {{gli|amino acids}}) found at each position in a common {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence alignment}} and obtained by comparing multiple closely related sequence alignments.</dd>

{{term|conservative replication}} <dd>A hypothetical mode of {{gli|DNA replication}} in which the two parental {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}} of the original {{gli|double-stranded DNA}} molecule ultimately remain hybridized to each other at the end of the replication process, with the two daughter strands forming their own separate molecule; hence one molecule is composed of both of the starting strands while the other is composed of the two newly synthesized strands. This is in contrast to {{gli|semiconservative replication}}, in which each molecule is a hybrid of one old and one new strand. See also ''{{gli|dispersive replication}}''.</dd>

{{term|conserved sequence}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence|nucleic acid}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} sequence that is highly similar or identical across many species or within a {{gli|genome}}, indicating that it has remained relatively unchanged through a long period of evolutionary time.</dd>

{{term|constitutive expression}} <dd>1.  The continuous {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} of a {{gli|gene}}, as opposed to {{gli|facultative expression}}, in which a gene is only transcribed as needed. A gene that is transcribed continuously is called a ''constitutive gene''.</dd> <dd>2.  A gene whose expression depends only on the efficiency of its {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoter}} in binding {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA polymerase}},<ref name="DoG7"/> and not on any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}} or other {{gli|gene regulation|regulatory elements}} which might {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|upregulation|promote}} or {{gli|downregulation|inhibit}} its transcription.</dd>

{{term|contact inhibition}} {{ghat|Also '''contact inhibition of growth''' or '''density-dependent inhibition'''.}} <dd>In {{gli|cell culture}}, the phenomenon by which most normal eukaryotic cells {{gli|adherent culture|adhering to a planar substratum}} cease to grow and {{gli|cell division|divide}} upon reaching a critical cell density, usually as they approach full {{gli|confluence}} or come into physical contact with other cells. As a result, many types of cells cultured on plates or in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Petri dishes}} will continue to proliferate until they cover the whole surface of the culture vessel, at which point the rate of cell division abruptly decreases or is arrested entirely, thus forming a confluent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monolayer}} with minimal overlap between neighboring cells, even if the nutrient medium remains plentiful, rather than stacking themselves on top of each other.<ref name="Rieger"/> {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transformation|Transformed}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|neoplastic}} cells tend not to respond to cell density in the same way and may continue to proliferate at high densities.<ref name="MacLean"/> This type of density-dependent inhibition of growth is similar to and may occur simultaneously with, but is nonetheless distinct from, the related phenomenon of contact inhibition of movement,<ref name="Lackie"/> whereby moving cells respond to physical contact by temporarily stopping and then reversing their direction of locomotion away from the point of contact.</dd>

{{term|contig}}{{anchor|contigs}} <dd>A continuous sequence of {{gli|genomic DNA}} generated by assembling cloned fragments by means of their overlapping sequences.<ref name="Lewin"/></dd>

{{term|cooperativity}} {{ghat|Also '''cooperative binding'''.}} <dd>A phenomenon observed in some {{gli|enzymes}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|receptor proteins}}, and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein complexes}} which have multiple {{gli|binding sites}}, whereby the binding of a {{gli|ligand}} to one or more sites apparently increases or decreases the affinity of one or more other binding sites for other ligands. This concept highlights the sensitive nature of the chemistry that governs interactions between biomolecules: the strength and specificity of interactions between protein and ligand are influenced, sometimes substantially, by nearby interactions (often {{gli|conformational changes}}) and by the local chemical environment in general. Cooperativity is frequently invoked to account for the non-linearity of data resulting from attempts to measure the association/dissociation constants of particular {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein–protein interactions}}.<ref name="Lackie"/></dd>

{{term|copy DNA (cDNA)}} <dd>See ''{{gli|cDNA|complementary DNA}}''.</dd>

{{term|copy error}}{{anchor|copy errors}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} resulting from a mistake made during {{gli|DNA replication}}.<ref name="DoG7"/></dd>

{{term|copy-number variation (CNV)}}{{anchor|copy number}} <dd>A phenomenon in which sections of a {{gli|genome}} are repeated and the number of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repeats}} varies between individuals in the population, usually as a result of {{gli|gene duplication|duplication}} or {{gli|deletion}} events that affect entire genes or sections of chromosomes. Copy-number variations play an important role in generating {{gli|genetic variation}} within a population.</dd>

{{term|coregulator}}{{anchor|coregulators}} <dd>A protein that works together with one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}} to regulate {{gli|gene expression}}.</dd>

{{term|corepressor}}{{anchor|corepressors}} <dd>A type of {{gli|coregulator}} that reduces (represses) the {{gli|expression}} of one or more genes by binding to and activating a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repressor}}.</dd>

{{term|cosmid}}{{anchor|cosmids}} <dd></dd>

{{term|cpDNA}} <dd>See ''{{gli|cpDNA|chloroplast DNA}}''.</dd>

{{term|CpG island}}{{anchor|CpG islands}} {{ghat|Also '''CG island''' and '''C-G island'''.}} <dd>A region of a {{gli|genome}} in which {{gli|CpG sites}} occur {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repeat|repetitively}} or with high frequency.</dd>

{{term|CpG site}}{{anchor|CpG sites|CpG|CG|C-G}} {{ghat|Also '''CG site''' and '''C-G site'''.}} <dd>A sequence of DNA in which a {{gli|cytosine}} nucleotide is immediately followed by a {{gli|guanine}} nucleotide on the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} in the 5'-to-3' {{gli|direction}}; the "p" in CpG refers simply to the intervening {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate}} group linking the two consecutive nucleotides.</dd>

{{term|CRISPR gene editing}} {{ghat|Also '''CRISPR/Cas9 gene editing'''.}} <dd></dd>

{{term|crista}}{{anchor|cristae}} {{ghat|(pl.) '''cristae'''}} <dd>Any of numerous folds or {{gli|invaginations}} in the inner {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondrial}} membrane, which give this membrane its characteristic wrinkled shape and increase the surface area across which {{gli|aerobic gas exchange}} and supporting {{gli|electron transport chain|electron transport}} reactions can occur. Cristae are studded with proteins such as ATP synthase and various cytochromes.</dd>

{{term|crossing over}} <dd>See ''{{gli|chromosomal crossover}}''.</dd>

{{term|crosslink}}{{anchor|crosslinks|crosslinking|crosslinked}} {{ghat|Also '''cross-link'''.}} <dd>Any chemical bond or series of bonds, normal or abnormal, natural or artificial, that connects two or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymeric}} molecules to each other, creating an even larger, often structurally rigid and mechanically durable {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|macromolecular}} complex. Crosslinks may consist of covalent, ionic, or intermolecular interactions, or even extensive physical entanglements of molecules, and may be reversible or irreversible; in polymer chemistry the term is often used to describe macrostructures that form predictably in the presence of a specific catalyst. In {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular biology}} the usage generally implies abnormal bonding (whether naturally occurring or experimentally induced) between different {{gli|biomolecules}} (or different parts of the same biomolecule) which are ordinarily separate, especially {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}. Crosslinking of DNA may occur between {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}} on opposite {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}} of a {{gli|dsDNA|double-stranded DNA}} molecule (''interstrand''), or between bases on the same strand (''intrastrand''), via the formation of covalent bonds that are stronger than the hydrogen bonds of normal {{gli|base pairing}}; these are common targets of {{gli|DNA repair}} pathways. Proteins are also susceptible to becoming crosslinked to DNA or to other proteins through bonds to specific surface residues, a process which is deliberately induced in many laboratory methods such as {{gli|fixation}} and which can be useful for studying {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein–protein interactions|interactions between proteins}} in their {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|native states}}. Crosslinks are generated by a variety of {{gli|exogenous}} and {{gli|endogenous}} agents, including chemical compounds and high-energy radiation, and tend to interfere with normal cellular processes such as {{gli|DNA replication}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}, meaning their persistence usually compromises cell health.</dd> [[File:Formaldehyde Crosslinking reaction.png|thumb|350px|DNA (black) and protein (blue) can undergo '''{{gli|crosslinking}}''' in the presence of sufficiently concentrated formaldehyde (red).]]

{{term|ctDNA}} <dd>1.  An abbreviation of {{gli|ctDNA|circulating tumor DNA}}.</dd> <dd>2.  An abbreviation of {{gli|cpDNA|chloroplast DNA}}.</dd>

{{term|C-terminus}}{{anchor|C-terminal|c-terminal}} {{ghat|Also '''carboxyl terminus'''.}} <dd>The end of a linear chain of {{gli|amino acids}} (i.e. a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptide}}) that is terminated by the free carboxyl group ({{chem|–COOH}}) of the last amino acid to be added to the chain during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}. This amino acid is said to be ''C-terminal''. By convention, sequences, domains, active sites, or any other structure positioned nearer to the C-terminus of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}} or the folded {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} it forms relative to others are described as {{gli|downstream}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|N-terminus}}''.</dd>

{{term|cut}} <dd></dd>

{{term|C-value}} <dd>The total amount of {{gli|DNA}} contained within a {{gli|haploid}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} (e.g. a {{gli|gamete}}) of a particular organism or species, expressed in number of {{gli|base pairs}} or in units of mass (typically picograms); or, equivalently, one-half the amount in a {{gli|diploid}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|somatic cell}}. For simple diploid {{gli|eukaryotes}} the term is often used interchangeably with {{gli|genome size}}, but in certain cases, e.g. in hybrid {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polyploids}} descended from parents of different species, the C-value may actually represent two or more distinct {{gli|genomes}} contained within the same nucleus. C-values apply only to {{gli|genomic DNA}}, and notably exclude {{gli|extrachromosomal DNA|extranuclear DNA}}.</dd>

{{term|C-value enigma}} {{ghat|Also '''C-value paradox'''.}} <dd>A term used to describe a diverse variety of questions regarding the immense variation in nuclear {{gli|C-value}} or {{gli|genome size}} among eukaryotic species, in particular the observation that genome size does not correlate with the perceived complexity of organisms, nor necessarily with the number of {{gli|genes}} they possess; for example, many single-celled protists have genomes containing thousands of times more DNA than the human genome. This was considered paradoxical until the discovery that eukaryotic genomes consist mostly of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ncDNA|non-coding DNA}}, which lacks genes by definition. The focus of the enigma has since shifted to understanding why and how eukaryotic genomes came to be filled with so much non-coding DNA, and why some genomes have a higher gene content than others.</dd>

{{term|cyclic adenosine monophosphate (cAMP)}} <dd></dd>

{{term|cyclosis}} <dd>See ''{{gli|cytoplasmic streaming}}''.</dd>

{{term|term=cytidine|content=cytidine ({{font|C|font=courier|size=large}}, Cyd)}} <dd>One of the four standard {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosides}} used in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules, consisting of a {{gli|cytosine}} {{gli|base}} with its N9 nitrogen {{gli|glycosidic bond|bonded}} to the C1 carbon of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} sugar. Cytosine bonded to {{gli|deoxyribose}} is known as {{gli|deoxycytidine}}, which is the version used in {{gli|DNA}}.</dd>

{{term|cytochemistry}} <dd>The branch of {{gli|cell biology}} involving the detection and identification of various cellular structures and components, in particular their {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|subcellular localization|localization within cells}}, using techniques of biochemical analysis and visualization such as chemical {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|staining}} and {{gli|immunostaining}}, spectrophotometry and spectroscopy, radioautography, and electron microscopy.</dd>

{{term|cytogenetics}} <dd>The branch of {{gli|genetics}} that studies how {{gli|chromosomes}} influence and relate to cell behavior and function, particularly during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}.</dd>

{{term|cytokine}}{{anchor|cytokines}} <dd>Any of a broad and loosely defined class of small {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptides}} which have functions in {{gli|cell signaling|intercellular signaling}} (primarily {{gli|autocrine}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|paracrine}}, and {{gli|endocrine}} pathways), typically by interacting with specific {{gli|cell surface receptor|receptors on the exterior surface of cells}}.<ref name="Alberts et al."/></dd>

{{term|cytokinesis}} <dd>The final stage of {{gli|cell division}} in both {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}, usually immediately following the division of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}}, during which the {{gli|cytoplasm}} of the parent cell is {{gli|cleaved}} and divided approximately evenly between two {{gli|daughter cells}}. In animal cells, this process occurs by the closing of a microfilament {{gli|contractile ring}} in the equatorial region of the dividing cell. Contrast ''{{gli|karyokinesis}}''.</dd>

{{term|cytology}}{{anchor|cytological}} <dd>The study of the morphology, processes, and life history of living {{gli|cells}}, particularly by means of light and electron microscopy.<ref name="MacLean"/> The term is also sometimes used as a synonym for the broader field of {{gli|cell biology}}.</dd>

{{term|cytolysis}} <dd>See ''{{gli|lysis}}''.</dd>

{{term|cytometer}} <dd></dd>

{{term|cytomics}} <dd>The interdisciplinary field that studies {{gli|cell biology}}, {{gli|cytology}}, and {{gli|biochemistry}} at the level of an individual cell by making use of single-cell molecular techniques and advanced microscopy to visualize the interactions of cellular components ''{{gli|in vivo}}''.<ref>{{#invoke:cite|journal|last1=Valet|first1=Günter|title=Cytomics: An entry to biomedical cell systems biology|journal=Cytometry Part A|volume=63A|issue=2|year=2005|pages=67–68|issn=1552-4922|doi=10.1002/cyto.a.20110|pmid=15657925 |doi-access=free}}</ref></dd>

{{term|cytoplasm}}{{anchor|cytoplasmic}} <dd>All of the material contained within a {{gli|cell}} excluding (in eukaryotes) the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}};<ref name="Alberts et al."/><ref name="Lackie"/> i.e. that part of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protoplasm}} which is enclosed by the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} but separated from the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoplasm}} by the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear envelope}}, consisting of the fluid {{gli|cytosol}} and the totality of its contents, including all of the cell's internal {{gli|compartmentalization|compartments}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelles}}, and substructures such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondria}}, {{gli|lysosomes}}, the {{gli|endoplasmic reticulum}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicles}} and {{gli|inclusions}}, and a network of filamentous {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microtubules}} known as the {{gli|cytoskeleton}}.<ref name="MacLean"/> Some definitions of cytoplasm exclude certain organelles such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vacuoles}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plastids}}. Composed of about 80 percent water, the numerous small molecules and macromolecular complexes dissolved or suspended within the cytoplasm give it characteristic viscoelastic and thixotropic properties, allowing it to behave variously as a gel or a liquid solution.<ref name="Rieger"/> Though continuous throughout the intracellular space, the cytoplasm can often be resolved into distinct phases of different density and composition, such as an {{gli|endoplasm}} and {{gli|ectoplasm}}.<ref name="Rieger"/> Most of the metabolic and biosynthetic activities of the cell take place in the cytoplasm, including {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein synthesis}} by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosomes}}. Despite their physical separation, the cytoplasm and the nucleus are mutually dependent upon each other, such that an isolated nucleus without cytoplasm is as incapable of surviving for long periods as is the {{gli|cytoplast|cytoplasm without a nucleus}}.<ref name="Rieger"/></dd>

{{term|cytoplasmic streaming}} {{ghat|Also '''protoplasmic streaming''' and '''cyclosis'''.}} <dd>The flow of the {{gli|cytoplasm}} inside a cell, driven by forces exerted upon cytoplasmic fluids by the {{gli|cytoskeleton}}. This flow functions partly to speed up the transport of molecules and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelles}} suspended in the cytoplasm to different parts of the cell, which would otherwise have to rely on passive diffusion for movement. It is most commonly observed in very large eukaryotic cells, for which there is a greater need for transport efficiency.</dd>

{{term|cytoplast}}{{anchor|cytoplasts}} <dd>An {{gli|enucleated}} eukaryotic cell; or all other cellular components besides the nucleus (i.e. the cell membrane, cytoplasm, organelles, etc.) considered collectively. The term is most often used in the context of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear transfer}} experiments, during which the cytoplast can sometimes remain viable in the absence of a nucleus for up to 48 hours.<ref name="Rieger"/></dd>

{{term|term=cytosine|content=cytosine ({{font|C|font=courier|size=large}})}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyrimidine}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase}} used as one of the four standard nucleobases in both {{gli|DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules. Cytosine forms a {{gli|base pair}} with {{gli|guanine}}.</dd>

{{term|cytosol}}{{anchor|cytosolic}} {{ghat|Also '''hyaloplasm''' and '''groundplasm'''.}} <dd>The soluble aqueous phase of the {{gli|cytoplasm}}, in which small particles such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosomes}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}}, and many other molecules are suspended or dissolved, excluding larger structures and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelles}} such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondria}}, {{gli|chloroplasts}}, {{gli|lysosomes}}, and the {{gli|endoplasmic reticulum}}.<ref name="MacLean"/></dd>

==D== {{glossary}} {{term|daughter cell}}{{anchor|daughter cells}} <dd>A {{gli|cell}} resulting from the {{gli|cell division|division}} of an initial progenitor, known as the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|parent cell}}. Generally two daughter cells are produced per division.<ref name="MacLean"/></dd>

{{term|Denoising Algorithm based on Relevance network Topology}} <dd>An unsupervised algorithm that estimates an activity score for a pathway in a gene expression matrix, following a denoising step.<ref name="dart" >{{#invoke:cite|journal|last=Jiao|first=yan|author2=Katherine Lawler |title=DART: Denoising Algorithm based on Relevance network Topology improves molecular pathway activity inference|journal=BMC Bioinformatics|date=19 October 2011|doi=10.1186/1471-2105-12-403|volume=12|article-number=403|pmid=22011170|pmc=3228554 |doi-access=free }}</ref></dd>

{{term|''de novo'' mutation}}{{anchor|de novo mutation}} <dd>A spontaneous {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} in the genome of an individual organism that is new to that organism's lineage, having first appeared in a {{gli|germ cell}} of one of the organism's parents or in the fertilized egg that develops into the organism; i.e. a mutation that was not present in either parent's genome.<ref name="DoG7"/></dd>

{{term|''de novo'' synthesis}}{{anchor|de novo synthesis}} <dd>The assembly of a synthetic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence}} from free {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} without relying on an existing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|template strand}}, i.e. ''de novo'', by any of a variety of laboratory methods. ''De novo'' synthesis makes it theoretically possible to construct completely {{gli|artificial gene synthesis|artificial molecules}} with no naturally occurring equivalent, and no restrictions on size or sequence. It is performed routinely in the commercial production of customized, made-to-order {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligonucleotide}} sequences such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primers}}.</dd>

{{term|deacetylation}}{{anchor|deacetylate|deacetylates|deacetylating|deacetylated}} <dd>The removal of an acetyl group ({{chem|–COCH|3}}) from a chemical compound, protein, or other biomolecule via hydrolysis of the covalent ester bond adhering it, either spontaneously or by {{gli|enzymatic}} catalysis. Deacetylation is the opposite of {{gli|acetylation}}.</dd>

{{term|decellularization}} <dd></dd>

{{term|dedifferentiation}}{{anchor|dedifferentiated}} <dd></dd>

{{term|degeneracy}}{{anchor|codon degeneracy}} <dd>The redundancy of the {{gli|genetic code}}, exhibited as the multiplicity of different {{gli|codons}} that specify the same {{gli|amino acid}}. For example, in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|standard genetic code}}, the amino acid serine is specified by six unique codons ({{font|UCA|font=courier|size=big}}, {{font|UCG|font=courier|size=big}}, {{font|UCC|font=courier|size=big}}, {{font|UCU|font=courier|size=big}}, {{font|AGU|font=courier|size=big}}, and {{font|AGC|font=courier|size=big}}). Codon degeneracy accounts for the existence of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|synonymous mutations}}.</dd>

{{term|degranulation}} <dd>The release of the contents of a secretory granule (usually antimicrobial or cytotoxic molecules) into an {{gli|extracellular}} space by the {{gli|exocytosis|exocytotic fusion}} of the granule with the cell's plasma membrane.<ref name="Lackie"/></dd>

{{term|deletion}}{{anchor|deletions}} {{ghat|Denoted in shorthand with the symbol '''''Δ'''''.}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} in which one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} are removed from a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence}}.</dd>

{{term|demethylation}}{{anchor|demethylate|demethylates|demethylating|demethylated}} <dd>The removal of a methyl group ({{chem|–CH|3}}) from a chemical compound, protein, or other biomolecule, either spontaneously or by {{gli|enzymatic}} catalysis. Demethylation is the opposite of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|methylation}}; both reactions play important roles in numerous biochemical processes, including in {{gli|gene regulation|regulating gene expression}}, as the methylation state of particular residues within particular proteins or nucleic acids can affect their structural {{gli|conformation}} in a way that alters their affinity for other molecules, making transcription at nearby genetic loci more or less likely.</dd>

{{term|denaturation}}{{anchor|denature|denatures|denatured|denaturing}} <dd>The process by which {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} lose their {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|quaternary}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tertiary}}, and/or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|secondary structures}}, either reversibly or irreversibly, through the application of some external chemical or mechanical stress, e.g. by heating, agitation, or exposure to a strong acid or base, all of which can disrupt intermolecular forces such as hydrogen bonding and thereby change or destroy chemical activity. Denatured proteins may be both a cause and a consequence of cell death. Denaturation may also be a normal process; the denaturation of {{gli|double-stranded DNA}} molecules, for example, which breaks the hydrogen bonds between {{gli|base pairs}} and causes the separation of the duplex molecule into two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single strands}}, is a necessary step in {{gli|DNA replication}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} and hence is routinely performed by enzymes such as {{gli|helicases}}. The same mechanism is also fundamental to laboratory methods such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|PCR}}.</dd>

{{term|dendrite}}{{anchor|dendrites}} {{defn|Any of multiple freely branching {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protoplasmic}} processes extending from a vertebrate nerve cell that receive electrical signals from other nerve cells or sensory receptors and integrate them in order to generate electrical impulses known as action potentials. These pulse-like changes in electrical polarity are then propagated along an {{gli|axon}} and transmitted to other cells.<ref name="Oxford B&MB"/>}}

{{term|deoxyadenosine}} {{ghat|Abbreviated in shorthand with ''{{font|dA|font=courier|size=big}}''.}} <dd>One of the four standard {{gli|deoxyribonucleosides}} used in {{gli|DNA}} molecules, consisting of an {{gli|adenine}} {{gli|base}} with its N9 nitrogen {{gli|glycosidic bond|bonded}} to the C1 carbon of a {{gli|deoxyribose}} sugar. Adenine bonded to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} forms an alternate compound known simply as {{gli|adenosine}}, which is used in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}.</dd>

{{term|deoxycytidine}} {{ghat|Abbreviated in shorthand with ''{{font|dC|font=courier|size=big}}''.}} <dd>One of the four standard {{gli|deoxyribonucleosides}} used in {{gli|DNA}} molecules, consisting of a {{gli|cytosine}} {{gli|base}} with its N9 nitrogen {{gli|glycosidic bond|bonded}} to the C1 carbon of a {{gli|deoxyribose}} sugar. Cytosine bonded to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} forms an alternate compound known simply as {{gli|cytidine}}, which is used in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}.</dd>

{{term|deoxyguanosine}} {{ghat|Abbreviated in shorthand with ''{{font|dG|font=courier|size=big}}''.}} <dd>One of the four standard {{gli|deoxyribonucleosides}} used in {{gli|DNA}} molecules, consisting of a {{gli|guanine}} {{gli|base}} with its N9 nitrogen {{gli|glycosidic bond|bonded}} to the C1 carbon of a {{gli|deoxyribose}} sugar. Guanine bonded to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} forms an alternate compound known simply as {{gli|guanosine}}, which is used in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}.</dd>

{{term|deoxyribonuclease (DNase)}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclease}} enzymes which catalyze the hydrolytic cleavage of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester bonds}} in {{gli|DNA}} molecules, thereby severing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}} of {{gli|deoxyribonucleotides}} and causing the degradation of DNA polymers into smaller components. Compare ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribonuclease}}''.</dd>

{{term|deoxyribonucleic acid (DNA)}}{{anchor|deoxyribonucleic acid|DNA}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymeric}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecule composed of a series of covalently linked {{gli|deoxyribonucleotides}}, each of which incorporates one of four {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}}: {{gli|adenine}} ({{font|A|font=courier|size=big}}), {{gli|guanine}} ({{font|G|font=courier|size=big}}), {{gli|cytosine}} ({{font|C|font=courier|size=big}}), and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine}} ({{font|T|font=courier|size=big}}). DNA is most often found in {{gli|dsDNA|double-stranded}} form, which consists of two {{gli|complementary}}, {{gli|antiparallel}} nucleotide chains in which each of the nucleobases on each individual {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|paired}} via hydrogen bonding with one on the opposite strand; this structure commonly assumes the shape of a {{gli|double helix}}. DNA can also exist in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssDNA|single-stranded}} form. By storing and encoding genetic information in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence}} of these nucleobases, DNA serves as the universal molecular basis of biological inheritance and the fundamental template from which all proteins, cells, and living organisms are constructed.</dd>

{{term|deoxyribonucleotide}}{{anchor|deoxyribonucleotides}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide}} containing {{gli|deoxyribose}} as its {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pentose}} sugar component, and the {{gli|monomer}} or subunit used to build {{gli|deoxyribonucleic acid}} (DNA) molecules. Deoxyribonucleotides canonically incorporate any of four {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nitrogenous bases}}: {{gli|adenine}} ({{font|A|font=courier|size=big}}), {{gli|guanine}} ({{font|G|font=courier|size=big}}), {{gli|cytosine}} ({{font|C|font=courier|size=big}}), and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine}} ({{font|T|font=courier|size=big}}). Compare ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribonucleotide}}''.</dd>

{{term|deoxyribose}} {{ghat|Also '''2-deoxyribose'''.}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monosaccharide}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pentose}} sugar derived from {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} by the replacement of the hydroxyl group attached to the C2 carbon with a single hydrogen atom. D-deoxyribose, in its cyclic ring form, is one of three main functional groups of {{gli|deoxyribonucleotides}} and hence of {{gli|DNA|deoxyribonucleic acid}} (DNA) molecules.</dd> 400px|thumb|right|'''{{gli|Deoxyribose}}''' differs from '''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}}''' only at the 2' carbon, where ribose has an oxygen atom that deoxyribose lacks (hence its name).

{{term|deoxythymidine}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymidine}}''.</dd>

{{term|dephosphorylation}}{{anchor|dephosphorylate|dephosphorylates|dephosphorylating|dephosphorylated}} <dd>The removal of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate}} group, {{chem|P|O|4|3-}}, from a chemical compound, protein, or other biomolecule, either spontaneously or by {{gli|enzymatic}} catalysis. Dephosphorylation is the opposite of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphorylation}}; both reactions are common molecular modifications involved in numerous biochemical pathways and processes, including in metabolism, where high-energy bonds to phosphate groups are used to transfer energy between molecules, and in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-translational modification}} of proteins, where the phosphorylation state of particular residues can affect the protein's affinity for other molecules or function as a {{gli|labelling|molecular signal}}.</dd>

{{term|depurination}} <dd>The spontaneous loss of one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|purine}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}} (either {{gli|adenine}} or {{gli|guanine}}) from a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecule, either {{gli|DNA}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}}, via the hydrolytic cleavage of the {{gli|glycosidic bond}} linking base and sugar, releasing a free purine nucleobase and a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside}}. {{gli|deoxyribonucleotide|Deoxyribonucleotides}} are especially prone to depurination. Loss of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyrimidine}} bases can also occur spontaneously but is far less common.</dd>

{{term|derivatization}}{{anchor|derivatize|derivatizes|derivatized|derivatizing}} <dd>The artificial modification of a molecule or protein with the intent of altering its solubility or other chemical properties so as to enable analysis (e.g. by mass spectroscopy or chromatography), or of {{gli|labelling}} it by attaching a detectable chemical moiety (e.g. a fluorescent tag) to make it easier to identify and track ''{{gli|in vivo}}''. Molecules modified in this way are described as derivatives of their naturally occurring counterparts and are said to have been ''derivatized''.<ref name="Lackie"/></dd>

{{term|desmosome}}{{anchor|desmosomes}} <dd>A specialized cell junction between neighboring {{gli|epithelial}} cells consisting of a network of keratin filaments and structural proteins bridging the gap between the plasma membranes.<ref name="Lackie">{{#invoke:cite|book|last1=Lackie |first1=J. M. |title=The Dictionary of Cell and Molecular Biology |date=2013 |publisher=Academic Press/Elsevier |location=Amsterdam |isbn=978-0-12-384931-1 |edition=5th}}</ref></dd>

{{term|destination vector}} <dd></dd>

{{term|desynapsis}} <dd>The failure of {{gli|homologous chromosomes}} that have {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|synapsis|synapsed}} normally during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pachynema}} to remain paired during {{gli|diplonema}}. Desynapsis is usually caused by the improper formation of {{gli|chiasmata}}.<ref name="DoG7">{{#invoke:cite|book|last1=King |first1=Robert C. |last2=Stansfield |first2=William D. |last3=Mulligan |first3=Pamela K. |title=A Dictionary of Genetics |date=2006 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-530762-7 |edition=7th |url=https://openlibrary.org/works/OL2942141W/A_dictionary_of_genetics}}</ref> Contrast ''{{gli|asynapsis}}''.</dd>

{{term|developmental biology}} <dd>The branch of biology that studies the various processes and phenomena by which organisms (particularly multicellular {{gli|eukaryotes}} but not necessarily excluding {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prokaryotes}}) grow and develop into mature forms capable of reproduction. In the broadest sense the field may encompass topics such as sexual and asexual reproduction, {{gli|gametogenesis}} and sporogenesis, {{gli|fertilization}}, embryogenesis, the renewal and {{gli|differentiation}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stem cells}} into specialized cell types, birth or hatching, metamorphosis, and the regeneration of mature tissues.</dd>

{{term|diakinesis}} <dd>In {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}, the fifth and final substage of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prophase|prophase I}}, following {{gli|diplonema}} and preceding {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metaphase I}}. During diakinesis, the chromosomes are further condensed, the two {{gli|centrosomes}} reach opposite poles of the cell, and the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spindle apparatus}} begins to extend from the poles to the equator.<ref name="DoG7"/></dd>

{{term|dicentric}} <dd>(of a linear {{gli|chromosome}} or chromosome fragment) Having two {{gli|centromere|centromeres}} instead of the normal one.<ref name="CoG"/></dd>

{{term|differential centrifugation}} <dd></dd>

{{term|differentiation}}{{anchor|differentiate|differentiates|differentiating|differentiated}} <dd>The process by which a eukaryotic cell changes from one {{gli|cell type}} to another, in particular from a non-specialized {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stem cell}} to a more specialized cell type which is then said to be ''differentiated''. This usually occurs by a carefully regulated series of {{gli|epigenetic}} modifications which change the specific set of {{gli|genes}} {{gli|expressed}} by the cell, turning certain genes "off" and others "on". These modifications result in a cascade of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotypic}} changes which can dramatically alter the cell's size, shape, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism}}, {{gli|cell membrane|membrane}} properties, and rate of {{gli|division}}, and therefore its functions, behaviors, and responsiveness to signals, permitting multicellular organisms to create a huge variety of functionally distinct cell types from a single {{gli|genome}}. Differentiation occurs repeatedly during an organism's development from a single-celled {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygote}} into a complex multicellular system of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissues}} and cell types, and continues to some extent after the organism reaches maturity in order to repair and replace damaged and dying cells. In most cases differentiation is irreversible, though some cells may also undergo {{gli|dedifferentiation}} in specific circumstances.</dd>

{{term|dimer}}{{anchor|dimers}} <dd>A molecular aggregate consisting of two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|subunits}}. The term is often used to describe a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein complex}} composed of two proteins, either the same protein (a {{gli|homodimer}}) or different proteins (a {{gli|heterodimer}}); or to an individual protein composed of two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptides}}. Compare ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monomer}}'', ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|trimer}}'', and ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tetramer}}''.</dd>

{{term|dinucleotide}}{{anchor|dinucleotides}} <dd>A molecular {{gli|dimer}} consisting of exactly two covalently linked {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}}; or any two nucleotides which are immediately adjacent to each other on the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} of a longer {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} polymer.</dd>

{{term|diploid}} {{ghat|Denoted in shorthand with the somatic number '''''2n'''''.}} <dd>(of a cell or organism) Having two {{gli|homologous chromosomes|homologous}} copies of each {{gli|chromosome}}. Contrast ''{{gli|haploid}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polyploid}}''.</dd>

{{term|diplonema}} {{ghat|Also '''diplotene stage'''.}} <dd>In {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}, the fourth of the five substages of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prophase|prophase I}}, following {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pachynema}} and preceding {{gli|diakinesis}}. During diplonema, the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|synaptonemal complex}} disassembles and the paired {{gli|homologous chromosomes}} begin to separate from one another, though they remain tightly bound at the {{gli|chiasmata}} where {{gli|crossover}} has occurred.</dd>

{{term|direct repeat}}{{anchor|direct repeats}} <dd>Any two or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repeat|repetitions}} of a specific {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence|sequence of nucleotides}} occurring in the same orientation (i.e. in precisely the same order and not {{gli|inverted repeat|inverted}}) and on the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}}, either separated by intervening nucleotides or not. An example is the sequence {{font|'''TACCG'''|font=courier|size=big|color=green}}{{font|nnnnnn|font=courier|size=big}}{{font|'''TACCG'''|font=courier|size=big|color=green}}, in which {{font|'''TACCG'''|font=courier|size=big|color=green}} occurs twice, though separated by six nucleotides that are not part of the repeated sequence. A direct repeat in which the repeats are immediately adjacent to each other is known as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tandem repeat}}.</dd>

{{term|directionality}}{{anchor|direction|directions|5' to 3'|3' to 5'|5'-to-3'|3'-to-5'}} <dd>The end-to-end orientation of a linear {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence}} of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} polymer or a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}}. The nomenclature used to indicate nucleic acid directionality is based on the chemical convention of identifying individual carbon atoms in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} or {{gli|deoxyribose}} sugars of nucleotides, specifically the {{gli|5' carbon}} and {{gli|3' carbon}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pentose}} ring. The sequence of nucleotides in a polymeric chain may be read or interpreted in the 5'-to-3' direction (i.e. starting from the terminal nucleotide in which the 5' carbon is not connected to another nucleotide, and proceeding to the other terminal nucleotide, in which the 3' carbon is not connected to another nucleotide) or in the opposite 3'-to-5' direction. Most types of nucleic acid synthesis, including {{gli|DNA replication}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}, build chains of nucleotides exclusively in the 5'-to-3' direction, because the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymerases}} involved can only catalyze the addition of free nucleotides to the open 3'-end of the previous nucleotide in the chain. Because of this, the convention when writing any nucleic acid sequence is to present it in the 5'-to-3' direction from left to right. In {{gli|double-stranded}} nucleic acids, the two paired strands must be {{gli|antiparallel|oriented in opposite directions}} in order to {{gli|base-pair}} with each other. Polypeptide directionality is similarly based on identifying the functional groups of {{gli|amino acids}}, specifically the amino group, which forms the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|N-terminus}}, and the carboxyl group, which forms the {{gli|C-terminus}}; amino acid sequences are assembled in the N-to-C direction during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}, and by convention are written in the same direction.</dd>

{{term|disaccharide}}{{anchor|disaccharides}} <dd>A {{gli|carbohydrate}} composed of two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monosaccharides}} (either the same or different) joined by a covalent {{gli|glycosidic bond}}.<ref name="Lackie"/> See also ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligosaccharide}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polysaccharide}}''.</dd>

{{term|dispersive replication}} <dd>A hypothetical mode of {{gli|DNA replication}} in which the pairing of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|template strands}} and newly synthesized strands is not consistent within the same daughter molecule; i.e. each of the replicated daughter molecules is a heterogeneous mixture, with some segments composed of the original template strands and others composed of the newly synthesized strands. This process implies that the pairing of strands does not occur uniformly at all {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication forks}}. Only the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|semiconservative replication|semiconservative}} mode of replication occurs naturally. See also ''{{gli|conservative replication}}''.</dd>

{{term|dissimilatory process}} <dd>Any exergonic process of microbial {{gli|catabolism}} by which redox-active chemical species participate in oxidation-reduction reactions (exchange of electrons) to provide the cell with energy needed for sustaining {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic}} activities. {{gli|exogenous|Exogenous}} substances are absorbed by the cell from its environment and then decomposed to release energy, with the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic waste|byproducts}} subsequently {{gli|excreted}} out of the cell. This is in contrast to an {{gli|assimilatory process}}, in which the atoms of the exogenous substances are reused in the synthesis of biomolecules or the fabrication of cellular components.</dd>

{{term|distance measure}} <dd>Any quantity used to measure the dissimilarity between the {{gli|gene expression}} levels of different {{gli|genes}}.<ref name="Priness">{{#invoke:cite|journal|last = Priness|first = I.|author2=Maimon, O. |author3=Ben-Gal, I. |title = Evaluation of gene-expression clustering via mutual information distance measure|journal = BMC Bioinformatics|volume = 8|date = 2007|article-number = 111|doi = 10.1186/1471-2105-8-111|pmid = 17397530|pmc = 1858704 | doi-access=free }}</ref></dd>

{{term|DNA}} <dd>See ''{{gli|dNA|deoxyribonucleic acid}}''.</dd>

{{term|DNA barcoding}} <dd>A method of taxonomic identification in which short DNA sequences from one or more specific genes are isolated from unidentified samples and then {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence alignment|aligned}} with a genomic {{gli|library|reference library}} in order to uniquely identify the species or other taxon from which the samples originated. The sequences used in the comparison are chosen carefully from genes that are both widely {{gli|conserved sequence|conserved}} and that show greater {{gli|genetic variation|variation}} between species than within species, e.g. the cytochrome c oxidase gene for eukaryotes or certain {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|rRNA|ribosomal RNA}} genes for prokaryotes. These genes are present in nearly all living organisms but tend to evolve different mutations in different species, such that a unique sequence variant can often be linked to one particular species, effectively creating a unique identifier akin to a retail barcode. DNA barcoding allows unknown specimens to be identified from otherwise indistinct tissues or body parts, where identification by morphology would be difficult or impossible, and the library of organismal barcodes is now comprehensive enough that even organisms previously unknown to science can often be {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phylogenetics|phylogenetically}} classified with confidence. The simultaneous identification of multiple different species from a mixed sample is known as ''metabarcoding''.</dd>

{{term|DNA condensation}} <dd>The process of compacting very long {{gli|DNA}} molecules into densely packed, orderly configurations such as {{gli|chromosomes}}, either ''{{gli|in vivo}}'' or ''{{gli|in vitro}}''.</dd>

{{term|DNA damage}} <dd></dd>

{{term|DNA fingerprinting}} <dd></dd>

{{term|DNA methylation}} <dd></dd>

{{term|DNA microarray}} <dd>A {{gli|high-throughput}} technology used to measure {{gli|expression}} levels of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA}} transcripts or to detect certain changes in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence|nucleotide sequence}}. It consists of an array of thousands of microscopic spots of {{gli|DNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligonucleotides}}, called ''features'', each containing picomoles of a specific DNA sequence. This can be a short section of a {{gli|gene}} or any other DNA element, and is used as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|probe}} to hybridize a {{gli|cDNA}}, cRNA, or {{gli|genomic DNA}} sample (called a ''target'') under {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stringency|high-stringency}} conditions. Probe-target {{gli|hybridization}} is usually detected and quantified by fluorescence-based detection of {{gli|labelling|fluorophore-labeled}} targets.</dd>

{{term|DNA polymerase}} <dd>Any of a class of {{gli|enzymes}} which synthesize {{gli|DNA}} molecules from individual {{gli|deoxyribonucleotides}}. DNA polymerases are essential for {{gli|DNA replication}} and usually work in pairs to create identical copies of the two {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}} of an original double-stranded molecule. They build long chains of DNA by adding nucleotides one at a time to the {{gli|3'-end}} of a DNA strand, usually relying on the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|template strand|template}} provided by the {{gli|complementary}} strand to copy the nucleotide sequence faithfully.</dd>

{{term|DNA repair}} <dd>The set of processes by which a cell identifies and corrects structural damage or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutations}} in the {{gli|DNA}} molecules that encode its {{gli|genome}}. The ability of a cell to repair its DNA is vital to the integrity of the genome and the normal functionality of the organism.</dd>

{{term|DNA replication}} <dd>The process by which a {{gli|DNA}} molecule copies itself, producing two identical copies of one original DNA molecule. This occurs by a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|semiconservative replication|semiconservative mechanism}} involving the separation of a double-stranded molecule into two individual strands, each of which then serves as a template for the synthesis of a new strand of complementary nucleotides. Replication of {{gli|chromosomes}} takes place during the {{gli|S phase}} of {{gli|interphase}}, though {{gli|extrachromosomal DNA}} molecules such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondrial DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmids}} may replicate independently at other times. DNA replication is the chief process by which genetic information is propagated in all living organisms and the central mechanism underlying biological inheritance.</dd> 350px|thumb|right|A diagram of the many components of '''{{gli|DNA replication}}'''

{{term|DNA sequencing}}{{anchor|sequencing}} <dd>The process of determining, by any of a variety of different methods and technologies, the order of the {{gli|bases}} in the long chain of nucleotides that constitutes a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence}} of {{gli|DNA}}.</dd>

{{term|DNA turnover}} <dd>Any mechanism by which {{gli|DNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequences}} are exchanged non-reciprocally (e.g. via {{gli|gene conversion}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transposition}}, or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|unequal crossing-over}}) that causes continual fluctuations in the {{gli|copy number}} of DNA {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motifs}} during an organism's lifetime. Such mechanisms are often major drivers of speciation between populations.<ref name="Rieger">{{#invoke:cite|book|last1=Rieger |first1=Rigomar |title=Glossary of Genetics: Classical and Molecular |date=1991 |publisher=Springer-Verlag |location=Berlin |isbn=3-540-52054-6 |edition=5th |url=https://openlibrary.org/works/OL4093818W/Glossary_of_genetics?edition=glossaryofgeneti0000rieg}}</ref></dd>

{{term|DNA-binding domain (DBD)}} <dd>A protein {{gli|domain}} containing at least one structural motif capable of recognizing and interacting with the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} of a {{gli|dsDNA|double-stranded}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssDNA|single-stranded}} {{gli|DNA}} molecule. DNA-binding domains may bind to specific sequences or have a non-specific affinity for DNA. They are the primary functional components of {{gli|DNA-binding proteins}}, including many {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}} and regulatory proteins.</dd> thumb|right|350px|The molecular structures of several common classes of '''{{gli|DNA-binding domains}}''' (grey), showing how they interact with the DNA double helix (blue)

{{term|DNA-binding protein (DBP)}}{{anchor|DNA-binding proteins}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} containing one or more {{gli|DNA-binding domain|domains}} capable of interacting chemically with one or more parts of a {{gli|DNA}} molecule, and consequently having a specific or general affinity for {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssDNA|single-}} and/or {{gli|dsDNA|double-stranded DNA}}. DNA-binding activity often depends on the presence and physical accessibility of a specific nucleobase sequence, and mostly occurs at the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|major groove}}, since it exposes more of the functional groups which uniquely identify the bases. Binding is also influenced by the spatial conformation of the DNA chain and the occupancy of other proteins near the binding site; many proteins cannot bind to DNA without first undergoing {{gli|conformational changes}} induced by interactions with other molecules.</dd>

{{term|DNase}} <dd>See ''{{gli|deoxyribonuclease}}''.</dd>

{{term|domain}}{{anchor|domains|protein domain|protein domains}} <dd>A discrete, usually contiguous region of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} (or the corresponding {{gli|amino acid}} sequence of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}}) which serves a particular function or is defined by particular physico-chemical properties (e.g. {{gli|hydrophobic}}, polar, non-polar, {{gli|globular protein|globular}}, etc.),<ref name="Lackie"/> and especially one which folds independently of the rest of the polypeptide into a characteristic, self-stabilizing spatial {{gli|conformation}} as part of the protein's {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|supersecondary structure}} and which contributes to or defines its biological activity. Large proteins are generally composed of multiple domains linked by short, intervening non-domain sequences.<ref name="Alberts et al."/> Domains are commonly grouped into classes with similar properties or functions, e.g. {{gli|DNA-binding domains}}. More broadly, the term may also be used to refer to a discrete structural entity within any biomolecule, including functionally or compositionally distinct subregions of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequences}} and {{gli|chromosomes}}.<ref name="Rieger"/></dd>

{{term|donor vector}} <dd></dd>

{{term|dosage compensation}} <dd>Any mechanism by which organisms neutralize the large difference in {{gli|gene dosage}} caused by the presence of differing numbers of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sex chromosomes}} in the different sexes, thereby equalizing the {{gli|expression}} of sex-linked genes so that the members of each sex receive the same or similar amounts of the {{gli|gene product|products}} of such genes. An example is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|X-inactivation}} in female mammals.</dd>

{{term|double helix}} <dd>The shape most commonly assumed by {{gli|dsDNA|double-stranded}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecules, resembling a ladder that has been twisted upon its long axis, with the rungs of the ladder consisting of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|paired}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}}. This {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|secondary structure}} is the most energetically stable conformation of the double-stranded forms of both {{gli|DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} under most naturally occurring conditions, arising as a consequence of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primary structure}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester backbone}} and the stacking of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} bonded to it. In {{gli|B-DNA}}, the most common DNA variant found in nature, the double helix has a right-handed twist with about 10 base pairs per full turn, and the molecular geometry results in an alternating pattern of "grooves" of differing widths (a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|major groove}} and a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|minor groove}}) between the parallel backbones.</dd> 350px|thumb|right|Double-stranded DNA most commonly exists in the shape of a '''{{gli|double helix}}'''.

{{term|double-strand break (DSB)}}{{anchor|double-strand breaks}} <dd>The loss of continuity of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate backbone|phosphate-sugar backbone}} in both strands of a {{gli|double-stranded DNA}} molecule, in particular when the two breaks occur at sites that are directly across from or very close to each other on the complementary strands.<ref name="Rieger"/> Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-strand break}}''.</dd>

{{term|double-stranded}}{{anchor|duplex}} <dd>Composed of two {{gli|antiparallel}}, {{gli|complementary}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecules or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}} (either {{gli|dsDNA|DNA–DNA}}, {{gli|dsRNA|RNA–RNA}}, or a {{gli|DNA–RNA hybrid}}) which are held together by hydrogen bonds between the complementary {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}} of each strand, known as {{gli|base pairing}}. Compare ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}}''.</dd>

{{term|double-stranded DNA (dsDNA)}}{{anchor|dsRNA}} <dd>Any {{gli|DNA}} molecule that is composed of two {{gli|antiparallel}}, {{gli|complementary}} {{gli|deoxyribonucleotide}} polymers, known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}}, which are bonded together by hydrogen bonds between the complementary {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}}. Though it is possible for DNA to exist as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssDNA|single strand}}, it is generally more stable and more common in double-stranded form. In most cases, the complementary {{gli|base pairing}} causes the twin strands to coil around each other in the shape of a {{gli|double helix}}.</dd>

{{term|double-stranded RNA (dsRNA)}}{{anchor|dsRNA}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecule that is composed of two {{gli|antiparallel}}, {{gli|complementary}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribonucleotide}} polymers, known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}}, which are bonded together by hydrogen bonds between the complementary {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobases}}. Though RNA usually occurs in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ssRNA|single-stranded form}}, it is also capable of forming duplexes in the {{gli|dsDNA|same way as DNA}}; an example is an {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA}} transcript pairing with an {{gli|asRNA|antisense version}} of the same transcript, which effectively {{gli|gene silencing|silences}} the gene from which the mRNA was transcribed by preventing translation. As in dsDNA, the {{gli|base pairing}} in dsRNA usually causes the twin strands to coil around each other in the shape of a {{gli|double helix}}.</dd>

{{term|downregulation}} {{ghat|Also '''repression''' or '''suppression'''.}} <dd>Any process, natural or artificial, which decreases the level of {{gli|gene expression}} of a certain {{gli|gene}}. A gene which is observed to be expressed at relatively low levels (such as by detecting lower levels of its {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA}} transcripts) in one sample compared to another sample is said to be ''downregulated''. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|upregulation}}''.</dd>

{{term|downstream}} <dd>Towards or closer to the {{gli|3'-end}} of a chain of nucleotides, or to the {{gli|C-terminus}} of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptide}} chain. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|upstream}}''.</dd>

{{term|dsDNA}} <dd>See ''{{gli|dsDNA|double-stranded DNA}}''.</dd>

{{term|dsRNA}} <dd>See ''{{gli|dsRNA|double-stranded RNA}}''.</dd>

{{term|duplex}} <dd>See ''{{gli|double-stranded}}''.</dd>

{{term|duplication}} <dd>The production of a second copy of part or all of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide sequence}} or {{gli|amino acid sequence}}, either naturally or artificially, and the retention of both copies; especially when both the copy and the original sequence are retained ''{{gli|in situ}}'' within the same molecule, often but not necessarily {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tandem|adjacent}} to each other. See also ''{{gli|gene duplication}}'', ''{{gli|chromosomal duplication}}'', and ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repeat}}''.</dd>

{{term|dyad}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sister chromatids}}''.</dd>

{{term|dysplasia}} <dd>The abnormal growth or development of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissue}} or organ; a change in the growth, behavior, or organization of cells within a tissue, or the presence of cells of an abnormal type, such that the tissue becomes disordered,<ref name="Alberts et al."/> an event which often precedes the development of cancer.</dd>

==E== {{glossary}} {{term|eat-me signal}} <dd>A molecule exposed on the surface of a cell which effectively tags the cell for {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phagocytosis}}, inducing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phagocytes}} to engulf or "eat" it. The presence of oxidized {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipids}} or phosphatidylserine, or the absence of sialic acid from cell surface {{gli|glycoproteins}} or {{gli|glycolipids}}, are all commonly used as eat-me signals in certain cell types. See also ''{{gli|find-me signal}}''.</dd>

{{term|ectopic}} <dd>Occurring or developing in an abnormal place or position or in an unusual form or manner; displaced, malpositioned, or produced in an unnatural context. For example, ectopic expression refers to the expression of a particular gene product in a cell or tissue where it is not normally expressed.<ref name="Oxford B&MB"/> Contrast ''{{gli|entopic}}''.</dd>

{{term|effector}}{{anchor|effectors}} {{ghat|Also '''modifier''' and '''modulator'''.}} <dd>Any small molecule or {{gli|ligand}} which by interacting with a particular {{gli|enzyme}} changes its catalytic activity but is not itself changed. A ''positive effector'' enhances the enzyme's activity while a ''negative effector'' reduces it.<ref name="Oxford B&MB"/></dd>

{{term|electron transport chain (ETC)}}{{anchor|electron transport chain}} <dd>The process by which electrons are transferred from electron donors to electron acceptors via a stepwise series of redox reactions carried out by dedicated {{gli|enzymes}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein complexes}}, especially as a component of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathways}} which convert chemical energy from food into a form that is readily accessible by the cell. Most electron transport chains begin by oxidizing molecules derived from {{gli|glycolysis}} such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|NADH}} and {{gli|FADH}}, converting them into a series of intermediate compounds via a specific sequence of independently catalyzed reactions, with the products of the previous reaction used as reactants in the next reaction until ultimately reaching a terminal electron acceptor. The particular compounds used as donors, intermediates, and acceptors vary widely between organisms and cell types; in {{gli|aerobic respiration}}, the terminal acceptor is diatomic oxygen ({{chem|O|2}}), whereas {{gli|anaerobic respiration}} uses other acceptors. In all variants, the free energy released by these reactions is coupled to the {{gli|chemiosmotic}} pumping of protons ({{chem|H|+}}) across a membrane in order to generate an electrochemical gradient which is then used to drive the production of {{gli|ATP}}, a process known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oxidative phosphorylation}}. In eukaryotes, electron transport chains are conducted by proteins embedded within the membranes of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondria}} and {{gli|chloroplasts}}, while in prokaryotes the relevant proteins are embedded within the {{gli|cell membrane}}.</dd> thumb|right|500px|A schematic layout of the '''{{gli|electron transport chain}}''' as it occurs in most animal mitochondria: {{gli|NADH}} and {{gli|FADH}} supplied by the {{gli|citric acid cycle}} donate their electrons to an enzyme embedded within the inner mitochondrial membrane, from which the electrons are then transferred through a series of other embedded enzymes, all of which use the free energy gained to pump protons into the intermembrane space, against their concentration gradient. The pressure to restore electrochemical equilibrium by {{gli|chemiosmosis|moving protons back}} across the membrane is exploited by {{gli|ATP synthase}}, which uses the free energy to catalyze the addition of a phosphate group to {{gli|ADP}}, converting it to {{gli|ATP}}.

{{term|electrophoresis}}{{anchor|electrophoretic|electrophoretically}} <dd>The physical separation of molecules, e.g. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, according to their movement through a fluid medium to which an electric field is applied, where the distance they travel is proportional to their size. Because of their negatively charged {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate backbones}}, nucleic acids are repelled by the negative electrode at one end of the medium and attracted to the positive electrode at the other end, which causes them to be pulled toward the latter over time; {{gli|denatured}} proteins and even whole cells may migrate through the medium in a similar manner. The speed at which the molecules migrate depends on their net electric charge and is inversely proportional to their overall size (i.e. the number of atoms they contain), such that very small molecules tend to move faster through the medium than very large molecules. Thus electrophoretic techniques, particularly {{gli|gel electrophoresis}} with agarose or polyacrylamide-based gels as the supporting medium, are widely used in molecular biology laboratories to quickly and conveniently isolate molecules of interest from heterogeneous mixtures and/or identify them based on their expected molecular weight. Reference markers containing molecules of known weight are commonly run alongside unknown samples to aid size-based identification. Electrophoresis is often combined with other techniques such as {{gli|labelling|immunolabelling and radiolabelling}}.<ref name="MacLean"/></dd>

{{term|electroporation}} {{ghat|Also '''electropermeabilization'''.}} <dd>A molecular biology technique in which a strong electric field is applied to living cells in order to temporarily increase the permeability of their cell membranes, allowing exogenous nucleic acids, proteins, or chemical compounds to easily pass through the membrane and thereby enter the cells. It is a common method of achieving {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transformation}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transfection}}.</dd>

{{term|elongation}} {{ghat|Also '''extension'''.}} <dd>The linear growth of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} polymer by the sequential addition of individual {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide}} monomers to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nascent}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}}, e.g. during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication}}, especially when it occurs by {{gli|complementary pairing}} with a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|template strand}}. The term is often used to describe steps in certain laboratory techniques such as the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymerase chain reaction}}.</dd>

{{term|elongation factor}}{{anchor|elongation factors}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} which, by binding to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosome}}, promotes {{gli|elongation}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}} chain during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}.<ref name="MacLean"/></dd>

{{term|embryo}}{{anchor|embryos|embryonic}} <dd>The developing organism that represents the earliest stages of {{gli|developmental biology|development}} in all sexually reproducing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|multicellular}} organisms, traditionally encompassing the period after {{gli|fertilization}} of an egg cell and formation of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygote}} but prior to birth, hatching, or metamorphosis. During this period, known as embryonic development, the single-cell zygote is transformed by repeated {{gli|cell divisions}} and rearrangements into a series of increasingly complex multicellular structures. For humans, the term "embryo" is only used until the ninth week after conception, after which time the embryo is known as a foetus; for most other organisms, including plants, "embryo" can be used more broadly to describe any early stage of the life cycle.</dd>

{{term|emergenesis}} <dd>The quality of genetic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|traits}} that results from a specific configuration of interacting {{gli|genes}}, rather than simply their combination.</dd>

{{term|endocytosis}} <dd>Any process by which a substance is {{gli|active transport|actively}} uptaken by or brought inside of a {{gli|cell}}, crossing the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} from an {{gli|extracellular space}} into an {{gli|intracellular space}}, which includes the subclasses of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pinocytosis}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phagocytosis}}, and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|receptor-mediated endocytosis|receptor-mediated}} processes. All of these involve surrounding an extracellular molecule, protein, or even another cell or organism with an extension or {{gli|invagination}} of the cell membrane, which then "buds off" or separates from the rest of the membrane on the cytoplasmic side, forming a membrane-enclosed {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicle}} containing the ingested materials. By this mechanism the material can cross the {{gli|lipid bilayer}} without being exposed to the hydrophobic space in between, instead remaining suspended in the fluid of the extracellular space. Many large, polar macromolecules which cannot simply diffuse across the membrane, such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolites}} and {{gli|hormones}}, are transported into the cell by endocytosis. It is distinguished from alternative routes such as passing through {{gli|channel protein|protein channels}} or being chaperoned by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transport proteins}}. The reverse process is called {{gli|exocytosis}}.</dd> thumb|right|500px|Different forms of '''{{gli|endocytosis}}'''

{{term|endogenous}} <dd>Originating or arising inside of an organism or cell; produced by the organism or cell itself, rather than sourced from the external environment; of or pertaining to native or internal factors or processes, to be distinguished from foreign or {{gli|exogenous}} factors or processes.<ref name="Oxford B&MB"/></dd>

{{term|endomembrane}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane}} surrounding an {{gli|intracellular}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelle}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicle}}, e.g. that of the {{gli|endoplasmic reticulum}}, {{gli|Golgi apparatus}}, {{gli|lysosome}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vacuole}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} (the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear envelope}}), etc.<ref name="MacLean"/></dd>

{{term|endonuclease}}{{anchor|endonucleases}} <dd>Any {{gli|enzyme}} whose activity is to cleave {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester bonds}} within a chain of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}}, including those that cleave relatively nonspecifically (without regard to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence|sequence}}) and those that cleave only at very specific sequences (so-called {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|restriction enzyme|restriction endonucleases}}). When recognition of a specific sequence is required, endonucleases make their cuts in the middle of the sequence. Contrast ''{{gli|exonuclease}}''.</dd>

{{term|endoplasmic reticulum (ER)}} <dd>The irregular network of unit membranes, continuous with the {{gli|outer nuclear membrane}}, that extends from the {{gli|nucleus}} into the {{gli|cytoplasm}} in most eukaryotic cells, where it serves important packaging and transport functions for newly synthesized macromolecules. The membranes interweave to form a mesh of tubular channels and flattened sacs called {{gli|cisternae}} which house a variety of enzymes that perform {{gli|post-translational modifications}} including {{gli|tagging}} proteins for {{gli|protein sorting|sorting}}. The outer surfaces of so-called {{gli|rough endoplasmic reticulum}} are studded with attached {{gli|ribosomes}} that serve as sites of protein synthesis, whereas {{gli|smooth endoplasmic reticulum}}, lacking ribosomes, functions in the synthesis of {{gli|lipids}} and steroid hormones and in the detoxification of metabolic wastes.<ref name="Oxford B&MB"/> Generally both types of ER occur together, though some cell types are characterized by different proportions of rough and smooth ER, depending on the activities of the cell.</dd>

{{term|endosome}}{{anchor|endosomes}} <dd>Any of a class of intracellular {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane-bound organelles}} which serve transportation and sorting functions in eukaryotic cells as part of the {{gli|endocytic cycle}}. They are formed when proteins or other macromolecules enter the cytoplasm inside {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicles}} invaginated from the {{gli|cell membrane}} or the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|trans-Golgi network}} by {{gli|endocytosis}}, after which they are shuttled across the cell to various destinations; e.g. endosomes carrying foreign molecules often fuse with {{gli|lysosomes}}, where the contents are then degraded.</dd>

{{term|enhancer}}{{anchor|enhancers}} <dd>A region of DNA near a {{gli|gene}} that can be bound by an {{gli|activator}} to increase {{gli|gene expression}} or by a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repressor}} to decrease expression.</dd>

{{term|enhancer RNA (eRNA)}} <dd>A subclass of {{gli|lncRNA|long non-coding RNAs}} transcribed from regions of DNA containing {{gli|enhancer}} sequences. The expression of a given eRNA generally correlates with the activity of the corresponding enhancer in enhancing transcription of its target genes, suggesting that eRNAs play an active role in gene regulation {{gli|cis|in ''cis''}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|trans|in ''trans''}}.</dd>

{{term|entopic}} <dd>Occurring or developing in the normal or natural place or position, as opposed to {{gli|ectopic}}.<ref name="Oxford B&MB"/></dd>

{{term|enucleate}}{{anchor|enucleates|enucleating|enucleated}} <dd>To artificially remove the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} from a {{gli|cell}}, e.g. by micromanipulation in the laboratory or by destroying it through irradiation with ultraviolet light, rendering the cell {{gli|anucleate}}.<ref name="MacLean"/></dd>

{{term|envagination}} <dd></dd>

{{term|enzyme}}{{anchor|enzymes|enzymatic}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} which acts as a {{gli|catalyst}} for a biological process by accelerating a specific chemical reaction, typically by binding one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substrate}} molecules and decreasing the activation energy necessary for the initiation of a particular reaction involving the substrate(s). Enzymatic catalysis often results in the chemical conversion of the substrate(s) into one or more products, which then inhibit or permit subsequent reactions. All {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathways}} consist of a series of individual reactions which each depend upon one or more specific enzymes to drive them forward at rates fast enough to sustain life.</dd>

{{term|enzyme-linked immunosorbent assay (ELISA)}}{{anchor|enzyme-linked immunosorbent assay|ELISA}} <dd>A {{gli|bioassay|biochemical assay}} designed to detect the presence of a particular {{gli|antigen}} or {{gli|ligand}} in a liquid sample using {{gli|enzymes}} conjugated to {{gli|antibodies}} capable of specifically binding the antigen. The antigen of interest is usually first immobilized by adhering to a solid support (e.g. a polystyrene microtiter plate), then one or more antigen-specific antibodies covalently bonded to a particular enzyme are added and any unbound antibody is washed away; when the attached enzyme's {{gli|substrate}} is subsequently added, the reaction between enzyme and substrate produces a detectable, quantifiable change in some measurable {{gli|biomarker}} (often a color change), thus reporting the presence of the targeted antigen in the sample. ELISA techniques are widely used as diagnostic tools in clinical medicine and academic research, as well as a form of quality control in many biotechnology industries.</dd>

{{term|epigenetics}}{{anchor|epigenetic}} <dd></dd>

{{term|epigenome}} <dd></dd>

{{term|episome}}{{anchor|episomes|episomal}} <dd>1.  Another name for a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmid}}, especially one that is capable of integrating into a {{gli|chromosome}}.</dd> <dd>2.  In eukaryotes, any non-integrated {{gli|extrachromosomal DNA|extrachromosomal}} circular {{gli|DNA}} molecule that is stably maintained and replicated in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} simultaneously with the rest of the host cell. Such molecules may include viral genomes, bacterial plasmids, and aberrant chromosomal fragments.</dd>

{{term|epistasis}}{{anchor|epistatic}} <dd>The collective action of multiple genes interacting during {{gli|gene expression}}. A form of gene action, epistasis can be either additive or multiplicative in its effects on specific {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotypic traits}}.</dd>

{{term|epitope}}{{anchor|epitopes}} {{ghat|Also '''antigenic determinant'''.}} <dd>The specific site or region within an {{gli|antigenic}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|macromolecule}} such as a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} or {{gli|carbohydrate}} which is recognized by B or T cells of the immune system, against which a specific {{gli|antibody}} is produced, and with which the antibody's {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|paratope}} specifically interacts or binds. In proteins, epitopes are typically {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motifs}} of 4–5 amino acid residues, sequential or discontiguous, which by virtue of the distinct spatial {{gli|conformation}} they adopt upon {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein folding}} are able to uniquely interact with a particular paratope. In this sense they may be considered {{gli|binding sites}}, though they do not necessarily overlap with ligand binding sites and need not be in any way relevant to the protein's normal function. Very large molecules may have multiple epitopes, each of which is recognized by a different antibody.</dd>

{{term|ergosome}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polysome}}''.</dd>

{{term|euchromatin}}{{anchor|euchromatic}} {{ghat|Also '''open chromatin'''.}} <dd>A relatively open, lightly compacted form of {{gli|chromatin}} in which {{gli|DNA}} is only sporadically bound in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosomes}} and thus broadly accessible to binding and manipulation by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} and other molecules. Euchromatic regions of a genome are often enriched in {{gli|genes}} and actively undergoing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}, in contrast to {{gli|heterochromatin}}, which is relatively gene-poor, nucleosome-rich, and less accessible to transcription machinery.</dd>

{{term|euploidy}} <dd>The condition of a cell or organism having an abnormal number of complete sets of {{gli|chromosomes}}, possibly excluding the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sex chromosomes}}. Euploidy differs from {{gli|aneuploidy}}, in which a cell or organism has an abnormal number of one or more specific individual chromosomes.</dd>

{{term|evolution}} <dd>The change in the {{gli|heredity|heritable}} characteristics of biological populations over successive generations. In the most traditional sense, it occurs by changes in the frequencies of {{gli|alleles}} in a population's {{gli|gene pool}}.</dd>

{{term|''ex vivo''}} <dd>Occurring outside of a cell or organism, as with observations made or experiments performed in or on cells or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissues}} which have been isolated or removed from their natural context to an external environment (usually a carefully controlled environment with minimal alteration of natural conditions, such as a {{gli|cell culture}} being grown in a laboratory). This is in contrast to ''{{gli|in vivo}}'' observations, which are made in an entirely natural context.</dd>

{{term|excision}} <dd>The enzymatic removal of a polynucleotide sequence from one or more strands of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}}, or of a polypeptide sequence from a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}}, typically implying both the breaking of the polymeric molecule in two locations and the subsequent rejoining of the two breakpoints after the sequence between them has been removed. The term may be used to describe a wide variety of processes performed by distinct enzymes, including most {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|splicing}} and {{gli|DNA repair}} pathways.<ref name="DoG7"/></dd>

{{term|exocytosis}} <dd>Any {{gli|active transport}} process by which a substance is secreted from or transported out of a {{gli|cell}}, crossing the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} from the {{gli|intracellular space|interior of the cell}} into the {{gli|extracellular space}}, especially that which occurs by the fusion of the membrane surrounding a secretory {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicle}} with the larger cell membrane. This fusion causes the intra-vesicular space to merge with the extracellular fluid, releasing the vesicle's contents on the exterior side of the cell without exposing them to the hydrophobic space between the {{gli|lipid bilayer}}. More narrowly the term may refer in particular to the bulk transport of a large amount of molecules out of the cell all at once, often {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolites}} or {{gli|hormones}} which are too large and polar to passively diffuse across the membrane themselves. The reverse process, whereby materials are invaginated into the cell, is known as {{gli|endocytosis}}.</dd>

{{term|exogenous}} <dd>Originating outside of an organism or cell; of or pertaining to foreign or external factors or processes, to be distinguished from native or {{gli|endogenous}} factors or processes.<ref name="Oxford B&MB"/></dd>

{{term|exome}} <dd>The entire set of {{gli|exons}} within a particular {{gli|genome}}, including {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|untranslated regions}} of mature mRNAs as well as {{gli|coding regions}}.</dd>

{{term|exon}}{{anchor|exons|exonic}} <dd>Any part of a {{gli|gene}} that encodes a part of the final mature {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA|messenger RNA}} produced by that gene after {{gli|introns}} have been removed by {{gli|alternative splicing}}. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts.</dd>

{{term|exon skipping}} <dd></dd>

{{term|exonuclease}}{{anchor|exonucleases}} <dd>Any {{gli|enzyme}} whose activity is to cleave {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester bonds}} within a chain of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}}, including those that cleave only upon recognition of a specific sequence (so-called {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|restriction enzyme|restriction exonucleases}}). Exonucleases make their cuts at either the {{gli|3'-end|3'}} or {{gli|5'-end}} of the sequence (rather than in the middle, as with {{gli|endonucleases}}).</dd>

{{term|exosome}} <dd>1.  (protein complex) An intracellular multi-protein complex which serves the function of degrading various types of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules.</dd> <dd>2.  (vesicle) A type of membrane-bound {{gli|extracellular vesicle}} produced in many eukaryotic cells by the inward budding of an {{gli|endosome}} and the subsequent fusion of the endosome with the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}}, causing the release of the vesicle into various extracellular spaces, including biological fluids such as blood and saliva, where they may serve any of a wide variety of physiological functions, from waste management to intercellular signaling.</dd>

{{term|exosome complex}} <dd>An intracellular multi-protein complex which serves the function of degrading various types of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules.</dd>

{{term|expression vector}} {{ghat|Also '''expression construct'''.}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vector}}, usually a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmid}} or viral vector, designed specifically for the {{gli|expression}} of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transgene}} {{gli|insert}} in a target cell, rather than for some other purpose such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular cloning|cloning}}.</dd> [[File:pET28a-T7-lacO-GFP.svg|thumb|right|350px|Plasmid map of a 3,756-{{gli|base pair|bp}} '''{{gli|expression vector}}''' used in the expression of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transgene}} that makes green fluorescent protein (GFP). The vector also includes a gene for the ''lac'' repressor (lacI) and a gene conferring resistance to the antibiotic kanamycin (KanR), as well as various {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoters}} for driving the expression of these genes.]]

{{term|extein}} <dd>Any part of an {{gli|amino acid sequence}} which is retained within a precursor {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}}, i.e. not excised by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-translational}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein splicing}}, and is therefore present in the mature {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}}, analogous to the {{gli|exons}} of RNA transcripts. Contrast ''{{gli|intein}}''.</dd>

{{term|extension}} <dd>See ''{{gli|elongation}}''.</dd>

{{term|extracellular}} <dd>Outside the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} of a {{gli|cell}} or cells; i.e. located or occurring externally to a cell. Contrast ''{{gli|intracellular}}''; see also ''{{gli|intercellular}}''.</dd>

{{term|extracellular fluid}} <dd></dd>

{{term|extracellular matrix (ECM)}}{{anchor|extracellular matrix}} {{ghat|Also '''intercellular matrix'''.}} <dd>The network of interacting {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|macromolecules}} and minerals secreted by and existing outside of and between cells in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|multicellular}} structures such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissues}} and {{gli|biofilms}}, forming a hydrated, mesh-like, semi-solid suspension which not only holds the cells together in an organized fashion but also provides structural and biochemical support, acting as an elastic, compressible buffer against external stresses as well as both regulating and influencing numerous aspects of cell behavior, among them {{gli|cell adhesion}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motility}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism}}, {{gli|cell division|division}}, and {{gli|cell signaling|cell-to-cell communication}}. The composition and properties of the ECM vary enormously between organisms and tissue types, but generally it takes the form of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polysaccharide}} gel in which various fibrous proteins (especially collagen and elastin), enzymes, and {{gli|glycoproteins}} are embedded. Cells themselves both produce the matrix components and respond constantly to local matrix composition, a source of environmental feedback which is critical for {{gli|differentiation}}, tissue organization, and development.<ref name="MacLean"/><ref name=ECB>{{#invoke:cite|book| vauthors = Alberts B, Bray D, Hopin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P | title = Essential cell biology | chapter-url = https://archive.org/details/essentialcellbio00albe | chapter-url-access = registration | chapter = Tissues and Cancer | location = New York and London | publisher = Garland Science | year = 2004 | isbn = 978-0-8153-3481-1 }}</ref></dd>

{{term|extrachromosomal DNA}} {{ghat|Also '''extranuclear DNA''' and '''cytoplasmic DNA'''.}} <dd>Any {{gli|DNA}} that is not found in {{gli|chromosomes}} or in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} of a cell and hence is not {{gli|genomic DNA}}. This may include the DNA contained in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmids}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelles}} such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mtDNA|mitochondria}} or {{gli|cpDNA|chloroplasts}}, or, in the broadest sense, DNA introduced by viral infection. Extrachromosomal DNA usually shows significant structural differences from nuclear DNA in the same organism.</dd>

==F== {{glossary}} {{term|facilitated diffusion}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|passive transport}} by which substances are conveyed across {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membranes}} more quickly than would be possible by ordinary passive diffusion alone, generally because {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane protein|proteins embedded within the membrane}} act as shuttles or pores, being arranged in such a way as to provide a {{gli|hydrophilic}} environment that is favorable for the movement of small polar molecules, which would otherwise be repulsed by the {{gli|hydrophobic}} interior of the {{gli|lipid bilayer}}.<ref name="Lackie"/></dd>

{{term|facultative expression}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} of a {{gli|gene}} only as needed, as opposed to {{gli|constitutive expression}}, in which a gene is transcribed continuously. A gene that is transcribed as needed is called a ''facultative gene''.</dd>

{{term|fatty acid}}{{anchor|fatty acids}} <dd>Any of a subclass of {{gli|lipid}} compounds consisting of a carboxylic acid bonded to an aliphatic chain of hydrocarbons, usually 4 to 28 carbon atoms in length, which may be either saturated (containing only single bonds between the carbon atoms) or unsaturated (containing one or more double bonds). In biological systems, fatty acid chains are commonly linked to other compounds via ester bonds, primarily in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|triglycerides}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipids}}, and derivatives of {{gli|cholesterol}}, all of which serve a wide variety of important cellular functions including as structural components of membranes and as energy sources in metabolic pathways.</dd>

{{term|fermentation}}{{anchor|ferment|ferments|fermenting|fermented}} <dd>Any {{gli|anaerobic}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathway}} in which organic molecules such as {{gli|glucose}} or other carbohydrates are {{gli|catabolized}} in the absence of oxygen in order to produce {{gli|ATP}}; or, in the broadest sense, any catabolic process in which organic compounds serve as both electron donors and acceptors.<ref>{{Cite journal |last1=Hackmann |first1=Timothy J. |last2=Zhang |first2=Bo |date=2023-09-29 |title=The phenotype and genotype of fermentative prokaryotes |journal=Science Advances |volume=9 |issue=39 |article-number=eadg8687 |doi=10.1126/sciadv.adg8687 |issn=2375-2548 |pmc=10530074 |pmid=37756392|bibcode=2023SciA....9G8687H}}</ref> This definition distinguishes fermentation from {{gli|aerobic respiration}}, where inorganic diatomic oxygen ({{chem|O|2}}) is the terminal electron acceptor, and from some types of {{gli|anaerobic respiration}}. Fermentation encompasses hundreds of different redox pathways which start and end with a huge variety of reactants and end-products, often branching from various steps in {{gli|glycolysis}}, with the most common fermentation products being lactate, acetate, ethanol, succinate, propionate, butyrate, carbon dioxide ({{chem|CO|2}}), and diatomic hydrogen ({{chem|H|2}}). It occurs in both prokaryotes and eukaryotes in conditions where exogenously supplied electron acceptors are unavailable, especially in oxygen-poor environments. Fermentation yields the equivalent of just 2 to 5 ATP per molecule of glucose, making it much less efficient than aerobic respiration, which can yield as much as 32 ATP per molecule of glucose. In multicellular organisms that primarily rely on aerobic respiration, such as animals, it is often employed as a contingency pathway; the term ''anaerobic glycolysis'' refers to the diversion of glycolysis intermediates to fermentation pathways when tissues cannot keep up with the demand for ATP due to insufficient oxygen supply.</dd>

{{term|filopodium}}{{anchor|filopodia}} <dd></dd>

{{term|find-me signal}} <dd>A molecule exposed on the surface of a cell destined for {{gli|apoptosis}} which is used to attract {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phagocytes}} to engulf and eliminate the cell by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phagocytosis}}. See also ''{{gli|eat-me signal}}''.</dd>

{{term|five-prime cap}} <dd>See ''{{gli|5' cap}}''.</dd>

{{term|five-prime end}} <dd>See ''{{gli|5'-end}}''.</dd>

{{term|five-prime untranslated region}} <dd>See ''{{gli|5' untranslated region}}''.</dd>

{{term|fixation}} <dd>1.  (histology) The preservation of biological material by treating it with a chemical {{gli|fixative}} that prevents or delays the natural postmortem processes of decay (e.g. {{gli|autolysis}} and putrefaction) which would otherwise eventually cause cells, tissues, and biomolecules to lose their characteristic structures and properties. Biological specimens are usually fixed with the broad objective of arresting or slowing biochemical reactions for long enough to study them in detail, essentially 'freezing' cellular processes in their natural state at a specific point in time, while minimizing disruption to existing structures and arrangements, all of which can improve subsequent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|staining}} and microscopy of the fixed samples. Though fixation tends to irreversibly terminate any ongoing reactions, thus killing the fixed cells, it makes it possible to study molecular details that occur too rapidly or transiently to observe in living samples. Common fixatives such as formaldehyde work by disabling {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteolytic}} enzymes, coagulating, insolubilizing, and/or {{gli|denaturing}} macromolecules, creating {{gli|crosslinks}} between them, and protecting specimens from decomposition by bacteria and fungi.</dd> <dd>2.  (population genetics) The process by which a single {{gli|allele}} for a particular {{gli|gene}} with multiple different alleles increases in {{gli|allele frequency|frequency}} in a given population such that it becomes permanently established as the only allele at that {{gli|locus}} within the population's {{gli|gene pool}}.</dd>

{{term|fixative}}{{anchor|fixatives}} <dd>Any chemical compound or solution that causes the {{gli|fixation}} of cells, tissues, or other microscopic structures by any mechanism, thus preserving them for long-term, detailed study by methods such as embedding, {{gli|staining}}, and microscopy. Common fixatives include dilute solutions of ethanol, acetic acid, formaldehyde, and osmium tetroxide, among others.<ref name="MacLean"/></dd>

{{term|flagellate}} <dd>(of a cell) Having one or more {{gli|flagella}}.</dd>

{{term|flagellum}}{{anchor|flagella}} {{ghat|(pl.) '''flagella'''}} <dd>A long, thin, hair-like appendage protruding from the surface of some cells, which serves locomotory functions by undulating in a way that propels the cell through its environment or by effecting the movement of {{gli|extracellular fluids}} and solutes past the cell surface. Many unicellular organisms, including some bacteria, protozoa, and algae, bear one or more flagella, and certain cell types in multicellular organisms, namely sperm cells, also have flagella. Eukaryotic flagella are essentially just longer versions of {{gli|cilia}}, often up to 150 micrometres (μm) in length, while bacterial flagella are typically smaller and completely different in structure and mechanism of action.<ref name="Alberts et al."/><ref name="MacLean"/></dd>

{{term|fluid mosaic model}} <dd>The prevailing scientific model of the structure and properties of {{gli|cell membranes}}, according to which the typical membrane consists of {{gli|lipid bilayer|back-to-back layers}} of amphipathic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane lipids}} (generally {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipids}} or {{gli|glycolipids}}) interspersed with a dynamic variety of embedded {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, {{gli|carbohydrates}}, and (especially in animal cells) {{gli|cholesterol}}, all of which behave as if suspended in a "two-dimensional liquid", constantly moving laterally between the lipids and interacting with each other and with the {{gli|cytoplasm}} and the {{gli|extracellular space}}. The membrane as a whole thus retains a fluidity and elasticity which allow it to change shape and adapt to the cell's environment.<ref>{{cite journal |last1=Singer |first1=S. J. |last2=Nicolson |first2=Garth L. |title=The Fluid Mosaic Model of the Structure of Cell Membranes: Cell membranes are viewed as two-dimensional solutions of oriented globular proteins and lipids. |journal=Science |date=18 February 1972 |volume=175 |issue=4023 |pages=720–731 |doi=10.1126/science.175.4023.720|pmid=4333397 }}</ref></dd>

{{term|fluorescence ''in situ'' hybridization (FISH)}}{{anchor|fluorescence in situ hybridization|FISH}} <dd>A type of {{gli|in situ hybridization|''in situ'' hybridization}} {{gli|assay}} where the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligonucleotide}} probes are {{gli|labelled}} with a {{gli|fluorophore|chemical compound}} that is naturally fluorescent when exposed to light at particular wavelengths, making it possible to detect the ''{{gli|in situ}}'' locations of complementary sequences with fluorescence microscopy. FISH is commonly used to visualize the physical locations of specific genes on chromosomes.</dd>

{{term|forward genetics}} <dd>An experimental approach in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular genetics}} in which a researcher starts with a specific known {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotype}} and attempts to determine the genetic basis of that phenotype by any of a variety of laboratory techniques, commonly by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutagenesis|inducing}} random {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutations}} in the organism's genome and then {{gli|screening}} for changes in the phenotype of interest. Observed phenotypic changes are assumed to have resulted from the mutation(s) present in the screened sample, which can then be {{gli|mapped}} to specific genomic {{gli|loci}} and ultimately to one or more specific {{gli|candidate genes}}. This methodology contrasts with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reverse genetics}}, in which a specific gene or its gene product is individually manipulated in order to identify the gene's function.</dd>

{{term|forward mutation}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} that changes a gene from {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|wild-type}} to mutant; the initial mutation which a {{gli|back mutation}} reverses.<ref name="Oxford B&MB"/></dd>

{{term|frameshift mutation}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} in a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence}} caused by the {{gli|insertion}} or {{gli|deletion}} of a number of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} that is not divisible by three. Because of the triplet nature by which nucleotides code for amino acids, a mutation of this sort causes a shift in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reading frame}} of the nucleotide sequence, resulting in the sequence of {{gli|codons}} downstream of the mutation site being completely different from the original.</dd>

{{term|freeze-drying}} <dd>See ''{{gli|lyophilization}}''.</dd>

{{term|Functional Genomics Data (FGED) Society}} {{ghat|Formerly known by the abbreviation '''MGED'''.}} <dd>An organization that works with others "to develop standards for biological research data quality, annotation and exchange" as well as software tools that facilitate their use.<ref name="urlFGED Homepage">{{#invoke:cite|web| url = http://fged.org/ | title = Functional Genomics Data Society – FGED Society }}</ref></dd>

==G== {{glossary}} {{term|G banding}} {{ghat|Also '''Giemsa banding''' or '''G-banding'''.}} <dd>A technique used in {{gli|cytogenetics}} to produce a visible {{gli|karyotype}} by staining the condensed chromosomes with Giemsa stain. The staining produces consistent and identifiable patterns of dark and light "bands" in regions of {{gli|chromatin}}, which allows specific chromosomes to be easily distinguished.</dd>

{{term|G1}} <dd></dd>

{{term|G2}} <dd></dd>

{{term|gamete}}{{anchor|gametes|gametic|gametic cell|gametic cells|gametogenesis}} <dd>A {{gli|haploid}} cell that is the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiotic}} product of a progenitor {{gli|germ cell}} and the final product of the {{gli|germ line}} in sexually reproducing multicellular organisms. Gametes are the means by which an organism passes its genetic information to its offspring; during fertilization, two gametes (one from each parent) are fused into a single {{gli|diploid}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygote}}.</dd>

{{term|gametogenesis}} <dd>The process by which eukaryotic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|precursor cell|precursor}} {{gli|germ cells}} divide and {{gli|differentiate}} into {{gli|haploid}} {{gli|gametes}}. Depending on the organism, gametes may be generated from haploid germ cells by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} or {{gli|diploid}} germ cells by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}.</dd>

{{term|GC content}} <dd>See ''{{gli|guanine-cytosine content}}''.</dd>

{{term|gDNA}} <dd>See ''{{gli|genomic DNA}}''.</dd>

{{term|gene}}{{anchor|genes}} <dd>Any segment or set of segments of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecule that contains the information necessary to produce a functional {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} transcript in a controlled manner. In living organisms, genes are often considered the fundamental units of {{gli|heredity}} and are typically encoded in {{gli|DNA}}. A particular gene can have multiple different versions, or {{gli|alleles}}, and a single gene can result in a {{gli|gene product}} that influences many different {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotypes}}.</dd>

{{term|gene cassette}} <dd>See ''{{gli|cassette}}''.</dd>

{{term|gene dosage}} <dd>The number of copies of a particular {{gli|gene}} present in a {{gli|genome}}. Gene dosage directly influences the amount of {{gli|gene product}} a cell is able to express, though a variety of controls have evolved which tightly {{gli|gene regulation|regulate}} {{gli|gene expression}}. Changes in gene dosage caused by mutations include {{gli|copy-number variations}}.</dd>

{{term|gene duplication}}{{anchor|duplication|duplicate|duplicates|duplicated|duplicating}} {{ghat|Also '''gene amplification'''.}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} defined as any {{gli|duplication}} of a region of {{gli|DNA}} that contains a {{gli|gene}}. Compare ''{{gli|chromosomal duplication}}''.</dd>

{{term|gene expression}}{{anchor|express|expression|expressed|expressing}} <dd>The set of processes by which the information encoded in a {{gli|gene}} is used in the synthesis of a {{gli|gene product}}, such as a protein or a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ncRNA|non-coding RNA}}, or otherwise made available to influence one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotypes}}; both the product and the gene encoding it are then said to be ''expressed''. Canonically, the first step is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}, which produces a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|messenger RNA}} molecule complementary to the {{gli|DNA}} molecule in which the gene is encoded. For protein-coding genes, the second step is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}, in which the messenger RNA is read by a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosome}} to produce a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}} and ultimately a protein. The information contained within a DNA sequence need not necessarily be transcribed and translated to exert an influence on molecular events, however: broader definitions encompass a huge variety of other ways in which genetic information can be expressed.</dd> thumb|right|500px|In the typical model of '''{{gli|gene expression}}''', genetic information (red) encoded in a DNA molecule is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcribed}} with help from nearby {{gli|cis-regulatory element|regulatory elements}} into a raw {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|messenger RNA}}, then processed into a mature form by the removal of {{gli|introns}} and the addition of a {{gli|5' cap}} and a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|poly-A tail}}, then finally {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translated}} into a polypeptide sequence which is folded into a functional protein.

{{term|Gene Expression Omnibus (GEO)}} <dd>A database of {{gli|high-throughput}} functional genomics and {{gli|gene expression}} data derived from experimental assays and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|next-generation sequencing}} and managed by the National Center for Biotechnology Information.<ref>{{#invoke:cite|journal|last1=Edgar |first1=R |last2=Domrachev |first2=M |last3=Lash |first3=AE |title=Gene Expression Omnibus: NCBI gene expression and hybridization array data repository. |journal=Nucleic Acids Research |date=1 January 2002 |volume=30 |issue=1 |pages=207–10 |doi=10.1093/nar/30.1.207 |pmid=11752295|pmc=99122 }}</ref><ref>{{#invoke:cite|journal|last1=Barrett |first1=T |last2=Wilhite |first2=SE |last3=Ledoux |first3=P |last4=Evangelista |first4=C |last5=Kim |first5=IF |last6=Tomashevsky |first6=M |last7=Marshall |first7=KA |last8=Phillippy |first8=KH |last9=Sherman |first9=PM |last10=Holko |first10=M |last11=Yefanov |first11=A |last12=Lee |first12=H |last13=Zhang |first13=N |last14=Robertson |first14=CL |last15=Serova |first15=N |last16=Davis |first16=S |last17=Soboleva |first17=A |title=NCBI GEO: archive for functional genomics data sets--update. |journal=Nucleic Acids Research |date=January 2013 |volume=41 |issue=Database issue |pages=D991-5 |doi=10.1093/nar/gks1193 |pmid=23193258|pmc=3531084 }}</ref></dd>

{{term|gene fusion}} <dd>The union, either by natural mutation or by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recombinant DNA|recombinant}} laboratory techniques, of two or more previously independent genes that code for different gene products such that they become subject to control by the same {{gli|gene regulation|regulatory}} systems. The resulting hybrid sequence is known as a {{gli|fusion gene}}.<ref name="DoG7"/></dd>

{{term|gene mapping}} <dd>Any of a variety of methods used to precisely identify the {{gli|locus|location}} of a particular {{gli|gene}} within a DNA molecule (such as a chromosome) and/or the physical or {{gli|linkage}} distances between it and other genes.</dd>

{{term|gene of interest (GOI)}}{{anchor|genes of interest}} <dd>A {{gli|gene}} being studied in a scientific experiment, especially one that is the focus of a {{gli|genetic engineering}} technique such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular cloning|cloning}}.</dd>

{{term|gene product}}{{anchor|gene products|products}} <dd>Any of the biochemical material resulting from the {{gli|expression}} of a {{gli|gene}}, most commonly interpreted as the functional {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA|mRNA transcript}} produced by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} of the gene or the fully constructed {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} produced by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}} of the transcript, though {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ncRNA|non-coding RNA}} molecules such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tRNA|transfer RNAs}} may also be considered gene products. A measurement of the quantity of a given gene product that is detectable in a cell or tissue is sometimes used to infer how active the corresponding gene is.</dd>

{{term|gene regulation}}{{anchor|regulation|regulate|regulating|regulated|regulatory}} <dd>The broad range of mechanisms used by cells to control the activity of their genes, especially to allow, prohibit, increase, or decrease the production or {{gli|expression}} of specific {{gli|gene products}}, such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}. Gene regulation increases an organism's versatility and adaptability by allowing its cells to express different gene products when required by changes in its environment. In multicellular organisms, the regulation of gene expression also drives {{gli|cellular differentiation}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|morphogenesis}} in the {{gli|embryo}}, enabling the creation of a diverse array of {{gli|cell types}} from the same {{gli|genome}}.</dd>

{{term|gene silencing}}{{anchor|silencing}} <dd>Any mechanism of {{gli|gene regulation}} which drastically reduces or completely prevents the {{gli|expression}} of a particular gene. Gene silencing may occur naturally during either {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}}. Laboratory techniques often exploit natural silencing mechanisms to achieve {{gli|gene knockdown}}.</dd>

{{term|gene therapy}} <dd>The insertion of a functional or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|wild-type}} gene or part of a gene into an organism (especially a patient) with the intention of correcting a {{gli|genetic disorder|genetic defect}}, either by direct substitution of the defective gene or by supplementation with a second, functional version.<ref name="Rieger"/></dd>

{{term|gene trapping}} <dd>A {{gli|high-throughput}} technology used to simultaneously inactivate, identify, and report the {{gli|expression}} of a target gene in a mammalian genome by introducing an insertional {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} consisting of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoter|promoterless}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reporter}} gene and/or a {{gli|selectable marker}} flanked by an upstream {{gli|gene splicing|splice}} site and a downstream {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polyadenylated}} termination sequence.</dd>

{{term|generation}}{{anchor|generations}} <dd>1.  In any given organism, a single reproductive cycle, or the phase between two consecutive reproductive events, i.e. between an individual organism's reproduction and that of the progeny of that reproduction; or the actual or average length of time required to complete a single reproductive cycle, either for a particular {{gli|lineage}} or for a population or species as a whole.</dd> <dd>2.  In a given population, those individuals (often but not necessarily living contemporaneously) who are equally removed from a given {{gli|common ancestor}} by virtue of the same number of reproductive events having occurred between them and the ancestor.<ref name="Rieger"/></dd>

{{term|genetic background}}{{anchor|genetic backgrounds}} <dd></dd>

{{term|genetic code}} <dd>A set of rules by which information encoded within {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}} is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translated}} into {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} by living cells. These rules define how sequences of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide}} triplets called {{gli|codons}} specify which {{gli|amino acid}} will be added next during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation|protein synthesis}}. The vast majority of living organisms use the same genetic code (sometimes referred to as the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|standard genetic code|"standard" genetic code}}) but variant codes do exist.</dd>

{{term|genetic disorder}}{{anchor|genetic disorders}} <dd>Any illness, disease, or other health problem directly caused by one or more abnormalities in an organism's {{gli|genome}} which are congenital (present at birth) and not acquired later in life. Causes may include a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} to one or more {{gli|genes}}, or a {{gli|chromosomal abnormality}} such as an {{gli|aneuploidy}} of a particular chromosome. The mutation responsible {{gli|de novo mutation|may occur spontaneously}} during embryonic development or may be {{gli|heredity|inherited}} from one or both parents, in which case the genetic disorder is also classified as a {{gli|hereditary disorder}}. Though the abnormality itself is present before birth, the actual disease it causes may not develop until much later in life; some genetic disorders do not necessarily guarantee eventual disease but simply {{gli|genetic predisposition|increase the risk}} of developing it.</dd>

{{term|genetic distance}} <dd>A measure of the genetic divergence between species, populations within a species, or individuals, used especially in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phylogenetics}} to express either the time elapsed since the existence of a {{gli|glossary=Glossary of genetics and evolutionary biology|common ancestor}} or the degree of differentiation in the {{gli|DNA sequences}} comprising the {{gli|genomes}} of each population or individual.</dd>

{{term|genetic engineering}} {{ghat|Also '''genetic modification''' or '''genetic manipulation'''.}} <dd>The direct, deliberate manipulation of an organism's genetic material using any of a variety of biotechnology methods, including the {{gli|insertion}} or {{gli|deletion|removal}} of {{gli|genes}}, the transfer of genes within and between species, the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} of existing sequences, and the construction of novel sequences using {{gli|artificial gene synthesis}}. Genetic engineering encompasses a broad set of technologies by which the genetic composition of individual cells, tissues, or entire organisms may be altered for various purposes, commonly in order to study the functions and {{gli|expression}} of individual genes, to produce hormones, vaccines, and other drugs, and to create {{gli|genetically modified organisms}} for use in research and agriculture.</dd>

{{term|genetic marker}}{{anchor|genetic markers|marker|markers}} <dd>A specific, easily identifiable, and usually highly {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polymorphism|polymorphic}} {{gli|gene}} or other {{gli|DNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence}} with a known location on a {{gli|chromosome}} that can be used to identify the individual or species possessing it.</dd>

{{term|genetic recombination}} <dd>Any reassortment or exchange of genetic material within an individual organism or between individuals of the same or different species, especially that which creates {{gli|genetic variation}}. In the broadest sense, the term encompasses a diverse class of naturally occurring mechanisms by which {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequences}} are copied or physically transferred into different genetic environments, including {{gli|homologous recombination}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} or as a normal part of {{gli|DNA repair}}; {{gli|horizontal gene transfer}} events such as {{gli|conjugation|bacterial conjugation}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transduction|viral transduction}}, or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transformation}}; or errors in {{gli|DNA replication}} or cell division. Artificial recombination is central to many {{gli|genetic engineering}} techniques which produce {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recombinant DNA}}.</dd>

{{term|genetic redundancy}} <dd>The redundant encoding of two or more distinct {{gli|gene products}} that ultimately perform the same biochemical function. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation|Mutations}} in one of these genes may have a smaller effect on fitness than might be expected, since the redundant genes often compensate for any {{gli|loss of function}} and obviate any {{gli|gain of function}}.</dd>

{{term|genetic regulatory network (GRN)}} <dd>A graph that represents the regulatory complexity of {{gli|gene expression}}. The vertices (nodes) are represented by various regulatory elements and {{gli|gene products}} while the edges (links) are represented by their interactions. These network structures also represent functional relationships by approximating the rate at which genes are {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcribed}}.</dd>

{{term|genetic testing}} {{ghat|Also '''DNA testing''' or '''genetic screening'''.}} <dd>A broad class of various procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage or ancestry, diagnose vulnerabilities to heritable diseases, or detect {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutant}} alleles associated with increased risks of developing {{gli|genetic disorders}}. Genetic testing is widely used in human medicine, agriculture, and biological research.</dd>

{{term|genetically modified organism (GMO)}} <dd>Any organism whose genetic material has been altered using {{gli|genetic engineering}} techniques, particularly in a way that does not occur naturally by mating or by natural {{gli|genetic recombination}}.</dd>

{{term|genetics}} <dd>The field of biology that studies {{gli|genes}}, {{gli|genetic variation}}, and {{gli|heredity}} in living organisms.</dd>

{{term|genome}}{{anchor|genomes|genomic}} <dd>1.  The entire complement of genetic material contained within the {{gli|chromosomes}} of an organism, {{gli|organelle}}, or virus.</dd> <dd>2.  The collective set of {{gli|genes}} or genetic {{gli|loci}} shared by every member of a population or species, regardless of the different {{gli|alleles}} that may be present at these loci in different individuals.</dd>

{{term|genome instability}}{{anchor|genomic instability}} <dd></dd>

{{term|genome size}} <dd>The total amount of {{gli|DNA}} contained within one copy of a {{gli|genome}}, typically measured by mass (in picograms or daltons) or by the total number of {{gli|base pairs}} (in {{gli|kilobases}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|megabases}}). For {{gli|diploid}} organisms, genome size is often used interchangeably with {{gli|C-value}}.</dd>

{{term|genome walking}} {{ghat|Also '''chromosome walking'''.}} <dd>A method of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|whole-genome sequencing}} in which the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequences}} of {{gli|genomic DNA}} fragments in a {{gli|library sample}} are assembled into a longer sequence, such as that of a full chromosome or the entire genome, by placing fragments with overlapping ends, known as {{gli|contigs}}, adjacent to each other. By repeating this procedure, one can hypothetically determine the correct arrangement of contigs for the entire sequence.<ref name="Oxford B&MB">{{cite book |editor1-last=Cammack |editor1-first=Richard |editor2-last=Atwood |editor2-first=Teresa |editor3-last=Campbell |editor3-first=Peter |editor4-last=Parish |editor4-first=Howard |editor5-last=Smith |editor5-first=Anthony |editor6-last=Vella |editor6-first=Frank |editor7-last=Stirling |editor7-first=John |title=Oxford Dictionary of Biochemistry and Molecular Biology |date=2008 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-172764-1 |edition=2nd}}</ref></dd>

{{term|genome-wide association study (GWAS)}}{{anchor|genome-wide association study|genome-wide association studies}} <dd></dd>

{{term|genomic DNA (gDNA)}} {{ghat|Also '''chromosomal DNA'''.}} <dd>The {{gli|DNA}} contained in {{gli|chromosomes}}, as opposed to the {{gli|extrachromosomal DNA}} contained in separate structures such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmids}} or organelles such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mtDNA|mitochondria}} or {{gli|cpDNA|chloroplasts}}.</dd>

{{term|genomic imprinting}} <dd>An {{gli|epigenetic}} phenomenon that causes {{gli|genes}} to be {{gli|expressed}} in a manner dependent upon the particular parent from which the gene was inherited. It occurs when epigenetic marks such as {{gli|DNA methylation|DNA}} or {{gli|histone methylation}} are established or "imprinted" in the {{gli|germ cells}} of a parent organism and subsequently maintained through cell divisions in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|somatic cells}} of the organism's progeny; as a result, a gene in the progeny that was inherited from the father may be expressed differently than another copy of the same gene that was inherited from the mother.</dd>

{{term|genomic island (GI)}}{{anchor|genomic island|genomic islands}} <dd>A region of a {{gli|genome}} that shows evidence of {{gli|horizontal gene transfer|horizontal transfer}} from another organism. The term is used especially in describing microbial genomes such as those of bacteria, where genomic islands having the same or similar sequences commonly occur in species or strains that are otherwise only distantly related, implying that they were not passed on through vertical descent from a common ancestor but through some form of lateral transfer such as {{gli|bacterial conjugation|conjugation}}. These islands often contain functional genes which confer adaptive traits such as {{gli|antibiotic resistance}}.</dd>

{{term|genomics}} <dd>An interdisciplinary field that studies the structure, function, evolution, mapping, and editing of entire {{gli|genomes}}, as opposed to individual {{gli|genes}}.</dd>

{{term|genotoxicity}} <dd>The ability of certain chemical agents to cause damage to genetic material within a living cell (e.g. through single- or double-stranded breaks, {{gli|crosslinking}}, or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|point mutations}}), which may or may not result in a permanent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}}. Though all {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutagens}} are genotoxic, not all genotoxic compounds are mutagenic.</dd>

{{term|genotype}}{{anchor|genotypes}} <dd>The entire complement of {{gli|alleles}} present in a particular individual's {{gli|genome}}, which gives rise to the individual's {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotype}}.</dd>

{{term|genotyping}} <dd>The process of determining differences in the {{gli|genotype}} of an individual by examining the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence|DNA sequences}} in the individual's {{gli|genome}} using {{gli|bioassays}} and comparing them to another individual's sequences or a reference sequence.</dd>

{{term|germ cell}}{{anchor|germ cells}} <dd>Any {{gli|cell}} that gives rise to the {{gli|gametes}} of a sexually reproducing organism. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished from {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|somatic cells}}, which are entirely separate from the {{gli|germ line}}.</dd>

{{term|germ line}} <dd>1.  In multicellular organisms, the subpopulation of cells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct from {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|somatic cells}}, which cannot pass on their genetic material except to their own immediate {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitotic}} daughter cells. Cells of the germ line are called {{gli|germ cells}}.</dd> <dd>2.  The {{gli|lineage}} of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.</dd>

{{term|gigabase (Gb)}}{{anchor|gigabases|Gb|Gbp}} <dd>A unit of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} length equal to one billion (1{{e|9}}) {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase|bases}} in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} molecules or one billion {{gli|base pairs}} in duplex molecules such as {{gli|dsDNA|double-stranded DNA}}.</dd>

{{term|glucogenic amino acid}}{{anchor|glucogenic amino acids}} <dd>Any {{gli|amino acid}} that can be converted into {{gli|glucose}} via {{gli|gluconeogenesis}}, as opposed to the {{gli|ketogenic amino acids}}, which can be converted into ketone bodies. In humans, 18 of the 20 amino acids are glucogenic; only leucine and lysine are not. Five amino acids (phenylalanine, isoleucine, threonine, tryptophan, and tyrosine) are both glucogenic and ketogenic.</dd>

{{term|gluconeogenesis (GNG)}}{{anchor|gluconeogenesis}} <dd>The chain of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic}} reactions that results in the generation of {{gli|glucose}} from some non-carbohydrate carbon substrates, including the {{gli|glucogenic amino acids}}. It is one of two primary {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathway|pathways}} used by most animals to maintain blood sugar levels (the other being {{gli|glycogenolysis}}), especially during periods of fasting, starvation, and intense exercise.</dd>

{{term|glucose}} <dd>A simple sugar with the molecular formula {{chem|C|6|H|12|O|6}} and the most abundant {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monosaccharide}} in nature, being the primary product of photosynthesis, where it is made in a sunlight-powered reaction of water with carbon dioxide. All living organisms are capable of metabolizing glucose via {{gli|glycolysis}}, an exergonic pathway which for most organisms is the primary means of obtaining chemical energy to power cellular activities.<ref name="MacLean"/> Metabolic glucose is usually stored in the form of large polymeric aggregates such as amylose in plants and {{gli|glycogen}} in animals, and is released by the breakdown of these polymers via {{gli|glycogenolysis}}.</dd>

{{term|glycocalyx}} {{ghat|Also '''pericellular matrix''' and '''cell coat'''.}} <dd>A fine, hair-like coating covering the outer surface of virtually all cells, composed of a layer of various branching {{gli|glycoproteins}} and {{gli|glycolipids}} which are embedded within and protrude from the {{gli|extracellular}} face of the {{gli|cell membrane}}.<ref>{{Cite journal |last=Möckl |first=Leonhard |date=2020 |title=The Emerging Role of the Mammalian Glycocalyx in Functional Membrane Organization and Immune System Regulation |journal=Frontiers in Cell and Developmental Biology |volume=8 |article-number=253 |doi=10.3389/fcell.2020.00253 |doi-access=free |issn=2296-634X |pmc=7174505 |pmid=32351961}}</ref> These molecules play critical roles in {{gli|cell–cell recognition}}, {{gli|cell signaling}}, and intercellular adhesion.<ref>Reitsma, Sietze. "The endothelial glycocalyx: composition, functions, and visualization." European Journal of Physiology. 2007. Vol. 454. Num. 3. p. 345–359</ref></dd>

{{term|glycogen}} <dd>A branched {{gli|polysaccharide}} composed of as many as 30,000 covalently bonded units of the {{gli|monosaccharide}} {{gli|glucose}} which functions as the primary form of short-term energy storage in most animal cells.<ref name="MacLean"/><ref name="Lackie"/> Glycogen reserves are especially abundant in muscle and liver cells,<ref name="Alberts et al."/> where they can be metabolized at-need into their component glucoses as a means of buffering blood sugar levels, a process known as {{gli|glycogenolysis}}.</dd>

{{term|glycogenolysis}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathway}} in which polymeric {{gli|glycogen}} molecules are broken down into individual {{gli|glucose}} monomers by the sequential removal of glucose units via phosphorolysis, a reaction catalyzed by the enzyme glycogen phosphorylase. Glycogenolysis is one of two primary pathways used in animal tissues to generate free glucose for the maintenance of blood sugar levels, the other being {{gli|gluconeogenesis}}.</dd>

{{term|glycolipid}}{{anchor|glycolipids}} <dd>Any of a subclass of {{gli|lipids}} consisting of a central polar molecule (most commonly glycerol or sphingosine) which is covalently attached to one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monosaccharides}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligosaccharides}} via {{gli|glycosidic bonds}}, as well as to one or more long, non-polar {{gli|fatty acid}} chains.<ref name="Alberts et al."/> Glycolipids are one of three major types of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane lipid}} comprising all biological membranes, along with {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipids}} and {{gli|cholesterol}}.</dd>

{{term|glycolysis}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolic pathway}} in which {{gli|carbohydrate}} sugars such as {{gli|glucose}} are broken down into simpler molecules, releasing chemical energy which can then be used for various cellular functions. In a series of ten enzyme-catalyzed reactions, each molecule of glucose is converted into two molecules of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyruvate}}, with the free energy liberated in this process simultaneously being used to form high-energy bonds in two molecules of reduced {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nicotinamide adenine dinucleotide}} (NADH) and two molecules of {{gli|adenosine triphosphate}} (ATP). In {{gli|aerobic}} conditions pyruvate and NADH are further oxidized in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitochondria}}; in {{gli|anaerobic}} conditions NADH itself subsequently reduces pyruvate to lactate.</dd> thumb|500px|'''{{gli|glycolysis|Glycolysis}}''' converts {{gli|glucose}} to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyruvate}} via a series of 10 steps, each catalyzed by a different enzyme and producing different intermediate metabolites. Steps 1 and 3 consume {{gli|ATP}} (blue arrows) and steps 7 and 10 produce ATP (yellow arrows); steps 6 through 10 occur twice per molecule of glucose.

{{term|glycoprotein}}{{anchor|glycoproteins}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} with one or more {{gli|carbohydrate}} molecules, typically short {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligosaccharide}} chains, covalently attached to one or more of its amino acid side chains.<ref name="Alberts et al."/> Proteins exposed on the outer surface of the {{gli|plasma membrane}} or secreted into the extracellular space are commonly modified in this way, after which they are said to be {{gli|glycosylated}}.</dd>

{{term|glycosidase}} <dd>Any of a class of {{gli|enzymes}} capable of breaking one or more {{gli|glycosidic bonds}} in {{gli|carbohydrate}} molecules, commonly found in {{gli|lysosomes}}.<ref name="MacLean"/></dd>

{{term|glycoside}}{{anchor|glycosides}} <dd>Any chemical compound consisting of a {{gli|carbohydrate}} molecule covalently bonded to another molecule containing a hydroxyl group (including other carbohydrates) via one or more {{chem|C–O}} {{gli|glycosidic bonds}}. When all of the compound's substituents are carbohydrates, the glycoside is a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polysaccharide}}.<ref name="MacLean"/></dd>

{{term|glycosidic bond}}{{anchor|glycosidic bonds}} <dd>A covalent ether bond that connects a carbon atom within a {{gli|carbohydrate}} molecule (e.g. a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monosaccharide}}) or a carbohydrate derivative to another substituent or functional group, which may or may not be another carbohydrate; such bonds form as the result of a dehydration reaction between hydroxyl groups on each molecule. A substance containing a glycosidic bond is known as a {{gli|glycoside}}.</dd>

{{term|glycosylation}}{{anchor|glycosylate|glycosylates|glycosylated|glycosylating}} <dd>The attachment of a carbohydrate molecule (e.g. {{gli|glucose}}) to an amino acid residue within a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptide}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} by covalent bonding, a process which takes place in or near the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|rough endoplasmic reticulum}}.<ref name="MacLean"/></dd>

{{term|Goldberg-Hogness box}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|TATA box}}''.</dd>

{{term|Golgi apparatus}} <dd></dd>

{{term|gRNA}} <dd>See ''{{gli|guide RNA}}''.</dd>

{{term|term=guanine|content=guanine ({{font|G|font=courier|size=large}})}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|purine}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase}} used as one of the four standard nucleobases in both {{gli|DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules. Guanine forms a {{gli|base pair}} with {{gli|cytosine}}.</dd>

{{term|guanine-cytosine content}} {{ghat|Also abbreviated '''GC-content'''.}} <dd>The proportion of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nitrogenous bases}} in a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} that are either {{gli|guanine}} ({{font|G|font=courier|size=big}}) or {{gli|cytosine}} ({{font|C|font=courier|size=big}}), typically expressed as a percentage. DNA and RNA molecules with higher GC-content are generally more thermostable than those with lower GC-content due to molecular interactions that occur during base stacking.<ref name ="Yakovchuk2006">{{#invoke:cite|journal|vauthors=Yakovchuk P, Protozanova E, Frank-Kamenetskii MD |title=Base-stacking and base-pairing contributions into thermal stability of the DNA double helix |journal=Nucleic Acids Res. |volume=34 |issue=2 |pages=564–74 |year=2006 |pmid=16449200 |pmc=1360284 |doi=10.1093/nar/gkj454 |url=}}</ref></dd>

{{term|term=guanosine|content=guanosine ({{font|G|font=courier|size=large}}, Guo)}} <dd>One of the four standard {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosides}} used in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules, consisting of a {{gli|guanine}} {{gli|base}} with its N9 nitrogen {{gli|glycosidic bond|bonded}} to the C1 carbon of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} sugar. Guanine bonded to {{gli|deoxyribose}} is known as {{gli|deoxyguanosine}}, which is the version used in {{gli|DNA}}.</dd>

{{term|guide RNA (gRNA)}} {{ghat|Also '''single guide RNA (sgRNA)'''.}} <dd>A short {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded RNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligonucleotide}} which complexes with Cas {{gli|endonucleases}} and, by annealing to a specific complementary sequence in a {{gli|DNA}} molecule, serves to "guide" these proteins to viral DNA introduced by foreign pathogens, which can then be digested and degraded as part of an adaptive immune defense employed by bacteria and archaea. Custom-made guide RNAs are designed by scientists to target specific genomic loci in CRISPR-Cas {{gli|gene editing}}.</dd>

==H== {{glossary}} {{term|hairpin}}{{anchor|hairpin loop|stem-loop}} {{ghat|Also '''hairpin loop''' or '''stem-loop'''.}} <dd>A characteristic {{gli|secondary structure}} that commonly forms in self-complementary {{gli|nucleic acid sequences}} by intramolecular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pairing}} between different parts of the same linear, {{gli|single-stranded}} molecule. The resulting conformation resembles a hairpin, where non-adjacent lengths of nucleotides form hydrogen bonds with each other, creating a local double-stranded {{gli|glossary=Glossary of cellular and molecular biology (0–L)|duplex}} (the "stem") which ends in a circle of unpaired nucleotides (the "loop"). Hairpin loops form readily in single-stranded DNA molecules containing {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inverted repeats}}<ref name="Oxford B&MB"/> and are especially common in large RNA molecules, where they play various roles in promoting or inhibiting the formation of other secondary structures, stabilizing {{gli|messenger RNAs}}, providing recognition sites for {{gli|RNA-binding proteins}}, or serving as {{gli|substrates}} for enzymes.<ref>Svoboda, P., & Cara, A. (2006). Hairpin RNA: A secondary structure of primary importance. Cellular and Molecular Life Sciences, 63(7), 901-908.</ref></dd> thumb|right|350px|The structure of a basic '''{{gli|hairpin loop}}''' in a single-stranded RNA molecule

{{term|haploid}} {{ghat|Denoted in shorthand with the somatic number '''''n'''''.}} <dd>(of a cell or organism) Having one copy of each {{gli|chromosome}}, with each copy not being part of a pair. Contrast ''{{gli|diploid}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polyploid}}''.</dd>

{{term|Hayflick limit}} <dd></dd>

{{term|helicase}}{{anchor|helicases}} <dd>Any of a class of {{gli|adenosine triphosphate|ATP}}-dependent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motor proteins}} that move directionally along the {{gli|DNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester backbone|backbone}} and catalyze the separation of the two complementary {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strands}} of {{gli|double-stranded}} molecules, permitting a wide variety of vital processes to take place, e.g. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}, {{gli|DNA replication|replication}}, and {{gli|DNA repair|repair}}.<ref name="Rieger"/></dd>

{{term|hemizygous}} <dd>In a {{gli|diploid}} organism, having just one {{gli|allele}} at a given {{gli|locus|genetic locus}} (where there would ordinarily be two). Hemizygosity may be observed when only one copy of a {{gli|chromosome}} is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele is {{gli|deletion|deleted}}, or when a gene is located on a {{gli|allosome|sex chromosome}} in the heterogametic sex (in which the sex chromosomes do not exist in matching pairs); for example, in human males with normal chromosomes, almost all {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|X-linked}} genes are said to be hemizygous because there is only one {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|X chromosome}} and few of the same genes exist on the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Y chromosome}}.</dd>

{{term|heredity}}{{anchor|inheritance|inherit|inherits|inherited}} {{ghat|Also '''inheritance'''.}} <dd>The storage, transfer, and expression of molecular information in biological organisms,<ref name="Rieger"/> as manifested by the passing on of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotypic traits}} from parents to their {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|offspring}}, either through sexual or asexual reproduction. Offspring cells or organisms are said to ''inherit'' the genetic information of their parents.</dd>

{{term|heterochromatin}}{{anchor|heterochromatic}} <dd>A compact, highly condensed form of {{gli|chromatin}} characterized chiefly by the close spatial proximity of adjacent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosomes}} and the consequent inaccessibility of intervening DNA sequences to {{gli|DNA-binding proteins}}, which contrasts with the more open and accessible form known as {{gli|euchromatin}}. The transcription of genes located within heterochromatic regions of chromosomes is therefore relatively limited, and so the formation of heterochromatin at specific loci is an important means of regulating {{gli|gene expression}}. Establishment of heterochromatin is associated with the {{gli|histone modification|modification}} of specific residues within specific {{gli|histones}}, such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|methylation}} of the ninth lysine residue of histone H3 (H3K9); the presence of these modifications at a specific locus signals the recruitment of other proteins which cause local DNA condensation. Many repetitive and structurally important regions of chromosomes are nearly always compacted in so-called {{gli|constitutive heterochromatin}}, while the compaction of {{gli|facultative heterochromatin}} is more temporary.</dd>

{{term|heterochromosome}} <dd>See ''{{gli|allosome}}''.</dd>

{{term|heterodimer}}{{anchor|heterodimers}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} or protein {{gli|domain}} composed of two different {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptides}} which are {{gli|dimer|paired with each other}} in the quaternary structure of a multimeric complex.<ref name="DoG7"/> Contrast ''{{gli|homodimer}}''.</dd>

{{term|heterogeneous nuclear RNA (hnRNA)}}{{anchor|hnRNA|heterogeneous nuclear RNA}} {{ghat|Also '''H-RNA'''.}} <dd>Any of a set of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules of widely variable size occurring in the nucleus and united by their rapid turnover rate during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein synthesis}}. HnRNA represents all of the various transcriptional products of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein-coding genes}}, including both of {{gli|exons}} and {{gli|introns}}, from raw, unprocessed {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primary transcripts}} to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA splicing|spliced}}, {{gli|5' cap|capped}}, and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polyadenylated}} mature {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|messenger RNAs}} and all of the intermediate forms in between.<ref name="Oxford B&MB"/></dd>

{{term|heterokaryon}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|multinucleate}} cell containing {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclei}} with different genotypes, resulting from the {{gli|cell fusion|fusion}} of two or more genetically distinct cells, either naturally (e.g. in certain types of sexual reproduction) or artificially (e.g. in genetic engineering).<ref name="MacLean"/></dd>

{{term|heterologous expression}} <dd>The {{gli|expression}} of a foreign {{gli|gene}} or any other foreign DNA sequence within a host organism which does not naturally contain the same gene. Insertion of foreign {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transgenes}} into heterologous hosts using {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recombinant DNA|recombinant}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vectors}} is a common biotechnology method for studying gene structure and function.</dd>

{{term|high-throughput}} <dd>Describing a method or system capable of {{gli|bioassay|assaying}} very large numbers of samples or of processing very large quantities of data extremely rapidly, generally by utilizing automation and miniaturization to greatly increase speed and efficiency. For example, ''high-throughput sequencing'' refers to modern {{gli|DNA sequencing}} technologies that can produce sequence reads for hundreds of millions of DNA fragments simultaneously, allowing scientists to {{gli|whole genome sequencing|sequence entire genomes}} quickly and inexpensively.<ref>{{cite web |last1=Cabuzu |first1=Daniela |title=What does 'high-throughput' mean in sequencing? |url=https://alitheagenomics.com/blog/what-does-high-throughput-mean-in-sequencing#:~:text=Written%20by%20Daniela%20Cabuzu,means%20in%20sequencing%20methods%20today. |website=Alithea Genomics}}</ref></dd>

{{term|histology}}{{anchor|histological}} <dd>The study or analysis of the microscopic anatomy of biological {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissues}} or of {{gli|cells}} within tissues, particularly by making use of specialized techniques to distinguish structures and functions based on visual morphology and differential staining. In practice the term is sometimes used more broadly to include {{gli|cytology}}.</dd>

{{term|histone}}{{anchor|histones}} <dd>Any of a class of highly alkaline {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} responsible for {{gli|DNA condensation|packaging}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear}} DNA into structural units called {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosomes}} in eukaryotic cells. Histones are the chief protein components of {{gli|chromatin}}, where they associate into {{gli|histone core|eight-membered complexes}} which act as "spools" around which the linear DNA molecule winds. They play a major role in {{gli|gene regulation}} and {{gli|expression}}.</dd>

{{term|histone core}} {{ghat|Also '''histone octamer''' and '''core particle'''.}} <dd>The complex of eight {{gli|histone}} proteins around which double-stranded DNA wraps within a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosome}}. The canonical histone octamer consists of two each of histones H2A, H2B, H3, and H4, which pair with each other symmetrically to form a ball-shaped cluster around which DNA winds through interactions with the histones' surface {{gli|domains}}, though {{gli|histone variant|variant histones}} may replace their analogues in certain contexts.</dd>

{{term|histone modification}}{{anchor|histone modifications}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-translational modification}} of {{gli|histone}} proteins by the chemical attachment of various molecules or functional groups to specific amino acid residues. Because histones form the {{gli|histone core|core}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosomes}}, the modification of exposed parts of their polypeptide chains is used to regulate gene expression by marking them with {{gli|labelling|molecular labels}} that signal the recruitment of other proteins to induce conformational changes that variously widen or condense the spacing of nucleosomes along strands of DNA, thereby changing the accessibility of nearby DNA sequences to transcriptional machinery. Histones are modified by many different labels, most commonly {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|methylation}}, {{gli|acetylation}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ubiquitination}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphorylation}}, and {{gli|citrullination}}.</dd>

{{term|histone variant}} <dd></dd>

{{term|hnRNA}} <dd>See ''{{gli|heterogeneous nuclear RNA}}''.</dd>

{{term|holocentric}} <dd>(of a linear {{gli|chromosome}} or chromosome fragment) Having no single {{gli|centromere}} but rather multiple {{gli|kinetochore}} assembly sites dispersed along the entire length of the chromosome. During cell division, the {{gli|chromatids}} of holocentric chromosomes move apart in parallel and do not form the classical V-shaped structures typical of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|monocentric}} chromosomes.</dd>

{{term|homeobox}}{{anchor|homeoboxes}} <dd>Any of a class of DNA {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequences}} approximately 180 base pairs in length occurring near the {{gli|3'‐end}} of certain eukaryotic genes and encoding a 60-amino acid domain, known as a {{gli|homeodomain}}, which is capable of {{gli|DNA-binding domain|binding to DNA or RNA}} via a characteristic helix-turn-helix motif. Homeobox-containing genes are translated into homeodomain-containing proteins, which commonly {{gli|regulate}} transcription or translation by binding to other genes or messenger RNAs containing {{gli|homeobox responsive elements}}. The products of many homeotic genes, exemplified by the {{gli|Hox genes}}, are of critical importance in developmental pathways.<ref name="DoG7"/></dd>

{{term|homeobox responsive element (HRE)}}{{anchor|homeodomain responsive element|homeobox responsive elements}} {{ghat|Also '''homeodomain responsive element'''.}} <dd>Any DNA or RNA {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequence}} that is specifically {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recognition sequence|recognized}} and bound by the {{gli|homeodomain}} of a homeodomain-containing protein.</dd>

{{term|homeodomain}}{{anchor|homeodomains}} <dd>A {{gli|DNA-binding domain|nucleic acid-binding domain}}, typically 60 {{gli|amino acids}} in length, found near the {{gli|C-terminus}} of certain eukaryotic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, characterized by a highly {{gli|conserved}} helix-turn-helix {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motif}} that binds with strong affinity to the backbone of specific {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recognition sequences}} in DNA or RNA molecules. A protein may have one or more homeodomains, each of which is specific to a different recognition sequence. Many homeodomain-containing proteins function as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}} by binding to sequences within {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoters}} and blocking or recruiting other proteins, such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA polymerase}} or {{gli|cofactors}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription initiation complex}}. Homeodomains are the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translated}} versions of {{gli|homeoboxes}}, though the terms are often used interchangeably.</dd>

{{term|homodimer}}{{anchor|homodimers}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} or protein {{gli|domain}} composed of two identical {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptides}} which are {{gli|dimer|paired with each other}} in the quaternary structure of a multimeric complex.<ref name="DoG7"/> Contrast ''{{gli|heterodimer}}''.</dd>

{{term|homologous chromosomes}}{{anchor|homologous chromosome|homolog|homologs}} {{ghat|Also '''homologs''' or '''homologues'''.}} <dd>A set of two matching {{gli|chromosomes}}, one maternal and one paternal, which pair up with each other inside the nucleus during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}. They have the same {{gli|genes}} at the same {{gli|loci}}, but may have different {{gli|alleles}}.</dd>

{{term|homologous recombination}} <dd>A type of {{gli|genetic recombination}} in which nucleotide sequences are exchanged between two similar or identical ("homologous") molecules of {{gli|DNA}}, especially that which occurs between {{gli|homologous chromosomes}}. The term may refer to the recombination that occurs as a part of any of a number of distinct cellular processes, most commonly {{gli|DNA repair}} or {{gli|chromosomal crossover}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}} in eukaryotes and {{gli|horizontal gene transfer}} in prokaryotes. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nonhomologous recombination}}''.</dd>

{{term|horizontal gene transfer (HGT)}} {{ghat|Also '''lateral gene transfer (LGT)'''.}} <dd>Any process by which genetic material is transferred between unicellular and/or multicellular organisms other than by vertical transmission from parent to offspring, e.g. bacterial conjugation.</dd>

{{term|housekeeping gene}}{{anchor|housekeeping genes}} <dd>Any {{gli|constitutive expression|constitutive gene}} that is {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcribed}} at a relatively constant level across many or all known conditions and cell types. The {{gli|gene product|products}} of housekeeping genes typically play critical roles in the maintenance of cellular integrity and basic metabolic function. It is generally assumed that their expression is unaffected by experimental or pathological conditions.</dd>

{{term|Hox genes}} <dd>A subset of highly {{gli|conserved}} {{gli|homeobox}}-containing {{gli|genes}} whose protein products function as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}} essential for the proper organization of the body plan in developing animal {{gli|embryos}}, ensuring that the correct structures are formed in the correct places. Hox genes are usually arranged on a chromosome in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tandem}} arrays and are {{gli|expressed}} sequentially during development, with the sequence of gene activation corresponding to their physical arrangement within the genome and/or the physical layout of the tissues in which they are expressed along the organism's anterior–posterior axis.<ref name="DoG7"/></dd>

{{term|Human Genome Project (HGP)}} <dd>A collaborative international scientific research project with the goal of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sequencing}} all of the {{gli|genomic DNA|chromosomal DNA}} and identifying and {{gli|gene mapping|mapping}} all of the {{gli|genes}} within human cells, and ultimately of assembling a complete {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reference genome}} for the human species. The project was launched in 1990 by a consortium of federal agencies, universities, and research institutions and was declared complete in 2003. Because each individual human being has a unique genome, the finished reference genome is a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mosaic}} of sequences obtained by sampling DNA from thousands of individuals across the world and does not represent any one individual.</dd>

{{term|hyaloplasm}} <dd>See ''{{gli|cytosol}}''.</dd>

{{term|hybrid}}{{anchor|hybrids}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|offspring}} that results from combining the qualities of two organisms of different genera, species, breeds, or varieties through sexual reproduction. Hybrids may occur naturally or artificially, as during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|selective breeding}} of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result in speciation.</dd>

{{term|hybridization}} <dd>1.  The process by which a {{gli|hybrid}} organism is produced from two organisms of different genera, species, breeds, or varieties.</dd> <dd>2.  The process by which two or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecules with {{gli|complementary}} nucleotide sequences {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pair}} with each other in solution, creating {{gli|double-stranded}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|triple-stranded}} molecules via the formation of hydrogen bonds between the complementary nucleobases of each strand. In certain laboratory contexts, especially ones in which long strands hybridize with short {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligonucleotide}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primers}}, hybridization is often referred to as {{gli|annealing}}.</dd> <dd>3.  A step in some experimental assays in which a single-stranded DNA or RNA preparation is added to an array surface and anneals to a {{gli|complementary}} {{gli|hybridization probe}}.</dd>

{{term|hybridization probe}}{{anchor|hybridization probes}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} {{gli|DNA}} or {{gli|RNA}} fragment (or a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid analogue}}) which is artificially {{gli|labelled}} with a radioactive or fluorescent compound or some other detectable marker and then allowed to {{gli|hybridize}} with {{gli|complementary}} DNA or RNA sequences in order to detect the presence of those complements in a heterogeneous sample or their specific ''{{gli|in situ}}'' {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|localization}}; or an {{gli|assay}} in which this procedure is performed. As with antibodies in {{gli|immunostaining}}, nucleic acid probes bind with high specificity to their target sequences, permitting visualization of the targets, if present, against a non-specific background, whether in a {{gli|blot|membrane blot}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microarray}} or even ''{{gli|in vivo}}''. A unique advantage of hybridization probes is that the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stringency}} of the hybridization reaction is easily modifiable by changing the temperature and salt concentration, making it possible for the same probe to bind to sequences with differing degrees of complementarity. Hybridization probes are employed in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Southern blotting}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|northern blotting}} and as part of many other laboratory methods. See also ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|probe}}''.</dd>

{{term|hydrophilic}} <dd>Soluble in or having an affinity for water or other polar compounds; describing a polar molecule, or a moiety or functional group within a molecule, which participates in intermolecular interactions such as hydrogen bonding with other polar molecules and therefore readily dissolves in polar solvents such as water or aqueous solutions.<ref name="Alberts et al."/> Unlike {{gli|hydrophobic}} compounds, hydrophilic compounds can form energetically favorable contacts with the aqueous phase of biological fluids and so can often be suspended directly in the {{gli|cytosol}} or exposed to extracellular spaces.<ref name="Lackie"/> Together, the contrasting properties of hydrophilicity and hydrophobicity play major roles in determining the structural {{gli|conformations}} and functions of most {{gli|biomolecules}}.</dd>

{{term|hydrophobic}} {{ghat|Sometimes used interchangeably with '''lipophilic'''.}} <dd>Having a low solubility in or affinity for water or other polar solvents; describing a non-polar molecule, or a moiety or functional group within a molecule, which cannot form energetically favorable interactions with polar compounds and which therefore tends to "avoid" or be repulsed by such compounds, instead clustering together with other hydrophobic molecules or arranging itself in a way that minimizes its exposure to its polar surroundings. This phenomenon is not so much due to the affinity of the hydrophobic molecules for each other as it is a consequence of the strong intermolecular forces that allow polar compounds such as water molecules to bond with each other; hydrophobic species are unable to form alternative bonds of equivalent strength with the polar compounds, hence they tend to be excluded from aqueous solutions by the tendency of the polar solvent to maximize interactions with itself. Hydrophobicity is a major determinant of countless chemical interactions in biological systems, including the spatial {{gli|conformations}} assumed by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|macromolecules}} such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} and {{gli|lipids}}, the binding of {{gli|ligands}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substrates}} to proteins, and the structure and properties of lipid {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membranes}}.<ref name="MacLean"/><ref name="Alberts et al."/> Contrast ''{{gli|hydrophilic}}''.</dd>

{{term|hypertonic}} <dd>Describing a solution containing a high concentration of dissolved solutes relative to another solution, i.e. having positive osmotic pressure, such that solvent will tend to move by osmosis across a semipermeable membrane from the solution of lower solute concentration to the solution of higher concentration until both solutions have equal concentrations. In a cell where the intracellular {{gli|cytosol}} is hypertonic relative to the surrounding {{gli|extracellular fluid}} (which by definition is {{gli|hypotonic}} relative to the cytosol), the solvent (water) will flow across the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} into the cytosol, filling the cell with extra water and diluting its contents until both sides of the membrane are {{gli|isotonic}}. Cells placed in severely hypotonic environments may be at risk of bursting due to the sudden inflow.</dd>

{{term|hypomorph}} <dd>A mutant {{gli|allele}} that permits a subnormal expression of the gene's normal phenotype, e.g. by encoding an unstable enzyme which degrades too quickly to fully serve its function but which nevertheless is functional in some limited capacity, being generated in quantities sufficient for its reaction to proceed slowly or at low levels.<ref name="DoG7"/></dd>

{{term|hypotonic}} <dd>Describing a solution containing a low concentration of dissolved solutes relative to another solution, i.e. having negative osmotic pressure, such that solvent will tend to move by osmosis across a semipermeable membrane from the solution of lower solute concentration to the solution of higher concentration until both solutions have equal concentrations. In a cell where the intracellular {{gli|cytosol}} is hypotonic relative to the surrounding {{gli|extracellular fluid}} (which by definition is {{gli|hypertonic}} relative to the cytosol), the solvent (water) will flow across the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} out of the cytosol, causing the cell to lose water until both sides of the membrane are {{gli|isotonic}}. Cells placed in severely hypertonic environments may be at risk of shriveling and desiccating due to the sudden outflow.</dd>

{{term|term=hypoxanthine|content=hypoxanthine ({{font|I|font=courier|size=large}})}} <dd>A naturally occurring, non-canonical {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|purine}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase}} that is used in some {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules and pairs with standard nucleobases in a phenomenon known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|wobble base pairing}}. Its {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside}} form is known as {{gli|inosine}}, which is the reason it is commonly abbreviated with the letter {{font|I|font=courier|size=big}} in sequence reads.</dd>

==I== {{glossary}} {{term|idiochromosome}} <dd>See ''{{gli|allosome}}''.</dd>

{{term|idiogram}} {{ghat|Also '''ideogram'''.}} <dd>A diagrammatic or schematic {{gli|karyotype}} of the entire set of {{gli|chromosomes}} within a cell or genome, in which annotated illustrations depict each chromosome in its most idealized form (e.g. with straight lines and obvious {{gli|centromeres}}) so as to facilitate the easy identification of sequences, structural features, and physical distances, which may be less apparent in photomicrographs of the actual chromosomes.</dd>

{{term|idiomere}} <dd>See ''{{gli|chromomere}}''.</dd>

{{term|immortalization}}{{anchor|immortalize|immortalizes|immortalizing|immortalized|immortalized cell|immortalized cells|immortalized cell line|immortalized cell lines}} <dd>The natural or artificial changing of a {{gli|cell line|cell population}} with a normally finite lifespan into one with a hypothetically infinite lifespan, capable of dividing indefinitely without {{gli|cellular senescence}} as long as essential nutrients are available and conditions are conducive for {{gli|cell division}}. Cells that undergo such a change are said to be ''immortalized''. Mutations that cause immortalization occur naturally in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|neoplasms}} that cause cancer but can also be induced artificially, which makes it possible to {{gli|culture}} certain cell lines ''{{gli|in vitro}}'' for prolonged periods. Immortalized cell lines are thus broadly useful for experimental purposes and in many biotechnology applications. Immortalized eukaryotic cells are commonly obtained by isolating them from a naturally occurring neoplasm (as with the human HeLa cell line), or may be generated from normal cells by introducing viral genes (as with HEK 293 cells), by artificially {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|overexpressing}} proteins required for immortality such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|telomerase}}, or by {{gli|cell fusion|fusing}} normal cells with cancer cells (as in the {{gli|hybridoma}} technologies used in the commercial production of {{gli|antibodies}}).<ref name="Oxford B&MB"/> Though {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stem cells}} are also capable of continuous self-renewal and are thus technically 'immortal', their immortalization is not abnormal because they are an ordinary part of the development of multicellular organisms.</dd>

{{term|immunofluorescence (IF)}}{{anchor|immunofluorescence}} <dd>A family of laboratory techniques in which a particular {{gli|antigen}} or {{gli|antibody}} is conjugated to a fluorescent dye and then allowed to bind specifically to its complementary antibody or antigen, if any exists, in a {{gli|cell culture|culture vessel}}, tissue section or smear, {{gli|hybridization probe}}, membrane {{gli|blot}}, or any other context. The presence or absence of the complement and its specific location(s) can be visualized by illuminating the sample with ultraviolet light and observing the fluorescence from the conjugated fluorophore, often under a microscope.<ref name="Oxford B&MB"/></dd>

{{term|immunogenic}} <dd>Capable of provoking or inducing an immune response, as with an {{gli|antigen}} or a vaccine.</dd>

{{term|immunohistochemistry (IHC)}}{{anchor|immunohistochemistry}} <dd>A branch of {{gli|histochemistry}} which makes use of {{gli|antibodies}} {{gli|conjugated}} to some kind of {{gli|labelling|molecular label}} in order to detect the presence or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|subcellular localization|localization}} of complementary antigenic structures in tissue samples.<ref name="Oxford B&MB"/> See also ''{{gli|immunostaining}}''.</dd>

{{term|immunoprecipitation (IP)}}{{anchor|immunoprecipitation}} <dd></dd>

{{term|immunostaining}} <dd>The use of an {{gli|antibody}} {{gli|conjugated}} to a {{gli|chromophore}} or {{gli|fluorophore}} to bind a specific {{gli|antigen}} within a target substance (e.g. a protein) and thereby make the substance visible amidst a background of non-specific substances, allowing for detection of the target in a biological sample. The term originally referred to antibody-based staining of tissue sections with strong dyes or colorants, known as {{gli|immunohistochemistry}}, but in modern usage encompasses a much broader range of laboratory methods united by their use of antibodies to {{gli|label}} specific biomolecules with visually conspicuous compounds.</dd>

{{term|''in silico''}} <dd>(of a scientific experiment or research) Conducted, produced, or analyzed by means of computer modeling or simulation, as opposed to a real-world trial.</dd>

{{term|''in situ''}} <dd>(of a scientific experiment or biological process) Occurring or made to occur in a natural, uncontrolled setting, or in the natural or original position or place, as opposed to in a foreign cell or tissue type or {{gli|in vitro|in an artificial environment}}.</dd>

{{term|''in situ'' hybridization (ISH)}}{{anchor|in situ hybridization|''in situ'' hybridization}} <dd>A {{gli|hybridization probe}} assay in which a {{gli|labeled}}, single-stranded DNA or RNA molecule or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid analogue}} containing a sequence that is {{gli|complementary}} to a particular DNA or RNA sequence is allowed to {{gli|hybridize}} with its complement ''{{gli|in situ}}'', i.e. in its natural context, such as within cells or tissue sections (as opposed to within homogeneous samples extracted from cells or tissues, where cellular or histological structure has been lost in the process of obtaining the sample), in order to {{gli|localization|reveal the precise location}} of the complementary sequence within this context. The label may be a radioactive compound, {{gli|fluorophore|fluorescent molecule}}, or {{gli|hapten}}, permitting detection by a variety of visualization techniques. ''In situ'' hybridization is commonly used to identify the physical locations of specific DNA sequences such as genes and regulatory elements on {{gli|chromosomes}}, which can provide insight into chromosomal structure and integrity; to determine the subcellular locations where various types of RNA accumulate and interact with other molecules; and to visualize the tissues and organs within an organism where specific genes are {{gli|expressed}} at various developmental stages (by probing for the genes' RNA transcripts).</dd>

{{term|''in vitro''}} <dd>(of a scientific experiment or biological process) Occurring or made to occur in a laboratory vessel or other controlled artificial environment, e.g. in a test tube or a petri dish, as opposed to {{gli|in vivo|inside a living organism}} or {{gli|in situ|in a natural setting}}.</dd>

{{term|''in vivo''}} <dd>(of a scientific experiment or biological process) Occurring or made to occur inside the cells or tissues of a living organism; or, in the broadest sense, in any natural, unmanipulated setting. Contrast ''{{gli|ex vivo}}'' and ''{{gli|in vitro}}''.</dd>

{{term|indel}}{{anchor|indels}} <dd>A term referring to either an {{gli|insertion|'''in'''sertion}} or a {{gli|deletion|'''del'''etion}} of one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase|bases}} in a nucleic acid sequence.</dd>

{{term|inducer}}{{anchor|inducers}} <dd>A protein that binds to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repressor}} (to disable it) or to an {{gli|activator}} (to enable it).</dd>

{{term|inducible gene}} <dd>A {{gli|gene}} whose {{gli|expression}} is either responsive to environmental change or dependent on its host cell's position within the cell cycle.</dd>

{{term|in-frame}} <dd>1.  (of a gene or sequence) Read or transcribed in the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reading frame}} as another gene or sequence; not requiring a shift in reading frame to be intelligible or to result in a functional {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptide}}.</dd> <dd>2.  (of a mutation) Not causing a {{gli|frameshift mutation|frameshift}}.<ref name="DoG7"/></dd>

{{term|inheritance}} <dd>See ''{{gli|heredity}}''.</dd>

{{term|initiation codon}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|start codon}}''.</dd>

{{term|initiation factor (IF)}}{{anchor|initiation factor|initiation factors}} <dd>Any of various {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} which bind to the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|small subunit|small}} or {{gli|large subunit}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosomes}} during the initiation of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}} and thereby play roles in regulating when and how protein synthesis occurs. Initiation factors are essential for assembly of the initiation complex and for {{gli|charged tRNA|charged}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transfer RNAs}} to properly associate with the ribosome and the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|messenger RNA}}. They are frequent targets of {{gli|activators}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repressors}} which can respectively increase or decrease the rate of translation. Though their functions are largely conserved, they are distinguished by the taxonomic domain in which they occur: bacterial initiation factors (IFs), archaeal initiation factors (aIFs), and eukaryotic initiation factors (eIFs).</dd>

{{term|term=inosine|content=inosine ({{font|I|font=courier|size=large}}, Ino)}} <dd>A naturally occurring, non-canonical {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoside}} consisting of a {{gli|hypoxanthine}} {{gli|base}} with its N9 nitrogen {{gli|glycosidic bond|bonded}} to the C1 carbon of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribose}} sugar. Inosine may be incorporated into certain {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} molecules such as the {{gli|anticodons}} of some {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transfer RNAs}}, and occurs as an intermediate in the breakdown of {{gli|adenosine}} to uric acid and in the recycling of adenosine by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|salvage pathways}}.<ref name="Oxford B&MB"/></dd>

{{term|insertion}}{{anchor|insertional|inserted|inserting}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} in which one or more {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|bases}} are added to a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence}}. Contrast ''{{gli|deletion}}''.</dd>

{{term|insertion sequence (IS)}}{{anchor|insertion sequences|insert|inserts}} {{ghat|Also '''insertion element''' or simply '''insert'''.}} <dd>Any nucleotide sequence that is {{gli|insertion|inserted}} naturally or artificially into another sequence. The term is used in particular to refer to the part of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transposable element}} that codes for those proteins directly involved in the transposition process, e.g. the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transposase}} enzyme. The coding region in a transposable insertion sequence is usually flanked by short {{gli|inverted repeats}}, and the structure of larger transposable elements may include a pair of flanking insertion sequences which are themselves inverted.</dd>

{{term|insertional mutagenesis}} <dd>The alteration of a DNA sequence by the {{gli|insertion}} of one or more nucleotides into the sequence, either naturally or artificially. Depending on the precise location of the insertion within the target sequence, insertions may partially or totally inactivate or even upregulate a {{gli|gene product}} or biochemical pathway, or they may be {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|neutral mutation|neutral}}, leading to no substantive changes at all. Many {{gli|genetic engineering}} techniques rely on the insertion of exogenous genetic material into host cells in order to study gene function and expression.<ref name="DoG7"/></dd>

{{term|insulator}} <dd>A specific DNA sequence that prevents a gene from being influenced by the {{gli|activation}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|repression}} of nearby genes.</dd>

{{term|integral membrane protein (IMP)}}{{anchor|integral membrane protein|integral membrane proteins}} {{ghat|Also '''intrinsic membrane protein'''.}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane proteins}} which are permanently embedded within or attached to the {{gli|cell membrane}} (as opposed to those which are {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peripheral membrane protein|attached only temporarily}}). Integral membrane proteins can be subclassified into {{gli|integral polytopic proteins}}, which span the entirety of the membrane, and {{gli|integral monotopic proteins}}, which adhere only to one side.</dd>

{{term|integral monotopic protein}}{{anchor|integral monotopic proteins}} <dd>Any of a class of {{gli|integral membrane proteins}} which are permanently attached to one side of the {{gli|cell membrane}} by any means but which do not completely span the membrane. Contrast ''{{gli|integral polytopic protein}}''.</dd>

{{term|integral polytopic protein}}{{anchor|integral polytopic proteins|transmembrane protein|transmembrane proteins}} {{ghat|Also '''transmembrane protein'''.}} <dd>Any of a class of {{gli|integral membrane proteins}} which span the entirety of the {{gli|cell membrane}}, extending from the interior or {{gli|cytosolic}} side of the membrane to the exterior or {{gli|extracellular}} side. Transmembrane proteins typically have hydrophilic {{gli|domains}} exposed to each side as well as one or more hydrophobic domains crossing the nonpolar space inside the {{gli|lipid bilayer}}, by which they are further classified as ''single-pass'' or ''multipass'' membrane proteins. As such many transmembrane proteins function as {{gli|channel proteins|gated channels}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transporters}} to permit or prohibit the movement of specific molecules or ions across the membrane, often undergoing conformational changes in the process, or as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|receptors}} in {{gli|cell signaling}} pathways. Contrast ''{{gli|integral monotopic protein}}''.</dd>

{{term|integration}} <dd></dd>

{{term|integron}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mobile genetic element}} consisting of a {{gli|gene cassette}} containing the gene for a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|site-specific recombination|site-specific recombinase}}, {{gli|integrase}}-specific recognition sites, and a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|promoter}} that governs the expression of one or more genes conferring adaptive traits on the host cell. Integrons usually exist in the form of circular {{gli|episomal}} DNA fragments, through which they facilitate the rapid adaptation of bacteria by enabling {{gli|horizontal gene transfer}} of antibiotic resistance genes between different bacterial species.<ref name="DoG7"/></dd>

{{term|intein}} <dd>Any sequence of one or more {{gli|amino acids}} within a precursor {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polypeptide}} that is excised by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein splicing}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-translational modification}} and is therefore absent from the mature protein, analogous to the {{gli|introns}} spliced out of RNA transcripts.<ref name="DoG7"/> Contrast ''{{gli|extein}}''.</dd>

{{term|intercalating agent}} <dd>Any chemical compound (e.g. ethidium bromide) that disrupts the alignment and {{gli|base pair|pairing of bases}} in the complementary strands of a {{gli|DNA}} molecule by {{gli|intercalation|inserting itself between the bases}}.<ref name="CoG"/></dd>

{{term|intercalation}}{{anchor|intercalate|intercalates|intercalated}} <dd>The insertion, naturally or artificially, of chemical compounds between the planar {{gli|bases}} of a {{gli|DNA}} molecule, which generally disrupts the hydrogen bonding necessary for {{gli|base pairing}}.</dd> [[File:Doxorubicin–DNA complex 1D12.png|thumb|350px|Two molecules of the chemotherapeutic drug doxorubicin '''{{gli|intercalated}}''' between the bases of a DNA molecule]]

{{term|intercellular}} <dd>Between two or more {{gli|cells}}. Contrast ''{{gli|intracellular}}''; see also ''{{gli|extracellular}}''.</dd>

{{term|intercistronic region}} <dd>Any DNA sequence that is located between the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|stop codon}} of one {{gli|gene}} and the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|start codon}} of the following gene in a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polycistronic}} transcription unit.<ref name="DoG7"/> See also ''{{gli|intergenic region}}''.</dd>

{{term|intercross}} <dd>A {{gli|cross}} in which both the male and female parents are {{gli|heterozygous}} at a particular {{gli|locus}}.<ref name="DoG7"/></dd>

{{term|intergenic region (IGR)}} <dd>Any sequence of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ncDNA|non-coding DNA}} that is located between functional {{gli|genes}}.</dd>

{{term|intergenic spacer (IGS)}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spacer}}''.</dd>

{{term|interkinesis}} {{ghat|Also '''interphase II'''.}} <dd>The abbreviated pause in activities related to cell division that occurs during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}} in some species, between the first and second meiotic divisions (i.e. meiosis I and meiosis II). No {{gli|DNA replication}} occurs during interkinesis, unlike during the normal {{gli|interphase}} that precedes meiosis I and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}}.<ref name="DoG7"/></dd>

{{term|internal ribosome entry site (IRES)}} <dd>A sequence present in some {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|messenger RNAs}} that permits recognition by the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosome}} and thus the initiation of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}} even in the absence of a {{gli|5' cap}}, which in eukaryotes is otherwise required for assembly of the initiation complex. IRES elements are often located in the {{gli|5' untranslated region}}, but may also be found in other positions.</dd>

{{term|International Union of Biochemistry and Molecular Biology (IUBMB)}}{{anchor|International Union of Biochemistry and Molecular Biology}} <dd>An international non-governmental organization devoted to promoting scientific research and education in the disciplines of {{gli|biochemistry}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular biology}}, primarily by standardizing biochemical nomenclature, developing and publishing laboratory methods, and awarding grants and fellowships to students and researchers.</dd>

{{term|interphase}} <dd>All stages of the {{gli|cell cycle}} excluding {{gli|cell division}}. A typical cell spends most of its life in interphase, during which it conducts everyday {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolism|metabolic activities}} as well as the complete {{gli|DNA replication|replication}} of its genome in preparation for {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}.</dd>

{{term|intracellular}} <dd>Within a {{gli|cell}} or cells; i.e. inside the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}}. Contrast ''{{gli|intercellular}}'' and ''{{gli|extracellular}}''.</dd>

{{term|intracrine}} <dd></dd>

{{term|intragenic region}} <dd>See ''{{gli|intron}}''.</dd>

{{term|intragenic suppression}} <dd></dd>

{{term|intrinsic membrane protein}} <dd>See ''{{gli|integral membrane protein}}''.</dd>

{{term|intrinsically disordered protein (IDP)}}{{anchor|intrinsically disordered protein|intrinsically disordered proteins}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} (or a region or {{gli|domain}} within a protein) that lacks any distinct, fixed three-dimensional structure or organization under physiological conditions, instead changing continuously and randomly between multiple transient {{gli|conformation|conformational states}} rather than folding into any one stable conformation, especially in the absence of specific macromolecular interaction partners. The majority of eukaryotic proteins contain domains with intrinsic structure alongside unstructured domains. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|peptide sequence|Peptide sequences}} lacking intrinsic order are generally characterized by high proportions of charged and hydrophilic amino acids and low proportions of hydrophobic amino acids, making them inherently flexible, accessible, and modifiable, which allows the same peptide sequence to have distinct functions across a wide variety of biochemical circumstances. They are frequently enriched in {{gli|binding site|binding motifs}} and are common targets of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-translational modifications}}, giving them important roles in {{gli|cell signaling}} pathways and as hubs in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein complexes}}.<ref>{{cite journal|author=Wright, P., Dyson, H.|title=Intrinsically disordered proteins in cellular signalling and regulation|journal=Nat Rev Mol Cell Biol|volume=16|pages=18–29|date=2015|issue=1 |doi=10.1038/nrm3920 |pmid=25531225 |pmc=4405151 |bibcode=2015NRMCB..16...18W }}</ref></dd>

{{term|intron}}{{anchor|introns|intronic}} {{ghat|Also '''intragenic region'''.}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence|nucleotide sequence}} within a functional {{gli|gene}} that is removed by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA splicing}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-transcriptional modification}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primary transcript}} and is therefore absent from the final mature mRNA. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts. Contrast ''{{gli|exon}}''.</dd>

{{term|intron-mediated recombination}} <dd>See ''{{gli|exon shuffling}}''.</dd>

{{term|intronic gene}} <dd>A {{gli|gene}} whose DNA sequence is nested within an {{gli|intron}} of another gene and hence surrounded by non-coding intronic sequences.<ref name="Rieger"/></dd>

{{term|invagination}} <dd>The infolding of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane}} toward the interior of a cell or organelle, or of a sheet of cells toward the interior of a developing {{gli|embryo}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tissue}}, or organ, forming a distinct membrane-lined pocket. In the case of individual cells, the invaginated pocket may proceed to separate from the source membrane entirely, creating a membrane-bound {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicle}} within the cell, as in {{gli|endocytosis}}.<ref name="MacLean"/></dd>

{{term|inverted repeat}}{{anchor|inverted repeats}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotide sequence}} followed {{gli|downstream}} on the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} by its own {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|reverse complement}}. The initial sequence and the reverse complement may be separated by any number of nucleotides, or may be immediately adjacent to each other; in the latter case, the composite sequence is also called a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|palindromic sequence}}. Inverted repeats are {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|self-complementary}} by definition, a property which involves them in many biological functions and dysfunctions. Contrast ''{{gli|direct repeat}}''.</dd>

{{term|ion channel}}{{anchor|ion channels}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transmembrane protein}} complex which forms an {{gli|channel protein|aqueous pore or channel}} spanning the {{gli|lipid bilayer}} of a membrane, through which specific inorganic, electrically charged ions can diffuse down their electrochemical gradients.<ref name="Alberts et al."/></dd>

{{term|ionophore}}{{anchor|ionophores}} <dd>Any chemical compound or macromolecule that facilitates the movement of ions across biological membranes, or more specifically, any chemical species that reversibly binds electrically charged atoms or molecules. Many ionophores are lipid-soluble {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}} that catalyze the transport of monovalent and divalent cations across the hydrophobic {{gli|lipid bilayers}} surrounding cells and vesicles.<ref name="MacLean"/></dd>

{{term|isochore}}{{anchor|isochores}} <dd>A large region of {{gli|genomic DNA}} with a relatively homogeneous composition of {{gli|base pairs}}, distinguished from other regions by the proportion of pairs that are {{font|{{gli|guanine|G}}|font=courier|size=big}}-{{font|{{gli|cytosine|C}}|font=courier|size=big}} or {{font|{{gli|adenine|A}}|font=courier|size=big}}-{{font|{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|thymine|T}}|font=courier|size=big}}. The genomes of most plants and vertebrates are composed of different classes of GC-rich and AT-rich isochores.<ref name="DoG7"/></dd>

{{term|isochromosome}} <dd>A type of abnormal {{gli|chromosome}} in which the arms of the chromosome are mirror images of each other. Isochromosome formation is equivalent to simultaneous {{gli|duplication}} and {{gli|deletion}} events such that two copies of either the {{gli|long arm}} or the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|short arm}} comprise the resulting chromosome.</dd>

{{term|term=isoelectric point (pH(I), pI)}}{{anchor|isoelectric point}} {{ghat|Also '''isoelectric pH'''.}} <dd>The pH at which a particular molecule, often a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}}, carries no net electrical charge, i.e. at which it is electrically neutral in the statistical mean. The concentration of protons (H+) in the surrounding environment affects how readily molecules gain or lose protons and thus their electrical properties. When the environmental pH is greater than the molecule's pI, the molecule is negatively charged, and when the pH is less than the pI, it is positively charged. Isoelectric point is therefore important for determining the behavior of molecules exposed to electric fields, as in {{gli|electrophoresis}} and ion chromatography. Proteins are least soluble at their isoelectric points because electrically neutral species do not repulse each other with electrostatic forces, such that they tend to aggregate and precipitate out of solution.<ref name="MacLean"/></dd>

{{term|isomerase}}{{anchor|isomerases}} <dd>Any of a class of {{gli|enzymes}} which catalyze the conversion of a molecule from one isomer to another, such that the product of the reaction has the same molecular formula as the original substrate but differs in the connectivity or spatial arrangement of its atoms.</dd>

{{term|isomeric genes}} <dd>Two or more {{gli|genes}} that are equivalent and redundant in the sense that, despite coding for distinct {{gli|gene products}}, they each result in the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotype}} when set within the same {{gli|genetic background}}. If several isomeric genes are present in a single {{gli|genotype}} they may be either cumulative or non-cumulative in their contributions to the phenotype.<ref name="Rieger"/></dd>

{{term|isotonic}} <dd>Describing a solution containing the same concentration of dissolved solutes as another solution, such that the two solutions have equal osmotic pressure. Isotonic solutions separated from each other by a semipermeable membrane (as with a cell, where the intracellular {{gli|cytosol}} is separated from the {{gli|extracellular fluid}} by the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}}) have no {{gli|biomolecular gradient|concentration gradient}} and thus will not exchange solvent by osmosis. Contrast ''{{gli|hypertonic}}'' and ''{{gli|hypotonic}}''.</dd>

==J== {{glossary}} {{term|jumping gene}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transposable element}}''.</dd>

{{term|junctional diversity}} <dd></dd>

{{term|junk DNA}} <dd>Any DNA sequence that appears to have no known biological function, or which acts in a way that has no positive or a net negative effect on the fitness of the {{gli|genome}} in which it is located. The term was once more broadly used to refer to all {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ncDNA|non-coding DNA}}, though much of this was later discovered to have a function; in modern usage it typically refers to broken or vestigial sequences and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|selfish genetic elements}}, including {{gli|introns}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pseudogenes}}, {{gli|intergenic region|intergenic DNA}}, and fragments of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transposons}} and retroviruses, which together constitute a large proportion of the genomes of most eukaryotes. Despite not contributing productively to the host organism, these sequences are able to persist indefinitely inside genomes because the disadvantages of continuing to copy them are too small to be acted upon by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|natural selection}}.</dd>

{{term|junk RNA}} <dd>Any RNA-encoded sequence, especially a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcript}}, that appears to have no known biological function, or whose function has no positive or a net negative effect on the fitness of the genome from which it is transcribed. Despite remaining {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation|untranslated}}, many {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ncRNA|non-coding RNAs}} still serve important functions, whereas junk RNAs are truly useless: often they are the product of accidental transcription of a {{gli|junk DNA}} sequence, or they may result from {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|post-transcriptional modification|post-transcriptional processing}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primary transcripts}}, as with {{gli|alternative splicing|spliced-out}} {{gli|introns}}. Junk RNA is usually quickly degraded by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribonucleases}} and other cytoplasmic enzymes.</dd>

==K== {{glossary}} {{term|karyogram}} <dd>A {{gli|karyotype}} which depicts the entire set of {{gli|chromosomes}} in a cell or organism by using photomicrographs of the actual chromosomes as they appear ''{{gli|in vivo}}'' (usually during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metaphase}}, in their most condensed forms), as opposed to the idealized illustrations of chromosomes used in {{gli|idiograms}}. The photomicrographs are often still arranged in pairs and by size for easier identification of particular chromosomes, whereas in the actual nucleus there is seldom any apparent organization.</dd>

{{term|karyokinesis}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}}''.</dd>

{{term|karyolymph}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosol}}''.</dd>

{{term|karyon}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}}''.</dd>

{{term|karyoplasm}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleoplasm}}''.</dd>

{{term|karyopyknosis}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyknosis}}''.</dd>

{{term|karyorrhexis}} <dd>The fragmentation and degeneration of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} of a dying cell, during which the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nuclear envelope}} is destroyed and the contents of the nucleus, including {{gli|chromatin}}, are dispersed throughout the {{gli|cytoplasm}} and degraded by enzymes. Karyorrhexis is usually preceded by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pyknosis}} and may occur as a result of {{gli|apoptosis}}, {{gli|cellular senescence}}, or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|necrosis}}.</dd>

{{term|karyosome}} {{ghat|Also '''karyosphere'''.}} <dd>A dense, organized bundle of {{gli|chromatin}} which forms in the oocyte nucleus during oogenesis in some female eukaryotes.<ref>{{#invoke:cite|journal| vauthors = Bogolyubov DS | title = Karyosphere (Karyosome): A Peculiar Structure of the Oocyte Nucleus | journal = International Review of Cell and Molecular Biology | volume = 337 | pages = 1–48 | date = 2018-01-01 | pmid = 29551157 | doi = 10.1016/bs.ircmb.2017.12.001 | publisher = Academic Press | isbn = 978-0-12-815195-2 | veditors = Galluzzi L }}</ref></dd>

{{term|karyotype}}{{anchor|karyotypes}} <dd>The number and appearance of {{gli|chromosomes}} within the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}} of a eukaryotic cell, especially as depicted in an organized {{gli|karyogram}} or {{gli|idiogram}} (in pairs and arranged by size and by position of the {{gli|centromere}}). The term is also used to refer to the complete set of chromosomes in a species or individual organism or to any test that detects this complement or measures the chromosome number.</dd> thumb|right|The '''{{gli|karyotype}}''' of a typical human male, as visualized in a karyogram using {{gli|G banding|Giemsa staining}}

{{term|ketogenesis}} <dd>The production of {{gli|ketone bodies}} via the {{gli|catabolism|metabolic decomposition}} of {{gli|fatty acids}} or {{gli|ketogenic amino acids}}, an exergonic process which is used to supply energy to certain tissues during periods of carbohydrate and protein insufficiency.</dd>

{{term|ketogenic amino acid}}{{anchor|ketogenic amino acids}} <dd>Any {{gli|amino acid}} that can be converted directly into {{gli|acetyl-CoA}}, which can then be oxidized for energy or used as a precursor for many {{gli|ketone bodies|compounds containing ketone groups}}. This is in contrast to the {{gli|glucogenic amino acids}}, which can be converted into {{gli|glucose}}. In humans, seven of the 20 amino acids are ketogenic, though only leucine and lysine are exclusively ketogenic; the other five (phenylalanine, isoleucine, threonine, tryptophan, and tyrosine) are both ketogenic and glucogenic.</dd>

{{term|ketolysis}} <dd>The {{gli|catabolism|metabolic decomposition}} of {{gli|ketone bodies}}, which releases energy that can be used to synthesize {{gli|ATP}}.</dd>

{{term|kilobase (kb)}}{{anchor|kilobases|kb|kbp}} <dd>A unit of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} length equal to one thousand (1{{e|3}}) {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleobase|bases}} in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|single-stranded}} molecules or one thousand {{gli|base pairs}} in duplex molecules such as {{gli|dsDNA|double-stranded DNA}}.</dd>

{{term|kinase}}{{anchor|kinases}} <dd>Any of a class of {{gli|enzymes}} which catalyze the transfer of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate}} groups from high-energy, phosphate-donating molecules such as {{gli|ATP}} to one or more specific {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substrates}}, a process known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphorylation}}. The opposite process, known as {{gli|dephosphorylation}}, is catalyzed by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphatase}} enzymes.</dd>

{{term|kinesis}} <dd>A non-specific, non-directional movement or change in activity by a cell or a population of cells in response to a stimulus, such that the rate of the movement or activity is dependent on the intensity of the stimulus but not on the direction from which the stimulus occurs. Kinesis refers particularly to cellular locomotion without directional bias, in contrast to {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|taxis}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|tropism}}.</dd>

{{term|kinetochore}}{{anchor|kinetochores}} <dd>A disc-shaped {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein complex}} which assembles around the {{gli|centromere}} of a {{gli|chromosome}} during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prometaphase}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mitosis}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}, where it functions as the attachment point for {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microtubules}} of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|spindle apparatus}}.</dd>

{{term|knob}} <dd>In {{gli|cytogenetics}}, an enlarged, heavily staining {{gli|chromomere}} that can be used as a visual marker, allowing specific chromosomes to be easily identified in the nucleus.<ref name="DoG7"/></dd>

{{term|knockdown (KD)}}{{anchor|knockdown|knockdowns}} <dd>A {{gli|genetic engineering}} method by which the normal rate of {{gli|expression}} of one or more of an organism's {{gli|genes}} is reduced or suppressed (though not necessarily completely turned off, as in {{gli|knockout}}), either through direct modification of a DNA sequence or through treatment with a reagent such as a short DNA or RNA {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oligonucleotide}} with a sequence {{gli|complementary}} to either an {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA}} transcript or a gene.</dd>

{{term|knockin (KI)}}{{anchor|knockin|knockins}} <dd>A {{gli|genetic engineering}} method in which one or more novel {{gli|genes}} are {{gli|inserted}} into an organism's genome, particularly when targeted to a specific {{gli|locus}}, or in which one or more existing genes are replaced by or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|substitution|substituted}} with novel genes.<ref>{{#invoke:cite|book|last=Gibson|first=Greg|title=A Primer Of Genome Science 3rd ed.|year=2009|publisher=Sinauer|location=Sunderland, Massachusetts|isbn=978-0-87893-236-8|pages=301–302}}</ref> This is in contrast to a {{gli|knockout}}, in which a gene is deleted or completely inactivated.</dd>

{{term|knockout (KO)}}{{anchor|knockout|knockouts}} <dd>A {{gli|genetic engineering}} method in which one or more specific {{gli|genes}} are inactivated or entirely {{gli|deletion|removed from}} an organism's genome, by any of a variety of mechanisms which disrupt their {{gli|expression}} at some point in the pathway that produces their {{gli|gene products}}, such that no functional gene products are produced. This allows researchers to study the function of a gene ''{{gli|in vivo}}'', by observing how the organism's {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phenotype}} changes when deprived of the gene's normal effects. A ''complete knockout'' permanently inactivates the gene; a ''conditional knockout'' allows the gene to be turned on or off at will, e.g. at specific times or in specific tissues, by linking the expression of the gene to some easily modifiable biochemical state or condition. In a ''heterozygous knockout'', only one of a diploid organism's two alleles is knocked out; in a ''homozygous knockout'', both copies are knocked out. Contrast ''{{gli|knockin}}''.</dd>

{{term|Kozak consensus sequence}} {{ghat|Also simply '''Kozak sequence'''.}} <dd>A highly {{gli|conserved sequence|conserved}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid sequence}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motif}} which functions as the recognition site for the initiation of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translation}} in most eukaryotic {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mRNA|messenger RNAs}}, generally a sequence of 10 bases immediately surrounding and inclusive of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|start codon}}: {{font|GCC{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|purine|R}}CCAUGG|font=courier|size=big}}. As the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|pre-initiation complex}} scans the transcript, recognition of this sequence (or a close variant) causes the complex to commit to full {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosome}} assembly and the start of translation. The Kozak sequence is distinct from other recognition sequences relevant to translation such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|ribosome binding site|ribosome binding sites}} and {{gli|internal ribosome entry site|internal ribosome entry sites}}.<ref name="Kozak">{{#invoke:cite|journal|last=Kozak|first=M.|date=February 1989|title=The scanning model for translation: an update|journal=The Journal of Cell Biology|volume=108|issue=2|pages=229–241|doi=10.1083/jcb.108.2.229|issn=0021-9525|pmc=2115416|pmid=2645293}}</ref></dd>

{{term|Krebs cycle}} <dd>See ''{{gli|citric acid cycle}}''.</dd>

==L== {{glossary}} {{term|labelling}} {{ghat|Also '''tagging'''.}} <dd>The chemical attachment of a highly selective substance, known as a ''label'', ''tag'', or ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|probe}}'', to a particular cell, protein, amino acid, or other molecule of interest, either naturally or artificially, ''{{gli|in vivo}}'' or ''{{gli|in vitro}}''. Natural labelling is a primary mechanism by which biomolecules specifically identify and interact with other biomolecules; important examples include {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|methylation}}, {{gli|acetylation}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphorylation}}, and {{gli|glycosylation}}. Labelling is also a common laboratory technique, where the label is typically a reactive derivative of a naturally fluorescent compound (e.g. green fluorescent protein), dye, enzyme, {{gli|antibody}}, radioactive molecule, or any other substance that makes its target distinguishable in some way. The labelled targets are thereby rendered distinct from their unlabelled surroundings, allowing them to be detected, identified, quantified, or isolated for further study.</dd>

{{term|lagging strand}} <dd>In {{gli|DNA replication}}, the nascent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} for which {{gli|DNA polymerase}}'s direction of synthesis is away from the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication fork}}, which necessitates a complex and discontinuous process in contrast to the streamlined, continuous synthesis of the other nascent strand, known as the {{gli|leading strand}}, which occurs simultaneously. Because DNA polymerase works only in the {{gli|5'}} to {{gli|3'}} direction, but the lagging strand's overall direction of chain elongation must ultimately be the opposite (i.e. 3' to 5', toward the replication fork), elongation must occur by an indirect mechanism in which a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primase}} enzyme synthesizes short {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primers}} complementary to the template DNA, and DNA polymerase then extends the primed segments into short chains of nucleotides known as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Okazaki fragments}}. The RNA primers are then removed and replaced with DNA, and the Okazaki fragments are joined by {{gli|DNA ligase}}.</dd>

{{term|lamella}}{{anchor|lamellae|lamellar}} <dd>1.  Any thin layer, membrane, or plate of tissue, occurring in a wide variety of structures of various scales and with various functions; e.g. a lamella made of a sheet of lipids forms a component of the {{gli|extracellular matrix}} between the cells of some tissues.</dd> <dd>2.  The leading edge of a motile cell, of which the lamellipodia is the most forward portion.</dd>

{{term|lampbrush chromosome}} <dd>A transcriptionally active, highly de-condensed morphology assumed by certain {{gli|chromosomes}} during the {{gli|diplotene stage}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiotic}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prophase|prophase I}} in the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|progenitor cells}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oocytes}} in female insects, amphibians, birds, and some other animals. Lampbrush chromosomes are conspicuous under the microscope because the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|synapsis|post-synaptic}} {{gli|homologous chromosomes|homologs}}, still attached at {{gli|chiasmata}}, are gigantically elongated into large loops of unpackaged {{gli|euchromatin}} extending laterally from a series of {{gli|chromomeres}}. Large numbers of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|messenger RNAs}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|non-coding RNAs}} are transcribed from the lateral loops, generating a rich pool of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcripts}} to be used in the immature oocyte and after fertilization, with functions in both {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|oogenesis}} and {{gli|embryogenesis}}. Because they allow individual {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription units}} to be directly visualized, lampbrush chromosomes are useful models for studying chromosome organization and genome structure and for constructing high-resolution chromosome maps.<ref>Morgan, G.T. (2002) "Lampbrush chromosomes and associated bodies: new insights into principles of nuclear structure and function." ''Chromosome Research''. 10: 177–200.</ref></dd> thumb|400px|A '''{{gli|lampbrush chromosome}}''' magnified 11,000 times with an electron microscope, showing the characteristic lateral loops containing transcriptionally active segments of DNA

{{term|lateral gene transfer (LGT)}} <dd>See ''{{gli|horizontal gene transfer}}''.</dd>

{{term|leader sequence}} <dd>See ''{{gli|5' untranslated region}}''.</dd>

{{term|leading strand}} <dd>In {{gli|DNA replication}}, the nascent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} for which both the direction of synthesis by {{gli|DNA polymerase}} and the direction of overall chain elongation are toward the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|replication fork}}; i.e. both occur in the {{gli|5'}} to {{gli|3'}} direction, resulting in a single, continuous elongation process with few or no interruptions. By contrast, the other nascent strand, known as the {{gli|lagging strand}}, is assembled in a discontinuous process involving the ligation of short {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|Okazaki fragment|DNA fragments}} synthesized in the opposite direction, away from the replication fork.<ref name="DoG7"/></dd>

{{term|left splicing junction}} {{ghat|Also '''donor splicing junction''' or '''donor splicing site'''.}} <dd>The boundary between the left end (by convention, the {{gli|5' end}}) of an {{gli|intron}} and the right ({{gli|3'}}) end of an adjacent {{gli|exon}} in a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|primary transcript|pre-mRNA transcript}}.</dd>

{{term|leptonema}} {{ghat|Also '''leptotene stage'''.}} <dd>In {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}, the first of five substages of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|prophase|prophase I}}, following {{gli|interphase}} and preceding {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|zygonema}}. During leptonema, the replicated chromosomes condense from diffuse {{gli|chromatin}} into long, thin strands that are much more visible within the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleus}}.</dd>

{{term|lethal mutation}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mutation}} that results in the premature death of the organism carrying it. {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recessive|Recessive}} lethal mutations are fatal only to {{gli|homozygous|homozygotes}}, whereas {{gli|dominant}} lethals are fatal even in {{gli|heterozygous|heterozygotes}}.<ref name="DoG7"/></dd>

{{term|leucine zipper (ZIP)}}{{anchor|leucine zippers}} <dd>A common structural {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|motif}} in {{gli|DNA binding|DNA-binding}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription factors}} and some other types of proteins, approximately 35 amino acids in length, characterized chiefly by the recurrence of the amino acid leucine every seven residues. When modeled in an idealized {{gli|alpha-helical}} conformation, the leucine residues are positioned in such a way that they can interdigitate with the same or similar motifs in an alpha helix belonging to another similar polypeptide, facilitating {{gli|dimerization}} and the formation of a complex resembling a zipper.<ref name="Lackie"/></dd>

{{term|ligand}}{{anchor|ligands}} <dd>In biochemistry, any molecule that binds to or interacts with a {{gli|binding site|specific site}} on a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} or other {{gli|biomolecule}}, usually reversibly via intermolecular forces;<ref name="Alberts et al."/> or any substance that forms a complex with a biomolecule as part of a biological process. The binding of specific ligands to DNA or proteins is important in many {{gli|biochemical pathways}}; for example, protein–ligand binding may result in the protein undergoing a {{gli|conformational change}} which alters its function or affinity for other molecules.</dd>

{{term|ligase}}{{anchor|ligases}} <dd>A class of {{gli|enzymes}} which catalyze the synthesis of large molecules such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}} by forming one or more chemical bonds between them, typically {{chem|C–C}}, {{chem|C–O}}, {{chem|C–S}}, or {{chem|C–N}} bonds via condensation reactions. An example is {{gli|DNA ligase}}, which catalyzes the formation of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester bonds}} between adjacent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} on the same strand of a DNA molecule, a reaction known as {{gli|ligation}}.</dd>

{{term|ligation}}{{anchor|ligations|ligate|ligates|ligated}} <dd>The joining of consecutive {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} in the same {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|strand}} of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acid}} molecule via the formation of a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphodiester bond}} between the {{gli|5'-end|5'-phosphoryl terminus}} of one nucleotide and the {{gli|3'-end|3'-hydroxyl terminus}} of an adjacent nucleotide, a condensation reaction catalyzed by enzymes known as {{gli|ligases}}.<ref name="MacLean"/> This reaction occurs in fundamentally the same way in all varieties of {{gli|DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|RNA}} anabolism, natural or artificial, whether the addition of individual nucleotides to a growing strand (as in {{gli|DNA replication}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcription}}), or the {{gli|DNA repair|repair}} of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nicks}} and {{gli|cuts}} in previously intact molecules, or the joining of separate nucleic acid fragments into a single molecule (as in {{gli|chromosomal crossover}}, {{gli|exon splicing}}, retroviral {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transposition}}, and all other forms of {{gli|genetic recombination}}, as well as artificial {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular cloning}} techniques). Ligation is the opposite of the catabolic reaction wherein phosphodiester bonds are cleaved by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleases}}. It also should not be confused with the non-covalent {{gli|base pairing}} that can occur between complementary strands; ligation refers specifically to the synthesis of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phosphate backbone}} of a single strand.</dd>

{{term|linkage}}{{anchor|linked|unlinked}} <dd>The tendency of DNA sequences which are physically near to each other on the same chromosome to be {{gli|heredity|inherited}} together during {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|meiosis}}. Because the physical distance between them is relatively small, the chance that any two nearby parts of a DNA sequence (often {{gli|loci}} or {{gli|genetic markers}}) will be separated on to different {{gli|chromatids}} during {{gli|chromosomal crossover}} is statistically very low; such loci are then said to be more ''linked'' than loci that are farther apart. Loci that exist on entirely different chromosomes are said to be perfectly ''unlinked''. The standard unit for measuring genetic linkage is the {{gli|centimorgan}} (cM).</dd>

{{term|linker DNA}} <dd>1.  A short, synthetic DNA duplex containing the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|restriction site|recognition sequence}} for a particular {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|restriction enzyme}}.<ref name="DoG7"/> In {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|molecular cloning}}, linkers are often deliberately included in {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|recombinant DNA|recombinant}} molecules in order to make them easily modifiable by permitting cleavage and {{gli|insertion}} of foreign sequences at precise locations. A segment of an engineered {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasmid}} containing many such restriction sites is sometimes called a {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|polylinker}}.</dd> <dd>2.  A section of chromosomal DNA connecting adjacent {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleosomes}} by binding to {{gli|histone}} H1.<ref name="DoG7"/></dd>

{{term|linking number}} <dd>The number of times that the two strands of a circular {{gli|double helix|double-helical}} {{gli|DNA}} molecule cross each other, equivalent to the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|twisting number}} (which measures the torsion of the double helix) plus the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|writhing number}} (which measures the degree of supercoiling). The linking number of a closed molecule cannot be changed without breaking and rejoining the strands. DNA molecules which are identical except for their linking numbers are known as ''topological isomers''.<ref name="DoG7"/></dd>

{{term|lipid}}{{anchor|lipids}} <dd>Any of a heterogeneous class of organic compounds, including {{gli|glycerides}} (fats), waxes, sterols, and some vitamins, united only by their amphipathic or {{gli|hydrophobic}} nature and consequently their very low solubility in water.<ref name="Lackie"/> Some lipids such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipids}} tend to form lamellar structures or micelles in aqueous environments, where they serve as the primary constituents of biological {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membranes}}. Others such as {{gli|fatty acids}} can be {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|metabolized}} for energy, have important functions in energy storage, or serve as {{gli|cell signaling|signaling}} molecules. Colloquially, the term "lipids" is sometimes used as a synonym for fats, though fats are more correctly considered a subclass of lipids.</dd> [[File:Common lipid types.svg|thumb|right|400px|Common examples of '''{{gli|lipids}}''' in biological systems include {{gli|cholesterols}}, {{gli|fatty acids}}, {{gli|triglycerides}}, and {{gli|phospholipids}} such as phosphatidylcholine.]]

{{term|lipid bilayer}}{{anchor|phospholipid bilayer|bilayer}} {{ghat|Also '''phospholipid bilayer'''.}} <dd>A lamellar structure composed of numerous amphipathic {{gli|lipid}} molecules packed together in two back-to-back sheets or layers, with their {{gli|hydrophobic}} {{gli|fatty acid}} "tails" directed inward and their {{gli|hydrophilic}} "heads" exposed on the outer surface. This is the basic structural motif for all biological {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membranes}}, including the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} surrounding all cells as well as the membranes surrounding {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelles}} and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicles}}. Though bilayers are sometimes colloquially described as ''phospholipid bilayers'', {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipids}} are just one of several classes of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|membrane lipids}} which form bilayers; most membranes are actually a {{gli|fluid mosaic model|fluid, heterogeneous mixture}} of phospholipids, {{gli|glycolipids}}, and {{gli|cholesterols}}, interspersed and studded with various other molecules such as {{gli|integral membrane protein|integral proteins}}.<ref name="Lackie"/></dd>

{{term|lipolysis}} <dd>The {{gli|metabolic pathway}} by which {{gli|triglycerides}} are broken down via hydrolysis into a glycerol molecule and free {{gli|fatty acid}} chains. This primarily takes place in {{gli|adipocytes}} to mobilize stored energy when demand is high or supply is low.</dd>

{{term|lipophilic}} <dd>See ''{{gli|hydrophobic}}''.</dd>

{{term|lipoprotein}}{{anchor|lipoproteins}} <dd>Any water-soluble {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|protein}} to which one or more lipid molecules are attached by covalent bonding to amino acid residues. Many classes of lipids can be conjugated to proteins, including {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|triacylglycerol|triacylglycerols}}, {{gli|cholesterols}}, and {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|phospholipids}}.<ref name="Oxford B&MB"/> Compare ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteolipid}}''.</dd>

{{term|liposome}}{{anchor|liposomes}} {{ghat|Also '''cellule''', '''spherule''', or '''spherulite'''.}} <dd>1.  Any small, natural {{gli|lipid}} globule, such as a micelle, occurring naturally in the cytoplasm;<ref name="Oxford B&MB"/> they are commonly formed by budding off from larger membrane-bound vesicles.</dd> <dd>2.  A small, spherical, artificial {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicle}} having at least one continuous {{gli|bilayer}} of lipid molecules enclosing some of the medium in which it is suspended.<ref>{{Cite journal |last1=Akbarzadeh |first1=A. |last2=Rezaei-Sadabady |first2=R. |last3=Davaran |first3=S. |last4=Joo |first4=S. W. |last5=Zarghami |first5=N. |last6=Hanifehpour |first6=Y. |last7=Samiei |first7=M. |last8=Kouhi |first8=M. |last9=Nejati-Koshki |first9=K. |date=22 February 2013 |title=Liposome: classification, preparation, and applications |journal=Nanoscale Research Letters |volume=8 |issue=1 |page=102 |doi=10.1186/1556-276X-8-102 |doi-access=free |issn=1931-7573 |pmc=3599573 |pmid=23432972 |bibcode=2013NRL.....8..102A}}</ref> Liposomes can be created in the laboratory by disrupting existing biological membranes and allowing complex lipids to form bilayer-bound vesicles in aqueous solution, usually with the aid of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|sonication}}. They are used experimentally as models of natural membranes and also therapeutically for the encapsulation and delivery of pharmaceutical compounds, enzymes, nutrients, nucleic acids, lipid-based nanoparticles (as in some vaccines), and many other agents between or inside of cells.<ref name="Oxford B&MB"/></dd>

{{term|lncRNA}} <dd>See ''{{gli|lncRNA|long non-coding RNA}}''.</dd>

{{term|locus}}{{anchor|loci}} {{ghat|Plural '''loci'''.}} <dd>A specific, fixed position on a {{gli|chromosome}} where a particular {{gli|gene}} or {{gli|genetic marker}} resides.</dd>

{{term|long arm}}{{anchor|long arms}} {{ghat|Denoted in shorthand with the symbol '''''q'''''.}} <dd>In condensed {{gli|chromosomes}} where the positioning of the {{gli|centromere}} creates two segments or "arms" of unequal length, the longer of the two arms of a {{gli|chromatid}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|short arm}}''.</dd>

{{term|long interspersed nuclear element (LINE)}} <dd>Any of a large family of non-{{gli|LTR}} {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|retrotransposons}} which together comprises one of the most widespread {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|mobile genetic elements}} in eukaryotic genomes. Each LINE {{gli|insertion sequence|insertion}} is on average about 7,000 base pairs in length.</dd>

{{term|long non-coding RNA (lncRNA)}} <dd>A class of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|non-coding RNA}} consisting of all {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transcripts}} of more than 200 {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleotides}} in length that are not {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|translated}}. This limit distinguishes lncRNA from the numerous smaller non-coding RNAs such as {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|microRNA}}. See also ''{{gli|lincRNA|long intervening non-coding RNA}}''.</dd>

{{term|lyonization}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (M–Z)|X-inactivation}}''.</dd>

{{term|lysis}}{{anchor|lytic|lysing|lysed}} <dd>The disruption and decomposition of the {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|plasma membrane}} surrounding a cell, or more generally of any membrane-bound {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|organelle}} or {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|vesicle}}, especially by {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|osmotic}}, {{gli|enzymatic}}, or other chemical or mechanical processes which compromise the membrane's integrity and thereby cause the unobstructed interchange of the contents of {{gli|intracellular}} and {{gli|extracellular}} spaces. Lysis generally implies the complete and irreversible loss of intracellular organization as a result of the release of the cell's internal components and the dilution of the {{gli|cytosol}}, and therefore the death of the cell. Such a cell is said to be ''lysed'', and a fluid containing the contents of lysed cells (usually including {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|nucleic acids}}, {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|proteins}}, and many other organic molecules) is called a ''lysate''. Lysis may occur both naturally and artificially, and is a normal part of the cellular life cycle.</dd>

{{term|lysosome}}{{anchor|lysosomes}} <dd></dd>

==References== <references />

==Further reading== *{{#invoke:cite|book|last=Budd |first=A. |title=Evolutionary Genomics |chapter=Introduction to Genome Biology: Features, Processes, and Structures |series=Methods in Molecular Biology |volume=855 |year=2012 |number=855 |pages=3–4|doi=10.1007/978-1-61779-582-4_1 |pmid=22407704 |isbn=978-1-61779-581-7 }}

==External links== *[https://www.genome.gov/genetics-glossary National Human Genome Research Institute (NHGRI) Talking Glossary of Genomic and Genetic Terms] *[https://www.swissbiopics.org/name/Animal_cell Interactive, labeled diagram of an animal cell from SwissBioPics]

{{DEFAULTSORT:Genetics Glossary}} * Glossary Genetics Category:Molecular-biology-related lists Category:Wikipedia glossaries using description lists