Glossary of cellular and molecular biology (M–Z)

{{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 molecular genetics, biochemistry, and microbiology.<ref>{{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:

*Glossary of cellular and molecular biology (0–L) lists terms beginning with numbers and those beginning with the letters A through L. *'''Glossary of cellular and molecular biology (M–Z)''' (this page) 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.

__NOTOC__ {{compact ToC|center=yes|num=yes|a=A|b=B|c=C|d=D|e=E|f=F|g=G|h=H|i=I|j=J|k=K|l=L|seealso=yes|refs=yes|extlinks=yes|nobreak=yes}}

==M== {{glossary}} {{term|M phase}} <dd>See ''{{gli|mitosis}}''.</dd>

{{term|macromolecule}}{{anchor|macromolecules|macromolecular}} <dd>Any very large molecule composed of dozens, hundreds, or thousands of covalently bonded atoms, especially one with biological significance. Many important {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biomolecules}}, such as {{gli|nucleic acids}} and {{gli|proteins}}, are {{gli|polymers}} consisting of a repeated series of smaller {{gli|monomers}}; others such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipids}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|carbohydrates}} may not be polymeric but are nevertheless large and complex molecules.</dd>

{{term|macronucleus}}{{anchor|macronuclei}} {{ghat|Also '''meganucleus'''.}} <dd>The larger of the two types of {{gli|nuclei}} which occur in pairs in the cells of some ciliated protozoa. Macronuclei are highly {{gli|polyploid}} and responsible for directing vegetative reproduction, in contrast to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} {{gli|micronuclei}}, which have important functions during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conjugation}}.<ref name="MacLean"/></dd>

{{term|macrophage}}{{anchor|macrophages}} <dd>Any of a class of relatively long-lived {{gli|phagocyte|phagocytic cells}} of the mammalian immune system which are activated in response to the presence of foreign materials in certain tissues and subsequently play important roles in antigen presentation, stimulating other types of immune cells, and killing or {{gli|phagocytosis|engulfing}} parasitic microorganisms, diseased cells, or tumor cells.<ref name="Lackie"/></dd>

{{term|major groove}} <dd></dd>

{{term|map-based cloning}} <dd>See ''{{gli|positional cloning}}''.</dd>

{{term|massively parallel sequencing}}{{anchor|next-generation sequencing|second-generation sequencing}} {{ghat|Also '''next-generation sequencing (NGS)''' and '''second-generation sequencing'''.}} <dd></dd>

{{term|medical genetics}} <dd>The branch of medicine and medical science that involves the study, diagnosis, and management of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic disorder|hereditary disorders}}, and more broadly the application of knowledge about human {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetics}} to medical care.</dd>

{{term|megabase (Mb)}}{{anchor|megabases|Mb|Mbp}} <dd>A unit of {{gli|nucleic acid}} length equal to one million (1{{e|6}}) {{gli|bases}} in {{gli|single-stranded}} molecules or one million {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pairs}} in duplex molecules such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsDNA|double-stranded DNA}}.</dd>

{{term|meiosis}}{{anchor|meiotic}} <dd>A specialized type of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}} that occurs exclusively in sexually reproducing eukaryotes, during which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} is followed by two consecutive rounds of division to ultimately produce four genetically unique {{gli|glossary=Glossary of cellular and molecular biology (0–L)|haploid}} daughter cells, each with half the number of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} as the original {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} parent cell. Meiosis only occurs in cells of the sex organs, and serves the purpose of generating haploid {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gametes}} such as sperm, eggs, or spores, which are later fused during fertilization. The two meiotic divisions, known as ''Meiosis I'' and ''Meiosis II'', may also include various {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination}} events between {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous chromosomes}}.</dd>

{{term|meiotic spindle}} <dd>See ''{{gli|spindle apparatus}}''.</dd>

{{term|melting}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (0–L)|denaturation}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded}} {{gli|nucleic acid}} into two {{gli|single strands}}, especially in the context of the {{gli|polymerase chain reaction}}.</dd>

{{term|membrane}}{{anchor|membranes}} <dd>A supramolecular aggregate of amphipathic {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipid}} molecules which when suspended in a polar solvent tend to arrange themselves into structures which minimize the exposure of their {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hydrophobic}} tails by sheltering them within a ball created by their own {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hydrophilic}} heads (i.e. a micelle). Certain types of lipids, specifically {{gli|phospholipids}} and other {{gli|membrane lipids}}, commonly occur as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipid bilayer|double-layered}} sheets of molecules when immersed in an aqueous environment, which can themselves assume approximately spherical shapes, acting as semipermeable barriers surrounding a water-filled interior space. This is the basic structure of the biological membranes enclosing all {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cells}}, {{gli|vesicles}}, and {{gli|membrane-bound organelles}}.</dd>

{{term|membrane lipid}}{{anchor|membrane lipids}} <dd></dd>

{{term|membrane potential}} <dd></dd>

{{term|membrane protein}}{{anchor|membrane proteins}} <dd>Any {{gli|protein}} that is closely associated either {{gli|glossary=Glossary of cellular and molecular biology (0–L)|integral monotopic protein|transiently}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|integral polytopic protein|permanently}} with the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipid bilayer}} {{gli|membrane}} surrounding a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell}}, {{gli|organelle}}, or {{gli|vesicle}}.<ref name="Alberts et al."/></dd>

{{term|membrane-bound organelle}}{{anchor|membrane-bound organelles}} <dd>An {{gli|organelle}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell compartmentalization|cellular compartment}} enclosed by its own dedicated lipid {{gli|membrane}}, separating its interior from the rest of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}.</dd>

{{term|messenger RNA (mRNA)}}{{anchor|messenger RNA|mRNAs}} <dd>Any of a class of {{gli|ssRNA|single-stranded}} {{gli|RNA}} molecules which function as molecular messengers, carrying sequence information encoded in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} genome to the {{gli|ribosomes}} where protein synthesis occurs. The primary products of {{gli|transcription}}, mRNAs are synthesized by {{gli|RNA polymerase}}, which builds a chain of {{gli|ribonucleotides}} that complement the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonucleotides}} of a {{gli|template strand|DNA template}}; in this way, the DNA sequence of a protein-coding {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} is effectively preserved in the {{gli|pre-mRNA|raw transcript}}, which is subsequently processed into a mature mRNA by a series of {{gli|post-transcriptional modifications}}.</dd> 400px|thumb|right|The structure of a typical mature protein-coding '''{{gli|messenger RNA}}''' or '''mRNA''', drawn approximately to scale. The coding sequence (green) is bounded by {{gli|untranslated regions}} at both the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5' UTR|5'-end}} (yellow) and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3' UTR|3'-end}} (pink). Prior to export from the nucleus, a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5' cap}} (red) and a {{gli|poly(A) tail|3' poly(A) tail}} (black) are added to help stabilize the mRNA and prevent its degradation by ribonucleases.

{{term|metabolic pathway}}{{anchor|metabolic pathways}} <dd>A stepwise series of biochemical reactions occurring within a cell, often but not necessarily {{gli|glossary=Glossary of cellular and molecular biology (0–L)|catalyzed}} by specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}}, that fulfills some activity or process related to {{gli|metabolism}}. The reactions are linked by the sharing of reactants, products, or intermediate compounds in consecutive steps, such that the product of one reaction is used as a reactant in a subsequent reaction. Byproducts are often removed from the cell as {{gli|metabolic waste}}. The overall pathway may be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anabolic}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|catabolic}}, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amphibolic}} in nature.<ref name="Oxford B&MB"/> In any actively metabolizing cell, an elaborate network of interconnected metabolic pathways is required to maintain {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homeostasis}}, with degradative catabolic processes providing the energy necessary to conduct anabolic biosynthesis; for example, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycolysis}}, the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|electron transport chain}}, and {{gli|oxidative phosphorylation}} provide the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP}} used in fatty acid synthesis. The flux of {{gli|metabolites}} through each pathway is regulated by the needs of the cell and the availability of {{gli|substrates}}.</dd>

{{term|metabolic waste}}{{anchor|metabolic wastes}} <dd></dd>

{{term|metabolism}}{{anchor|metabolic}} <dd>The complete set of chemical reactions which sustain and account for the basic processes of life in all living cells,<ref name="MacLean"/> especially those involving: 1) the conversion of energy from food into energy available for cellular activities; 2) the breakdown of food into simpler compounds which can then be used as {{gli|substrates}} to build complex {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biomolecules}} such as {{gli|proteins}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipids}}, and {{gli|nucleic acids}}; and 3) the degradation and excretion of toxins, byproducts, and other unusable compounds known as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|metabolic wastes}}. In a broader sense the term may include ''all'' chemical reactions occurring in living organisms, even those which are not strictly necessary for life but instead serve accessory functions. Many specific cellular activities are accomplished by {{gli|metabolic pathways}} in which one chemical is ultimately transformed through a stepwise series of reactions into another chemical, with each reaction {{gli|glossary=Glossary of cellular and molecular biology (0–L)|catalyzed}} by a specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzyme}}. Most metabolic reactions can be subclassified as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|catabolic}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anabolic}}.</dd>

{{term|metabolite}}{{anchor|metabolites}} <dd>An intermediate or end product of {{gli|metabolism}}, especially degradative metabolism ({{gli|glossary=Glossary of cellular and molecular biology (0–L)|catabolism}});<ref name="MacLean"/> or any substance produced by or taking part in a metabolic reaction. Metabolites include a huge variety of small molecules generated by cells from various {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biochemical pathway|pathways}} and having various functions, including as inputs to other pathways and reactions, as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|signaling}} molecules, and as stimulators, inhibitors, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cofactors}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}}. Metabolites may result from the degradation and elimination of naturally occurring compounds as well as of synthetic compounds such as pharmaceuticals.</dd>

{{term|metabolome}} <dd>The complete set of {{gli|small molecule|small-molecule}} chemical compounds within a cell, organelle, or any other biological sample, including both {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endogenous}} molecules (e.g. individual {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} and {{gli|nucleotides}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|fatty acids}}, organic acids, amines, {{gli|monosaccharide|simple sugars}}, {{gli|vitamins}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antibiotics}}, etc.) and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exogenous}} molecules (e.g. drugs, toxins, environmental contaminants, and other xenobiotics).</dd>

{{term|metacentric}} <dd>(of a linear {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} or chromosome fragment) Having a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromere}} positioned in the middle of the chromosome, resulting in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatid}} arms of approximately equal length.<ref name="CoG">{{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|metaphase}} <dd>The stage of {{gli|mitosis}} and {{gli|meiosis}} that occurs after {{gli|prometaphase}} and before {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anaphase}}, during which the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromeres}} of the replicated chromosomes align along the equator of the cell, with each {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinetochore}} attached to the {{gli|mitotic spindle}}.</dd>

{{term|methylation}}{{anchor|methylate|methylates|methylating|methylated}} <dd>The covalent attachment of a methyl group ({{chem|–CH|3}}) to a chemical compound, protein, or other biomolecule, either spontaneously or by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymatic}} catalysis. Methylation is one of the most widespread natural mechanisms by which {{gli|nucleic acids}} and {{gli|proteins}} are {{gli|glossary=Glossary of cellular and molecular biology (0–L)|labelled}}. The {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA methylation|methylation of nucleobases}} in a DNA molecule inhibits recognition of the methylated sequence by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding proteins}}, which can effectively {{gli|silence}} the expression of genes. Specific {{gli|residues}} within {{gli|glossary=Glossary of cellular and molecular biology (0–L)|histones}} are also commonly methylated, which can change {{gli|nucleosome}} positioning and similarly {{gli|activate}} or {{gli|repress}} nearby loci. The opposite reaction is {{gli|glossary=Glossary of cellular and molecular biology (0–L)|demethylation}}.</dd>

{{term|methyltransferase}}{{anchor|methyltransferases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (M–Z)|transferase}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} which catalyze the covalent bonding of a methyl group ({{chem|–CH|3}}) to another compound, protein, or biomolecule, a process known as {{gli|methylation}}.</dd>

{{term|MicroArray and Gene Expression (MAGE)}} <dd>A group that "aims to provide a standard for the representation of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA microarray}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} data that would facilitate the exchange of microarray information between different data systems".<ref name="pmid17087822">{{cite journal | display-authors = 4| author = Rayner TF| author2 = Rocca-Serra P| author3 = Spellman PT| author4 = Causton HC| author5 = Farne A| author6 = Holloway E| author7 = Irizarry RA| author8 = Liu J| author9 = Maier DS| author10 = Miller M| author11 = Petersen K| author12 = Quackenbush J| author13 = Sherlock G| author14 = Stoeckert CJ| author15 = White J| author16 = Whetzel PL| author17 = Wymore F| author18 = Parkinson H| author19 = Sarkans U| author20 = Ball CA| author21 = Brazma A | title = A simple spreadsheet-based, MIAME-supportive format for microarray data: MAGE-TAB | journal = BMC Bioinformatics | volume = 7| article-number = 489 | date = 2006 | pmid = 17087822 | pmc = 1687205 | doi = 10.1186/1471-2105-7-489 | doi-access = free}}</ref></dd>

{{term|microbody}}{{anchor|microbodies}} <dd>Any of a diverse class of small {{gli|membrane-bound organelles}} or {{gli|vesicles}} found in the cells of many eukaryotes, especially plants and animals, usually having some specific metabolic function and occurring in great numbers in certain specialized cell types. {{gli|peroxisome|Peroxisomes}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glyoxysomes}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycosomes}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hydrogenosomes}} are often considered microbodies.</dd>

{{term|microchromosome}} <dd>A type of very small {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}}, generally less than 20,000 {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pairs}} in size, present in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|karyotypes}} of some organisms.</dd>

{{term|microdeletion}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomal}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deletion}} that is too short to cause any apparent change in morphology under a light microscope, though it may still be detectable with other methods such as {{gli|sequencing}}.</dd>

{{term|microfilament}}{{anchor|microfilaments}} <dd>A long, thin, flexible, rod-like structure composed of polymeric strands of proteins, usually actins, that occurs in abundance in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}} of eukaryotic cells, forming part of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoskeleton}}. Microfilaments comprise the cell's structural framework. They are modified by and interact with numerous other cytoplasmic proteins, playing important roles in cell stability, motility, contractility, and facilitating changes in cell shape, as well as in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytokinesis}}.</dd>

{{term|micronucleus}}{{anchor|micronuclei}} <dd>The smaller of the two types of {{gli|nuclei}} that occur in pairs in the cells of some ciliated protozoa. Whereas the larger {{gli|macronucleus}} is {{gli|polyploid}}, the micronucleus is {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} and generally transcriptionally inactive except for the purpose of sexual reproduction, where it has important functions during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conjugation}}.<ref name="MacLean">{{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|microRNA (miRNA)}} <dd>A type of small, {{gli|single-stranded RNA|single-stranded}}, {{gli|ncRNA|non-coding RNA}} molecule that functions in {{gli|post-transcriptional}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|regulation}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}}, particularly RNA {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene silencing|silencing}}, by base-pairing with complementary sequences in {{gli|mRNA|mRNA transcripts}}, which typically results in the cleavage or destabilization of the transcript or inhibits its {{gli|translation}} by ribosomes.</dd>

{{term|microsatellite}}{{anchor|microsatellites}} {{ghat|Also '''short tandem repeat (STR)''' or '''simple sequence repeat (SSR)'''.}} <dd>A type of {{gli|satellite DNA}} consisting of a relatively short {{gli|nucleotide sequence|sequence}} of {{gli|tandem repeats}}, in which certain {{gli|motifs}} (ranging in length from one to six or more {{gli|nitrogenous base|bases}}) are repeated, typically 5–50 times. Microsatellites are widespread throughout most organisms' genomes and tend to have higher mutation rates than other regions. They are classified as {{gli|variable number tandem repeat}} (VNTR) DNA, along with longer {{gli|minisatellites}}.</dd>

{{term|microsome}}{{anchor|microsomes}} <dd>A small intracellular {{gli|vesicle}} derived from fragments of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endoplasmic reticulum}} observed in cells which have been homogenized.<ref name="Alberts et al."/></dd>

{{term|microspike}}{{anchor|microspikes}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|filopodium}}''.</dd>

{{term|microtome}}{{anchor|microtomy}} <dd>An instrument used to cut extremely thin slices of material, known as ''microsections'' or simply ''sections'', preparatory to observation under a microscope.<ref name="Oxford B&MB"/> Sections of tissues and cells are usually 50 nanometres (nm) to 100 micrometres (μm) in width. The process of cutting them is known as ''microtomy''.</dd>

{{term|microtrabecula}}{{anchor|microtrabeculae}} {{ghat|(pl.) '''microtrabeculae'''}} <dd>A fine protein filament of the cytoskeleton. Multiple filaments form the microtrabecular network.<ref name="MacLean"/></dd>

{{term|microtubule}}{{anchor|microtubules}} <dd>Any of the long, generally straight, hollow tubes, about 24 nanometers in diameter and composed of interwoven polymeric filaments of the protein tubulin, found in the cytoplasm of many eukaryotic cells, where they are involved in maintaining the cell's shape and structural integrity as well as in force generation for cellular or {{gli|organellar}} locomotion (as with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cilia}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|flagella}}). They also comprise the {{gli|spindle apparatus}} critical to {{gli|mitosis}} and {{gli|meiosis}}. Microtubules are rigid but transient all-purpose structural members which can be rapidly assembled and disassembled at the cell's needs. Many different microtubule-associated proteins interact with them.<ref name="Oxford B&MB"/> See also ''{{gli|microfilament}}''.</dd>

{{term|microtubule-organizing center (MTOC)}}{{anchor|microtubule-organizing center}} <dd>A region near the center of a eukaryotic cell typically consisting of two {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centrioles}} oriented at right angles to each other and surrounded by a {{gli|protein complex|complex}} of associated proteins, which functions as the site of initiation for the assembly of {{gli|microtubules}}.<ref name="Oxford B&MB"/></dd>

{{term|microvesicle}}{{anchor|microvesicles}} {{ghat|Also '''ectosome''' and '''microparticle'''.}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extracellular vesicle}} released when an evagination of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}} "buds off" into the extracellular space. Microvesicles vary in size from 30–1,000 nanometres in diameter and are thought to play roles in many physiological processes, including {{gli|glossary=Glossary of cellular and molecular biology (0–L)|intercellular communication}} by shuttling molecules such as RNA and proteins between cells.<ref>{{cite journal |vauthors=Yáñez-Mó M, Siljander PR, Andreu Z, Zavec AB, Borràs FE, Buzas EI, Buzas K, Casal E, Cappello F, Carvalho J, Colás E, Cordeiro-da Silva A, Fais S, Falcon-Perez JM, Ghobrial IM, Giebel B, Gimona M, Graner M, Gursel I, Gursel M, Heegaard NH, Hendrix A, Kierulf P, Kokubun K, Kosanovic M, Kralj-Iglic V, Krämer-Albers EM, Laitinen S, Lässer C, Lener T, Ligeti E, Linē A, Lipps G, Llorente A, Lötvall J, Manček-Keber M, Marcilla A, Mittelbrunn M, Nazarenko I, Nolte-'t Hoen EN, Nyman TA, O'Driscoll L, Olivan M, Oliveira C, Pállinger É, Del Portillo HA, Reventós J, Rigau M, Rohde E, Sammar M, Sánchez-Madrid F, Santarém N, Schallmoser K, Ostenfeld MS, Stoorvogel W, Stukelj R, Van der Grein SG, Vasconcelos MH, Wauben MH, De Wever O |title=Biological properties of extracellular vesicles and their physiological functions |journal=J Extracell Vesicles |volume=4 |article-number=27066 |date=2015 |pmid=25979354 |pmc=4433489 |doi=10.3402/jev.v4.27066 }}</ref></dd>

{{term|microvillus}}{{anchor|microvilli}} <dd>A small, slender, tubular cytoplasmic projection, generally 0.2–4 micrometres long and 0.1 micrometres in diameter,<ref name="Rieger"/> protruding from the surface of some animal cells and supported by a central core of {{gli|microfilaments}}. When present in large numbers, such as on {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epithelial}} cells lining the respiratory and alimentary tracts, they form a dense brush border which presumably serves to increase each cell's absorptive surface area.<ref name="MacLean"/><ref name="Lackie"/></dd>

{{term|mid body}} <dd>The centrally constricted region that forms across the central axis of a cell during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytokinesis}}, constricted by the closing of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|contractile ring}} until the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|daughter cells}} are finally separated,<ref name="MacLean"/> but occasionally persisting as a tether between the two cells for as long as a complete {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell cycle}}.<ref name="Rieger"/></dd>

{{term|middle lamella}} <dd>In plant cells, the outermost layer of the {{gli|cell wall}}; a continuous, unified layer of extracellular pectins which is the first layer deposited by the cell during {{gli|cytokinesis}} and which serves to cement together the primary cell walls of adjacent cells.<ref name="Alberts et al."/></dd>

{{term|Minimal information about a high-throughput sequencing experiment (MINSEQE)}} <dd>A commercial standard developed by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|FGED}} for the storage and sharing of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|high-throughput}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|sequencing}} data.<ref name="minseqe">{{cite web | author = Functional Genomics Data Society | title = Minimum Information about a high-throughput SEQuencing Experiment | date = June 2012 | url = http://fged.org/projects/minseqe/ | access-date = 2022-11-13 | archive-date = 2022-12-06 | archive-url = https://web.archive.org/web/20221206120352/https://www.fged.org/projects/minseqe }}</ref></dd>

{{term|Minimum information about a microarray experiment (MIAME)}} <dd>A commercial standard developed by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|FGED}} and based on {{gli|MAGE}} in order to facilitate the storage and sharing of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} data.<ref name="pmid18629115">{{cite journal | author = Oliver S | title = On the MIAME Standards and Central Repositories of Microarray Data | journal = Comparative and Functional Genomics | volume = 4 | issue = 1 | page = 1 | date = 2003 | pmid = 18629115 | pmc = 2447402 | doi = 10.1002/cfg.238 }}</ref><ref name="pmid19484163">{{cite journal | author = Brazma A | title = Minimum Information About a Microarray Experiment (MIAME)--successes, failures, challenges | journal = ScientificWorldJournal | volume = 9 | pages = 420–3 | date = 2009 | pmid = 19484163 | doi = 10.1100/tsw.2009.57 | pmc = 5823224 | doi-access = free }}</ref></dd>

{{term|minisatellite}}{{anchor|minisatellites}} <dd>A region of {{gli|repetitive DNA|repetitive}}, {{gli|non-coding DNA|non-coding}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomic DNA}} in which certain DNA {{gli|motifs}} (typically 10–60 bases in length) are {{gli|tandem repeat|tandemly repeated}} (typically 5–50 times). In the human genome, minisatellites occur at more than 1,000 loci, especially in {{gli|glossary=Glossary of genetics (M−Z)|centromeres}} and {{gli|telomeres}}, and exhibit high mutation rates and high variability between individuals. Like the shorter {{gli|microsatellites}}, they are classified as {{gli|variable number tandem repeats}} (VNTRs) and are a type of {{gli|satellite DNA}}.</dd>

{{term|minor groove}} <dd></dd>

{{term|minus-strand}} <dd>See ''{{gli|template strand}}''.</dd>

{{term|miRNA}} <dd>See ''{{gli|miRNA|microRNA}}''.</dd>

{{term|mismatch}}{{anchor|mismatches}} {{ghat|Also '''mispairing'''.}} <dd>An incorrect {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pair|pairing}} of {{gli|nucleobases}} on {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} {{gli|strands}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}}; i.e. the presence in one strand of a duplex molecule of a base that is not complementary (by Watson–Crick pairing rules) to the base occupying the corresponding position in the other strand, which prevents normal hydrogen bonding between the bases. For example, a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine}} paired with a {{gli|thymine}} would be a mismatch, as guanine normally pairs with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine}}.<ref name="DoG7">{{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></dd>

{{term|mismatch repair (MMR)}} <dd></dd>

{{term|missense mutation}} <dd>A type of {{gli|point mutation}} which results in a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} that codes for a different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} than in the unmutated sequence. Compare ''{{gli|nonsense mutation}}''.</dd>

{{term|mistranslation}} <dd>The insertion of an incorrect {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} in a growing {{gli|peptide}} chain during {{gli|translation}}, i.e. the inclusion of any amino acid that is not the one specified by a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} in an {{gli|mRNA}} transcript. Mistranslation may originate from a {{gli|mischarged}} {{gli|transfer RNA}} or from a malfunctioning {{gli|ribosome}}.<ref name="DoG7"/></dd>

{{term|mitochondrial DNA (mtDNA)}} <dd>The set of DNA molecules contained within {{gli|mitochondria}}, usually one or more circular {{gli|plasmids}} representing a semi-autonomous {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} which is physically separate from and functionally independent of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomal DNA}} in the cell's nucleus. The mitochondrial genome encodes many unique enzymes found only in mitochondria.</dd>

{{term|mitochondrial fusion}} <dd></dd>

{{term|mitochondrion}}{{anchor|mitochondria|mitochondrial}} {{ghat|(pl.) '''mitochondria'''; also formerly '''chondriosome'''.}} <dd>A highly pleiomorphic {{gli|membrane-bound organelle}} found in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}} of nearly all eukaryotic cells, usually in large numbers in the form of sausage-shaped structures 5–10 micrometres in length,<ref name="Rieger"/> enclosed by a double membrane, with the inner membrane infolded in an elaborate series of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cristae}} so as to maximize surface area. Mitochondria are the primary sites of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP}} synthesis, where ATP is regenerated from {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ADP}} via {{gli|oxidative phosphorylation}}, as well as many supporting pathways, including the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|citric acid cycle}} and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|electron transport chain}}.<ref name="Lackie"/> Like other {{gli|plastids}}, mitochondria contain {{gli|mitochondrial DNA|their own genome}} encoded in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|circular DNA}} molecules which replicate independently of the {{gli|nuclear genome}}, as well as their own unique set of {{gli|transcription factors}}, {{gli|polymerases}}, {{gli|ribosomes}}, {{gli|transfer RNAs}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|aminoacyl-tRNA synthetases}} with which to direct transcription and translation of their genes. The majority of the {{gli|structural proteins}} found in mitochondria are encoded by nuclear genes, however, such that mitochondria are only partially autonomous.<ref name="MacLean"/> These observations suggest mitochondria evolved from symbiotic {{gli|prokaryotes}} living inside eukaryotic cells.</dd> thumb|right|400px|Diagram of a '''{{gli|mitochondrion}}''' found in an animal cell

{{term|mitogen}}{{anchor|mitogens}} <dd>Any substance or stimulus that promotes or induces {{gli|mitosis}}, or more generally which causes cells to re-enter the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell cycle}}.<ref name="Lackie"/></dd>

{{term|mitophagy}} <dd>The selective degradation of {{gli|mitochondria}} by means of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|autophagy}}; i.e. the mitochondrion initiates its own degradation. Mitophagy is a regular process in healthy populations of cells by which defective or damaged mitochondria are recycled, preventing their accumulation. It may also occur in response to the changing {{gli|metabolic}} needs of the cell, e.g. during certain developmental stages.</dd>

{{term|mitosis}}{{anchor|mitotic}} {{ghat|Also '''M phase'''.}} <dd>In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|eukaryotic}} cells, the part of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell cycle}} during which the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division|division}} of the {{gli|nucleus}} takes place and replicated {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} are separated into two distinct nuclei. Mitosis is generally preceded by the {{gli|S phase}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|interphase}}, when the cell's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|DNA is replicated}}, and either occurs simultaneously with or is followed by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytokinesis}}, when the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}} and {{gli|plasma membrane}} are divided into two new {{gli|glossary=Glossary of cellular and molecular biology (0–L)|daughter cells}}. Colloquially, the term "mitosis" is often used to refer to the entire process of cell division, not just the division of the nucleus.</dd>

{{term|mitotic index (MI)}}{{anchor|mitotic index}} <dd>The proportion of cells within a sample which are undergoing {{gli|mitosis}} at the time of observation, typically expressed as a percentage or as a value between 0 and 1. The number of cells dividing by mitosis at any given time can vary widely depending on organism, {{gli|tissue}}, developmental stage, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell culture|culture}} media, among other factors.<ref name="MacLean"/></dd>

{{term|mitotic recombination}} {{ghat|Also '''somatic {{gli|glossary=Glossary of cellular and molecular biology (0–L)|crossing over}}'''.}} <dd>The abnormal {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination|exchange of genetic material}} between {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous chromosomes}} during {{gli|mitosis}} (as opposed to {{gli|meiosis}}, where it occurs normally). {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous recombination|Homologous recombination}} during mitosis is relatively uncommon; in the laboratory, it can be induced by exposing dividing cells to high-energy electromagnetic radiation such as X rays. As in meiosis, it can separate {{gli|glossary=Glossary of cellular and molecular biology (0–L)|heterozygous}} alleles and thereby propagate potentially significant changes in zygosity to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|daughter cells}}, though unless it occurs very early in development this often has little or no phenotypic effect, since any phenotypic variance shown by mutant lineages arising in terminally differentiated cells is generally masked or compensated for by neighboring {{gli|wild-type}} cells.<ref name="MacLean"/></dd>

{{term|mitotic rounding}} <dd>The process by which most animal cells undergo an overall change in shape during or preceding {{gli|mitosis}}, abandoning the various complex or elongated shapes characteristic of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|interphase}} and rapidly contracting into a rounded or spherical morphology that is more conducive to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}}. This phenomenon has been observed both ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|in vivo}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|in vitro}}''.</dd>

{{term|mitotic segregation}} <dd></dd>

{{term|mitotic spindle}} <dd>See ''{{gli|spindle apparatus}}''.</dd>

{{term|mixoploidy}} <dd>The presence of more than one different {{gli|ploidy}} level, i.e. more than one number of sets of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}, in different cells of the same cellular population.<ref name="DoG7"/></dd>

{{term|mobile genetic element (MGE)}} <dd>Any genetic material that can move between different parts of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} or be transferred from one species or {{gli|replicon}} to another within a single {{gli|glossary=Glossary of cellular and molecular biology (0–L)|generation}}. The many types of MGEs include {{gli|transposable elements}}, bacterial {{gli|plasmids}}, bacteriophage elements which integrate into host genomes by viral {{gli|transduction}}, and {{gli|self-splicing introns}}.</dd>

{{term|mobilome}} <dd>The complete set of {{gli|mobile genetic elements}} within a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, cell, species, or other taxon, including all {{gli|transposons}}, {{gli|plasmids}}, {{gli|prophages}}, and other self-splicing nucleic acid molecules.</dd>

{{term|molecular biology}} <dd>The branch of biology that studies biological activity at the molecular level, in particular the various mechanisms underlying the biological processes that occur in and between {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cells}}, including the structures, properties, synthesis, and modification of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biomolecules}} such as {{gli|proteins}} and {{gli|nucleic acids}}, their interactions with the chemical environment and with other biomolecules, and how these interactions explain the observations of classical biology (which in contrast studies biological systems at much larger scales).<ref name="Astbury">{{cite journal|last=Astbury|first=W. T.|date=June 1961|title=Molecular Biology or Ultrastructural Biology ?|journal=Nature|language=en|volume=190|issue=4781|page=1124|doi=10.1038/1901124a0|pmid=13684868|bibcode=1961Natur.190.1124A|s2cid=4172248|issn=1476-4687|doi-access=free}}</ref> Molecular biology relies largely on laboratory techniques of physics and chemistry to manipulate and measure microscopic phenomena. It is closely related to and overlaps with the fields of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell biology}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biochemistry}}, and {{gli|molecular genetics}}.</dd>

{{term|molecular cloning}} <dd>Any of various {{gli|molecular biology}} methods designed to {{gli|replicate}} a particular molecule, usually a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} {{gli|nucleic acid sequence|sequence}} or a {{gli|protein}}, many times inside the cells of a natural host. Commonly, a {{gli|recombinant DNA}} fragment containing a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene of interest}} is {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligated}} into a {{gli|plasmid}} {{gli|vector}}, which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|competent}} bacterial cells are then induced to uptake in a process known as {{gli|transformation}}. The bacteria, carrying the recombinant plasmid, are then allowed to proliferate naturally in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell culture}}, so that each time the bacterial cells divide, the plasmids are replicated along with the rest of the bacterial genome. Any functioning gene of interest within the plasmid will be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}} by the bacterial cells, and thereby its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene products}} will also be cloned. The plasmids or gene products, which now exist in many copies, may then be extracted from the bacteria and purified. Molecular cloning is a fundamental tool of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering}} employed for a wide variety of purposes, often to study {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}}, to amplify a specific gene product, or to generate a {{gli|selectable marker|selectable phenotype}}.</dd> 400px|thumb|right|An outline of how '''{{gli|molecular cloning}}''' works

{{term|molecular genetics}} <dd>A branch of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetics}} that employs methods and techniques of {{gli|molecular biology}} to study the structure and function of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene products}} at the molecular level. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|classical genetics}}''.</dd>

{{term|monad}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|haploid}} set of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} as it exists inside the {{gli|nucleus}} of an immature {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gamete|gametic cell}} such as an ootid or spermatid, i.e. a cell which is a product of {{gli|meiosis}} but is not yet a mature gamete.<ref name="DoG7"/></dd>

{{term|monocentric}} <dd>(of a linear {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} or chromosome fragment) Having only one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromere}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|dicentric}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|holocentric}}''.</dd>

{{term|monoclonal}} <dd>Describing cells, proteins, or molecules descended or derived from a single {{gli|glossary=Glossary of cellular and molecular biology (0–L)|clone}} (i.e. from the same genome or genetic lineage) or made in response to a single unique compound. Monoclonal {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antibodies}} are raised against only one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antigen}} or can only recognize one unique {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epitope}} on the same antigen. Similarly, the cells of some {{gli|tissues}} and {{gli|neoplasms}} may be described as monoclonal if they are all the asexual progeny of one original {{gli|parent cell}}.<ref name="MacLean"/> Contrast ''{{gli|polyclonal}}''.</dd>

{{term|monocyte}}{{anchor|monocytes}} <dd>A type of large {{gli|glossary=Glossary of cellular and molecular biology (0–L)|leukocyte}} of the mononuclear phagocyte system in mammals, characterized by pale-staining cytoplasm and a kidney-shaped or horseshoe-shaped nucleus. Monocytes are derived from {{gli|pluripotent}} {{gli|stem cells}} in bone marrow and become {{gli|macrophages}} in other tissues.<ref name="Oxford B&MB"/></dd>

{{term|monokaryotic}} <dd>(of a cell) Having a single {{gli|nucleus}}, as opposed to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anucleate|no nucleus}} or {{gli|multinucleate|multiple nuclei}}.</dd>

{{term|monomer}}{{anchor|monomers}} <dd>A molecule or compound which can exist individually or serve as a building block or {{gli|subunit}} of a larger {{gli|macromolecular}} aggregate known as a {{gli|polymer}}.<ref name="Alberts et al."/> Polymers form when multiple monomers of the same or similar molecular species are connected to each other by chemical bonds, either in a linear chain or a non-linear conglomeration. Examples include the individual {{gli|nucleotides}} which form {{gli|nucleic acid}} polymers, the individual {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} which form {{gli|polypeptides}}, and the individual proteins which form {{gli|protein complexes}}.</dd>

{{term|monoploid}}{{anchor|monoploidy}} <dd></dd>

{{term|monosaccharide}}{{anchor|monosaccharides}} <dd>Any of a class of organic compounds which are the simplest forms of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|carbohydrates}} and the most basic structural subunits or {{gli|monomers}} from which larger carbohydrate {{gli|polymers}} such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|disaccharides}}, {{gli|oligosaccharides}}, and {{gli|polysaccharides}} are composed. With few exceptions, all monosaccharides are variations on the empirical formula {{chem|(|C|H|2|O|)|''n''}}, where {{chem|''n''}} typically ranges from 3 (trioses) to 7 (heptoses).<ref name="Lackie"/> Common examples include {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glucose}}, {{gli|ribose}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}}.</dd>

{{term|monosomy}} <dd>The abnormal and frequently pathological presence of only one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} of a normal {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} pair. It is a type of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|aneuploidy}}.</dd>

{{term|Morpholino}} {{ghat|Also '''phosphorodiamidate Morpholino oligomer'''.}} <dd>A synthetic {{gli|nucleic acid analogue}} connecting a short sequence of {{gli|nucleobases}} into an artificial {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antisense}} {{gli|oligomer}}, used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering}} to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|knockdown}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|pairing}} with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} sequences in naturally occurring RNA or DNA molecules, especially {{gli|mRNA|mRNA transcripts}}, thereby inhibiting interactions with other biomolecules such as proteins and {{gli|ribosomes}}. Morpholino oligomers are not themselves {{gli|translated}}, and neither they nor their hybrid duplexes with RNA are attacked by {{gli|nucleases}}; also, unlike the negatively charged {{gli|phosphate backbone|phosphates}} of normal nucleic acids, the synthetic backbones of Morpholinos are electrically neutral, making them less likely to interact non-selectively with a host cell's charged proteins. These properties make them useful and reliable tools for artificially generating {{gli|mutant}} phenotypes in living cells.<ref name="DoG7"/></dd>

{{term|mosaicism}} <dd>The presence of two or more populations of cells with different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genotypes}} in an individual organism which has developed from a single fertilized egg. A mosaic organism can result from many kinds of genetic phenomena, including {{gli|nondisjunction}} of chromosomes, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endoreduplication}}, or mutations in individual {{gli|stem cell}} lineages during the early development of the embryo. Mosaicism is similar to but distinct from {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chimerism}}.</dd>

{{term|motif}}{{anchor|motifs}} <dd>Any distinctive or recurring {{gli|sequence}} of {{gli|nucleotides}} in a {{gli|nucleic acid}} or of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} in a {{gli|peptide}} that is or is conjectured to be biologically significant, especially one that is reliably {{gli|recognition site|recognized}} by other biomolecules or which has a {{gli|secondary structure|three-dimensional structure}} that permits unique or characteristic chemical interactions such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA binding}}.<ref name="DoG7"/> In nucleic acids, motifs are often short (three to ten nucleotides in length), highly {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conserved sequences}} which act as recognition sites for {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding proteins}} or RNAs involved in the regulation of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}}.</dd>

{{term|motor protein}}{{anchor|motor proteins}} <dd>Any {{gli|protein}} which converts chemical energy derived from the hydrolysis of {{gli|nucleoside triphosphates}} such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|GTP}} into mechanical work in order to effect its own locomotion, by propelling itself along a filament or through the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}.<ref name="Alberts et al."/></dd>

{{term|mRNA}} <dd>See ''{{gli|mRNA|messenger RNA}}''.</dd>

{{term|mtDNA}} <dd>See ''{{gli|mtDNA|mitochondrial DNA}}''.</dd>

{{term|multicellular}} <dd>Composed of more than one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell}}. The term is used especially to describe organisms or tissues consisting of many cells descendant from the same original {{gli|parent cell}} which work together in an organized way, but may also describe groups of nominally single-celled organisms such as protists and bacteria which live symbiotically with each other in large colonies. Contrast ''{{gli|unicellular}}''.</dd>

{{term|multimer}}{{anchor|multimeric}} <dd>An aggregate of two or more molecular entities, identical or non-identical, held together by non-covalent forces;<ref name="Oxford B&MB"/> e.g. a {{gli|protein complex}}.</dd>

{{term|multinucleate}} <dd>(of a cell) Having more than one {{gli|nucleus}} within a single {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell}}; i.e. having multiple nuclei occupying the same {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}.</dd>

{{term|multiomics}} <dd>The integration of data from multiple "omics" technologies (e.g. data from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epigenome}}, {{gli|transcriptome}}, {{gli|proteome}}, {{gli|metabolome}}, etc.) in order to study complex biological relationships, discover novel associations between biological entities, pinpoint relevant {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biomarkers}}, or build elaborate models of physiology and disease.</dd>

{{term|multiple cloning site (MCS)}}{{anchor|polylinker}} {{ghat|Also '''polylinker'''.}} <dd>A locus or sequence within a {{gli|plasmid}} {{gli|vector}} which contains multiple unique {{gli|restriction sites}} recognized by various {{gli|restriction endonucleases}}, which makes it possible for scientists to target the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|insertion}} of a DNA fragment (often a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene cassette}}) specifically to that locus and in the desired orientation, by {{gli|restriction digest|digesting}} the insert and the vector with the same endonuclease(s) and then {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligating}} them together via compatible restriction ends, a technique known as {{gli|restriction cloning}}.<ref name="Oxford B&MB"/> Commercial plasmids designed for cloning commonly incorporate one or more multiple cloning sites.</dd>

{{term|mutagen}}{{anchor|mutagens}} <dd>Any physical or chemical agent that {{gli|mutagenesis|changes the genetic material}} (usually {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}}) of an organism and thereby increases the frequency of {{gli|mutations}} above natural background levels.</dd>

{{term|mutagenesis}} <dd>1. The process by which the genetic information of an organism is changed, resulting in a {{gli|mutation}}. Mutagenesis may occur spontaneously or as a result of exposure to a {{gli|mutagen}}.</dd> <dd>2. In {{gli|molecular biology}}, any laboratory technique by which one or more genetic mutations are deliberately {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering|engineered}} in order to produce a {{gli|mutant}} gene, regulatory element, gene product, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetically modified organism}} so that the functions of a genetic locus, process, or product can be studied in detail.</dd>

{{term|mutant}}{{anchor|mutants}} <dd>An organism, gene product, or phenotypic {{gli|glossary=Glossary of genetics and evolutionary biology|trait}} resulting from a {{gli|mutation}}, of a type that would not be observed naturally in {{gli|wild-type}} specimens.</dd>

{{term|mutation}}{{anchor|mutations|mutate|mutates|mutated}} <dd>Any permanent change in the {{gli|nucleotide sequence}} of a strand of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}}, or in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} sequence of a {{gli|peptide}}. Mutations play a role in both normal and abnormal biological processes; their natural occurrence is integral to the process of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|evolution}}. They can result from errors in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replication}}, chemical damage, exposure to high-energy radiation, or manipulations by {{gli|mobile genetic element|mobile genetic elements}}. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA repair|Repair}} mechanisms have evolved in many organisms to correct them. By understanding the effect that a mutation has on {{gli|phenotype}}, it is possible to establish the function of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or sequence in which it occurs.</dd>

{{term|mutator gene}} <dd>Any mutant {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or sequence that increases the spontaneous {{gli|mutation rate}} of one or more other genes or sequences. Mutators are often {{gli|transposable elements}}, or may be mutant {{gli|glossary=Glossary of cellular and molecular biology (0–L)|housekeeping genes}} such as those that encode {{gli|glossary=Glossary of cellular and molecular biology (0–L)|helicases}} or proteins involved in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA proofreading|proofreading}}.<ref name="DoG7"/></dd>

{{term|mutein}} <dd>A {{gli|mutant}} {{gli|protein}}, i.e. a protein whose {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} sequence differs from that of the normal because of a {{gli|mutation}}.</dd>

{{term|muton}} <dd>The smallest unit of a DNA molecule in which a physical or chemical change can result in a {{gli|mutation}} (conventionally a single {{gli|nucleotide}}).<ref name="DoG7"/></dd>

==N== {{glossary}} {{term|''n'' orientation}} <dd>One of two possible orientations by which a linear DNA fragment can be inserted into a {{gli|vector}}, specifically the one in which the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene maps}} of both fragment and vector have the same orientation.<ref name="DoG7"/> Contrast ''{{gli|u orientation}}''.</dd>

{{term|NAD}} <dd>See ''{{gli|nicotinamide-adenine dinucleotide}}''.</dd>

{{term|NADP}} <dd>See ''{{gli|nicotinamide-adenine dinucleotide phosphate}}''.</dd>

{{term|nanoinjection}} <dd>A laboratory technique involving the use of a microscopic lance or nanopipette (typically about 100 nanometres in diameter) in the presence of an electric field in order to deliver DNA or RNA directly into a cell, often a {{gli|zygote}} or early {{gli|embryo}}, via an {{gli|electrophoretic}} mechanism. While submerged in a pH-buffered solution, a positive electric charge is applied to the lance, attracting negatively charged nucleic acids to its surface; the lance then penetrates the cell membrane and the electric field is reversed, applying a negative charge which repels the accumulated nucleic acids away from the lance and thus into the cell. Compare ''{{gli|microinjection}}''.</dd>

{{term|nascent}} <dd>In the process of being synthesized; incomplete; not yet fully processed or mature. The term is commonly used to describe {{gli|strands}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}} which are actively undergoing synthesis during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replication}} or {{gli|transcription}}, or sometimes a complete, fully transcribed RNA molecule before any {{gli|post-transcriptional modifications}} have been made (e.g. {{gli|polyadenylation}} or {{gli|RNA editing}}), or a {{gli|peptide}} chain actively undergoing {{gli|translation}} by a {{gli|ribosome}}.<ref name="DoG7"/></dd>

{{term|ncAA}} <dd>See ''{{gli|non-canonical amino acid}}''.</dd>

{{term|ncDNA}} <dd>See ''{{gli|ncDNA|non-coding DNA}}''.</dd>

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

{{term|negative (-) sense strand}} <dd>See ''{{gli|template strand}}''.</dd>

{{term|negative control}} {{ghat|Also '''negative regulation'''.}} <dd>The inhibition or deactivation of some biological process caused by the presence of a specific molecular entity (e.g. a {{gli|repressor}}), in the absence of which the process is not inhibited and thus can proceed normally.<ref name="Oxford B&MB"/> In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation}}, for example, a repressor may bind to an {{gli|operator}} upstream from a coding sequence and prevent access by {{gli|transcription factors}} and/or {{gli|RNA polymerase}}, thereby blocking the gene's {{gli|transcription}}. This is contrasted with {{gli|positive control}}, in which the presence of an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inducer}} is necessary to switch on transcription.<ref name="Rieger">{{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|negative supercoiling}} <dd>The {{gli|supercoiling}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecule in the direction opposite to the turn of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double helix}} itself (e.g. a left-handed coiling of a helix with a right-handed turn).<ref name="Rieger"/> Contrast ''{{gli|positive supercoiling}}''.</dd>

{{term|next-generation sequencing (NGS)}} <dd>See ''{{gli|massively parallel sequencing}}''.</dd>

{{term|nick}}{{anchor|nicks|nicking|nicked}} <dd>A break or discontinuity in the {{gli|phosphate backbone}} of one {{gli|strand}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecule, i.e. where a {{gli|phosphodiester bond}} is hydrolyzed but no nucleotides are removed; such a molecule is said to be ''nicked''. A nick is a {{gli|single-strand break}}, where despite the break the DNA molecule is not ultimately broken into multiple fragments, which contrasts with a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cut}}, where {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-strand break|both strands are broken}}. Nicks may be caused by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA damage}} or by dedicated {{gli|nucleases}} known as {{gli|nicking enzymes}}, which nick DNA at random or specific sites. Nicks are frequently placed by the cell as markers identifying target sites for enzyme activity, including in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}, {{gli|transcription}}, and {{gli|mismatch repair}}, and also to release torsional stress from {{gli|overwinding|overwound}} DNA molecules, making them important in manipulating {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA topology}}.<ref name="Rieger"/></dd>

{{term|nick translation}} <dd></dd>

{{term|nickase}} <dd>Another name for a {{gli|nicking enzyme}}, especially one that has been artificially engineered to create single-stranded breaks (i.e. {{gli|nicks}}) by altering the cleavage activity of an endonuclease that normally creates double-stranded breaks, e.g. Cas9 nickase (nCas9).<ref>{{cite journal|last1=Klermund|first1=Julia|last2=Rhiel|first2=Manuel|last3=Kocher|first3=Thomas|last4=Chmielewski|first4=Kay Ole|last5=Bischof|first5=Johannes|last6=Andrieux|first6=Geoffroy|last7=el Gaz|first7=Melina|last8=Hainzl|first8=Stefan|last9=Boerries|first9=Melanie|last10=Cornu|first10=Tatjana I.|last11=Koller|first11=Ulrich|last12=Cathomen|first12=Toni|title=On- and off-target effects of paired CRISPR-Cas nickase in primary human cells|journal=Molecular Therapy|pages=1298–1310|doi=10.1016/j.ymthe.2024.03.006|date=May 2024 |volume=32 |issue=5 |pmid=38459694 |pmc=11081867 }}</ref></dd>

{{term|nicking enzyme}}{{anchor|nicking enzymes}} {{ghat|Also '''nicking endonuclease''' and '''nickase'''.}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endonuclease}} enzymes capable of generating a {{gli|single-stranded break}} in a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecule, i.e. a {{gli|nick}}, either at random or at a specific {{gli|recognition sequence}}, by breaking a {{gli|phosphodiester bond}} linking adjacent nucleotides.</dd>

{{term|nicotinamide adenine dinucleotide (NAD)}}{{anchor|NAD}} <dd></dd>

{{term|nicotinamide adenine dinucleotide phosphate|nicotinamide adenine dinucleotide phosphate (NADP+, NADP)}} <dd></dd>

{{term|nitrogenous base}}{{anchor|nitrogenous bases}} {{ghat|Sometimes used interchangeably with '''{{gli|nucleobase}}''' or simply '''base'''.}} <dd>Any organic compound containing a nitrogen atom that has the chemical properties of a base. {{gli|nucleobase|Five particular nitrogenous bases}} – {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine}} ({{font|A|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine}} ({{font|G|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine}} ({{font|C|font=courier|size=big}}), {{gli|thymine}} ({{font|T|font=courier|size=big}}), and {{gli|uracil}} ({{font|U|font=courier|size=big}}) – are especially relevant to biology because they are components of {{gli|nucleotides}}, which are the primary {{gli|monomers}} that make up {{gli|nucleic acids}}.</dd>

{{term|non-canonical amino acid (ncAA)}}{{anchor|non-canonical amino acid|non-canonical amino acids|non-standard amino acid|non-standard amino acids}} {{ghat|Also '''non-standard amino acid'''.}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}}, natural or artificial, that is not one of the 20 or 21 {{gli|proteinogenic amino acids}} encoded by the {{gli|standard genetic code}}. There are hundreds of such amino acids, many of which have biological functions and are specified by alternative codes or incorporated into proteins accidentally by {{gli|mistranslation|errors in translation}}. Many of the best known naturally occurring ncAAs occur as intermediates in the metabolic pathways leading to the standard amino acids, while others have been made synthetically in the laboratory.<ref>"[https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Boundless)/07%3A_Microbial_Genetics/7.06%3A_Translation-_Protein_Synthesis/7.6D%3A_The_Incorporation_of_Nonstandard_Amino_Acids 7.6D: The Incorporation of Nonstandard Amino Acids]". ''LibreTexts Biology''.</ref></dd>

{{term|non-coding DNA (ncDNA)}} <dd>Any segment of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} that does not {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic code|encode}} a sequence that may ultimately be {{gli|transcribed}} and {{gli|translated}} into a {{gli|protein}}. In most organisms, only a small fraction of the genome consists of protein-coding DNA, though the proportion varies greatly between species. Some non-coding DNA may still be transcribed into functional {{gli|ncRNA|non-coding RNA}} (as with {{gli|tRNA|transfer RNAs}}) or may serve important developmental or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|regulatory}} purposes; other regions (as with so-called "{{gli|glossary=Glossary of cellular and molecular biology (0–L)|junk DNA}}") appear to have no known biological function.</dd>

{{term|non-coding RNA (ncRNA)}}{{anchor|non-coding RNA|non-coding RNAs}} <dd>Any molecule of {{gli|RNA}} that is not ultimately {{gli|translated}} into a {{gli|protein}}. The {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} sequence from which a functional non-coding RNA is {{gli|transcribed}} is often referred to as an "RNA gene". Numerous types of non-coding RNAs essential to normal genome function are produced constitutively, including {{gli|tRNA|transfer RNA}} (tRNA), {{gli|rRNA|ribosomal RNA}} (rRNA), {{gli|miRNA|microRNA}} (miRNA), and {{gli|siRNA|small interfering RNA}} (siRNA); other non-coding RNAs (sometimes described as "junk RNA") have no known function and are likely the product of spurious transcription.</dd>

{{term|non-coding strand}} <dd>See ''{{gli|template strand}}''.</dd>

{{term|nondisjunction}} <dd>The failure of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous chromosomes}} or {{gli|sister chromatids}} to {{gli|segregate}} properly during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}}. Nondisjunction results in daughter cells that are {{gli|glossary=Glossary of cellular and molecular biology (0–L)|aneuploid}}, containing abnormal numbers of one or more specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}. It may be caused by a variety of factors.</dd>

{{term|non-homologous end joining (NHEJ)}} <dd></dd>

{{term|nonrepetitive sequence}} <dd>Broadly, any {{gli|nucleotide sequence}} or region of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} that does not contain {{gli|repeated sequences}}, or in which repeats do not comprise a majority; or any segment of DNA exhibiting the {{gli|reassociation kinetics}} expected of a unique sequence.<ref name="DoG7"/></dd>

{{term|nonsense mutation}} {{ghat|Also '''point-nonsense mutation'''.}} <dd>A type of {{gli|point mutation}} which results in a premature {{gli|stop codon}} in the {{gli|transcribed}} {{gli|mRNA}} sequence, thereby causing the premature termination of {{gli|translation}}, which results in a truncated, incomplete, and often non-functional {{gli|protein}}.</dd>

{{term|nonsense suppressor}} <dd>A factor which can inhibit the effects of a {{gli|nonsense mutation}} (i.e. a premature stop codon) by any mechanism, usually either a mutated {{gli|transfer RNA}} which can bind the mutated stop codon or some kind of {{gli|ribosomal}} mutation.<ref>{{cite book| last1 = Hartwell| first1 = Leland |first2=L. |last2=Hood |first3=M. |last3=Goldberg |first4=A. |last4=Reynolds |first5=L. |last5=Silver |first6=R. |last6=Veres |title = Genetics: From Genes to Genomes| publisher = McGraw-Hill| year = 2004 | url = http://highered.mcgraw-hill.com/sites/0072919302/information_center_view0/| isbn = 978-0-07-246248-7| page = 267 |oclc=50417228 }}</ref></dd>

{{term|nonsynonymous mutation}} {{ghat|Also '''nonsynonymous substitution''' or '''replacement mutation'''.}} <dd>A type of {{gli|mutation}} in which the {{gli|substitution}} of one {{gli|nucleotide}} base for another results, after {{gli|transcription}} and {{gli|translation}}, in an amino acid sequence that is different from that produced by the original unmutated gene. Because nonsynonymous mutations always result in a biological change in the organism, they are often subject to strong {{gli|glossary=Glossary of evolutionary biology|selection pressure}}. Contrast ''{{gli|synonymous mutation}}''.</dd>

{{term|non-transcribed spacer (NTS)}} <dd>See ''{{gli|spacer}}''.</dd>

{{term|northern blotting}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|blotting}} method in molecular biology used to detect {{gli|RNA}} in a sample. Compare ''{{gli|Southern blotting}}'', ''{{gli|western blotting}}'', and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|eastern blotting}}''.</dd>

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

{{term|N-terminus}}{{anchor|N-terminal|N terminus|n terminus|N terminal|n terminal|n-terminal}} {{ghat|Also '''amine terminus''' and '''amino terminus'''.}} <dd>The end of a linear chain of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} (i.e. a {{gli|peptide}}) that is terminated by the free amine group ({{chem|–NH|2}}) of the first amino acid added to the chain during {{gli|translation}}. This amino acid is said to be ''N-terminal''. By convention, sequences, domains, active sites, or any other structure positioned nearer to the N-terminus of the {{gli|polypeptide}} or the folded {{gli|protein}} it forms relative to others are described as {{gli|upstream}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|C-terminus}}''.</dd>

{{term|nuclear DNA}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecule contained within the {{gli|nucleus}} of a eukaryotic cell, most prominently the DNA in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}. It is sometimes used interchangeably with {{gli|genomic DNA}}.</dd>

{{term|nuclear envelope}}{{anchor|nuclear membrane}} <dd>A sub-cellular barrier consisting of two concentric {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipid bilayer}} {{gli|membranes}} that surrounds the {{gli|nucleus}} in eukaryotic cells. The nuclear envelope is sometimes simply called the "nuclear membrane", though the structure is actually composed of two distinct membranes, an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inner nuclear membrane|inner membrane}} and an {{gli|outer nuclear membrane|outer membrane}}.</dd>

{{term|nuclear equivalence}} <dd>The principle that the nuclei of essentially all {{gli|glossary=Glossary of cellular and molecular biology (0–L)|differentiated}} cells of a mature multicellular organism are genetically identical to each other and to the nucleus of the {{gli|zygote}} from which they descended; i.e. they all contain the same genetic information on the same chromosomes, having been replicated from the original zygotic set with extremely high fidelity. Even though all adult {{gli|somatic cells}} have the same set of genes, cells can nonetheless differentiate into distinct {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell types}} by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressing}} different subsets of these genes. Though this principle generally holds true, the reality is slightly more complex, as mutations such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|insertions}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deletions}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|duplications}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|translocations}} as well as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chimerism}}, {{gli|mosaicism}}, and various types of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination}} can all cause different somatic lineages within the same organism to be genetically non-identical.</dd>

{{term|nuclear export signal (NES)}}{{anchor|nuclear export signal}} <dd></dd>

{{term|nuclear lamina}} <dd>A fibrous network of proteins lining the inner, {{gli|nucleoplasmic}} surface of the {{gli|nuclear envelope}}, composed of filaments similar to those that make up the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoskeleton}}. It may function as a scaffold for the various contents of the nucleus including {{gli|nuclear proteins}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}.<ref name="Lackie"/></dd>

{{term|nuclear localization signal (NLS)}}{{anchor|nuclear localization signal}} {{ghat|Also '''nuclear localization sequence'''.}} <dd>An {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid sequence}} within a {{gli|protein}} which serves as a molecular signal marking the protein for {{gli|nuclear transport|transport}} into the {{gli|nucleus}}, typically consisting of one or more short motifs containing positively charged amino acid residues exposed on the mature protein's surface (especially lysines and arginines). Though all proteins are {{gli|translated}} in the cytoplasm, many whose primary biological activities occur inside the nucleus (e.g. {{gli|transcription factors}}) require nuclear localization signals identifiable by {{gli|molecular chaperones}} in order to cross the {{gli|nuclear envelope}}. Contrast ''{{gli|nuclear export signal}}''.</dd>

{{term|nuclear matrix}} {{ghat|Also '''nucleoskeleton'''.}} <dd>A mesh-like latticework of protein polymers and {{gli|microfilaments}} suspended in the {{gli|nucleoplasm}} in the {{gli|nuclei}} of eukaryotic cells, akin to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoskeleton}} in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}. The nuclear matrix functions as a scaffold and an anchor for large DNA molecules such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} and for the macromolecular complexes that perform essential nuclear activities such as {{gli|transcription}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}.<ref name="Lackie"/></dd>

{{term|nuclear membrane}} <dd>See ''{{gli|nuclear envelope}}''.</dd>

{{term|nuclear pore}}{{anchor|nuclear pores|nuclear pore complex|nuclear pore complexes}} <dd>A {{gli|protein complex|complex}} of {{gli|membrane proteins}} that creates an opening in the {{gli|nuclear envelope}} through which certain molecules and ions are {{gli|nuclear transport|permitted to pass}} and thereby enter or exit the {{gli|nucleus}} (analogous to the {{gli|channel proteins}} in the {{gli|cell membrane}}). The nuclear envelope typically has thousands of pores to selectively regulate the exchange of specific materials between the {{gli|nucleoplasm}} and the {{gli|cytoplasm}}, including {{gli|mRNA|messenger RNAs}}, which are transcribed in the nucleus but must be translated in the cytoplasm, as well as {{gli|nuclear proteins}}, which are synthesized in the cytoplasm but must return to the nucleus to serve their functions.<ref name="Alberts et al."/><ref name="Lackie"/></dd>

{{term|nuclear protein}}{{anchor|nuclear proteins}} <dd>Any {{gli|protein}} that is naturally found in or {{gli|localizes}} to the cell's {{gli|nucleus}} (as opposed to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}} or elsewhere).</dd>

{{term|nuclear RNA (nRNA)}} <dd>Any {{gli|RNA}} molecule located within a cell's {{gli|nucleus}}, whether associated with chromosomes or existing freely in the nucleoplasm, including {{gli|snRNA|small nuclear RNA}} (snRNA), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|eRNA|enhancer RNA}} (eRNA), and all newly transcribed {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hnRNA|immature RNAs}}, {{gli|mRNA|coding}} or {{gli|ncRNA|non-coding}}, prior to their export to the cytosol (hnRNA).</dd>

{{term|nuclear transfer}} <dd></dd>

{{term|nuclear transport}} <dd>The mechanisms by which molecules cross the {{gli|nuclear envelope}} surrounding a cell's nucleus. Though small molecules and ions can cross the membrane freely, the entry and exit of larger molecules is tightly regulated by {{gli|nuclear pores}}, so that most {{gli|macromolecules}} such as RNAs and proteins require association with transport factors in order to be {{gli|molecular chaperone|chaperoned}} across.</dd>

{{term|nuclease}}{{anchor|nucleases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} capable of cleaving {{gli|phosphodiester bonds}} connecting adjacent {{gli|nucleotides}} in a nucleic acid molecule (the opposite of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligase}}). Nucleases may {{gli|nick|nick one}} {{gli|strand}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cut|cut both}} strands of a duplex molecule, and may cleave randomly or at specific recognition sequences. They are ubiquitous and imperative for normal cellular function, and are also widely employed in laboratory techniques.</dd>

{{term|nucleic acid}}{{anchor|nucleic acids}} <dd>A long, {{gli|polymeric}} {{gli|macromolecule}} made up of smaller {{gli|monomers}} called {{gli|nucleotides}} which are chemically linked to one another in a chain. Two specific types of nucleic acid, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} and {{gli|RNA}}, are common to all living organisms, serving to encode the genetic information governing the construction, development, and ordinary processes of all biological systems. This information, contained within the order or {{gli|nucleic acid sequence|sequence of the nucleotides}}, is {{gli|translated}} into {{gli|proteins}}, which direct all of the chemical reactions necessary for life.</dd>

{{term|nucleic acid sequence}}{{anchor|nucleic acid sequences|nucleotide sequence|nucleotide sequences|sequence|sequences|DNA sequence|DNA sequences}} <dd>The precise order of consecutively linked {{gli|nucleotides}} in a {{gli|nucleic acid}} molecule such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}}. Long sequences of nucleotides are the principal means by which biological systems store genetic information, and therefore the accurate {{gli|replication}}, {{gli|transcription}}, and {{gli|translation}} of such sequences is of the utmost importance, lest the information be lost or corrupted. Nucleic acid sequences may be equivalently referred to as sequences of nucleotides, {{gli|nitrogenous bases}}, {{gli|nucleobases}}, or, in {{gli|double-stranded|duplex molecules}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pairs}}, and they correspond directly to sequences of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codons}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}}.</dd>

{{term|nucleobase}}{{anchor|base|bases|nucleobases}} {{ghat|Sometimes used interchangeably with '''{{gli|nitrogenous base}}''' or simply '''base'''.}} <dd>Any of the five primary or canonical {{gli|nitrogenous bases}} – {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine}} ({{font|A|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine}} ({{font|G|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine}} ({{font|C|font=courier|size=big}}), {{gli|thymine}} ({{font|T|font=courier|size=big}}), and {{gli|uracil}} ({{font|U|font=courier|size=big}}) – that form {{gli|nucleosides}} and {{gli|nucleotides}}, the latter of which are the fundamental building blocks of {{gli|nucleic acids}}. The ability of these bases to form {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pairs}} via hydrogen bonding, as well as their flat, compact three-dimensional profiles, allows them to "stack" one upon another and leads directly to the long-chain structures of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} and {{gli|RNA}}. When writing sequences in shorthand notation, the letter ''N'' is often used to represent a nucleotide containing a generic or unidentified nucleobase.</dd>

{{term|nucleoid}}{{anchor|nucleoids}} {{ghat|Also '''prokaryon'''.}} <dd>An irregularly shaped region which contains most or all of the genetic material in prokaryotic cells such as bacteria, but is not enclosed by a {{gli|nuclear membrane}} as in eukaryotes.</dd>

{{term|nucleolin}} <dd>The primary {{gli|protein}} of which the eukaryotic {{gli|nucleolus}} is composed, thought to play important roles in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatin}} decondensation, transcription of {{gli|ribosomal RNA}}, and {{gli|ribosome}} assembly.</dd>

{{term|nucleolonema}} <dd>The central region of the {{gli|nucleolus}}, composed of dense, convoluted fibrillar material.<ref name="MacLean"/></dd>

{{term|nucleolus}}{{anchor|nucleoli}} <dd>An {{gli|organelle}} within the {{gli|nucleus}} of eukaryotic cells which is composed of proteins, DNA, and RNA and serves as the site of {{gli|ribosome}} synthesis.</dd>

{{term|nucleoplasm}}{{anchor|nucleoplasmic}} {{ghat|Also '''karyoplasm'''.}} <dd>All of the material enclosed within the {{gli|nucleus}} of a cell by the {{gli|nuclear envelope}}, analogous to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}} enclosed by the main {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}}. Like the cytoplasm, the nucleoplasm is composed of a gel-like substance (the {{gli|nucleosol}}) in which various {{gli|organelles}}, {{gli|nuclear proteins}}, and other {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biomolecules}} are suspended, including nuclear DNA in the form of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}, the {{gli|nucleolus}}, {{gli|nuclear bodies}}, and free {{gli|nucleotides}}.</dd>

{{term|nucleoprotein}}{{anchor|nucleoproteins}} <dd>Any {{gli|protein}} that is chemically bonded to or conjugated with a {{gli|nucleic acid}}. Examples include {{gli|ribosomes}}, {{gli|nucleosomes}}, and many {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}}.</dd>

{{term|nucleosidase}}{{anchor|nucleosidases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} which catalyze the decomposition of {{gli|nucleosides}} into their component {{gli|nitrogenous bases}} and {{gli|pentose}} sugars.<ref name="DoG7"/></dd>

{{term|nucleoside}}{{anchor|nucleosides}} <dd>An organic molecule composed of a {{gli|nitrogenous base}} bonded to a {{gli|pentose|five-carbon sugar}} (either {{gli|ribose}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}}). A {{gli|nucleotide}} additionally includes one or more {{gli|phosphate groups}}.</dd>

{{term|nucleosol}} {{ghat|Also '''karyolymph''' or '''nuclear hyaloplasm'''.}} <dd>The soluble, liquid portion of the {{gli|nucleoplasm}} (analogous to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosol}} of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}).</dd>

{{term|nucleosome}}{{anchor|nucleosomes|nucleosomal}} <dd>The basic structural subunit of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatin}} used in {{gli|DNA packaging|packaging}} nuclear DNA such as chromosomes, consisting of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|histone octamer|core particle of eight}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|histone}} proteins around which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} is wrapped in a manner akin to thread wound around a spool. The technical definition of a nucleosome includes a segment of DNA about 146 base pairs in length which makes 1.67 left-handed turns as it coils around the histone core, as well as a stretch of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|linker DNA}} (generally 38–80 bp) connecting it to an adjacent core particle, though the term is often used to refer to the core particle alone. Long series of nucleosomes are further condensed by association with histone H1 into higher-order structures such as {{gli|30-nm fibers}} and ultimately {{gli|supercoiled}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatids}}. Because the histone–DNA interaction limits access to the DNA molecule by other proteins and RNAs, the precise positioning of nucleosomes along the DNA sequence plays a fundamental role in controlling whether or not genes are {{gli|transcribed}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}}, and hence mechanisms for moving and ejecting nucleosomes have evolved as a means of {{gli|regulating}} the expression of particular loci.</dd>

{{term|nucleosome-depleted region (NDR)}} <dd>A region of a genome or chromosome in which long segments of DNA are bound by few or no {{gli|nucleosomes}}, and thus exposed to manipulation by other proteins and molecules, especially implying that the region is {{gli|transcriptionally}} active.</dd>

{{term|nucleotidase}} <dd>Any of a class of phosphoric monoester hydrolase {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} which catalyze the hydrolysis of a {{gli|nucleotide}} into a {{gli|nucleoside}} and {{gli|orthophosphate}}. Such enzymes may or may not distinguish {{gli|ribonucleotides}} from {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonucleotides}}, i.e. their function may not be sugar-specific.<ref name="Oxford B&MB"/></dd>

{{term|nucleotide}}{{anchor|nucleotides}} {{ghat|Also '''nucleoside monophosphate (NMP)'''.}} <dd>An organic molecule that serves as the fundamental {{gli|monomer}} or subunit of {{gli|nucleic acid}} polymers, including {{gli|RNA}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}}. Each nucleotide is composed of three connected functional groups: a {{gli|nitrogenous base}}, a {{gli|pentose|five-carbon sugar}} (either {{gli|ribose}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}}), and a single {{gli|phosphate group}}. Though technically distinct, the term "nucleotide" is often used interchangeably with nitrogenous base, {{gli|nucleobase}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pair}} when referring to the {{gli|nucleic acid sequence|sequences}} that make up nucleic acids. Compare ''{{gli|nucleoside}}''.</dd> 450px|thumb|right|The '''{{gli|nucleobases}}''' (blue) are the five specific {{gli|nitrogenous bases}} canonically used in DNA and RNA. A nucleobase bonded to a {{gli|pentose}} sugar (either {{gli|ribose}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}}; yellow) is known as a '''{{gli|nucleoside}}''' (yellow + blue). A nucleoside bonded to a single {{gli|phosphate group}} (red) is known as a nucleoside monophosphate (NMP) or a '''{{gli|nucleotide}}''' (red + yellow + blue). When not incorporated into a nucleic acid chain, free nucleosides can bind multiple phosphate groups: two phosphates yields a {{gli|nucleoside diphosphate}} (NDP), and three yields a {{gli|nucleoside triphosphate}} (NTP).

{{term|nucleotide sequence}} <dd>See ''{{gli|nucleic acid sequence}}''.</dd>

{{term|nucleus}}{{anchor|nuclei|nuclear}} {{ghat|pl. '''nuclei'''}} <dd>A large spherical or lobular {{gli|organelle}} surrounded by a {{gli|nuclear envelope|dedicated membrane}} which functions as the main storage compartment for the genetic material of eukaryotic cells, including the DNA comprising {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}, as well as the site of RNA synthesis during {{gli|transcription}}. The vast majority of eukaryotic cells have a single nucleus, though some cells may have {{gli|multinucleate|more than one nucleus}}, either temporarily or permanently, and in some organisms there exist certain cell types (e.g. mammalian {{gli|glossary=Glossary of cellular and molecular biology (0–L)|erythrocytes}}) which lose their nuclei upon reaching maturity, effectively becoming {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anucleate}}. The nucleus is one of the defining features of eukaryotes; the cells of prokaryotes such as bacteria lack nuclei entirely.<ref name="MacLean"/></dd>

==O== {{glossary}} {{term|occluding junction}} <dd>See ''{{gli|tight junction}}''.</dd>

{{term|ochre}} <dd>One of three {{gli|stop codons}} used in the {{gli|standard genetic code}}; in {{gli|RNA}}, it is specified by the nucleotide triplet {{font|UAA|font=courier|size=big}}. The other two stop codons are named {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amber}} and {{gli|opal}}.</dd>

{{term|Okazaki fragments}} <dd>{{gli|oligonucleotide|Short sequences}} of {{gli|nucleotides}} which are synthesized discontinuously by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerase}} and later linked together by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA ligase}} to create the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lagging strand}} during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}. Okazaki fragments are the consequence of the unidirectionality of DNA polymerase, which only works in the 5' to 3' direction.</dd>

{{term|oligo dT}} <dd>A short, {{gli|single-stranded DNA}} {{gli|oligonucleotide}} consisting of a sequence of repeating {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxythymidine}} (dT) nucleotides. Oligo dTs are commonly {{gli|glossary=Glossary of cellular and molecular biology (0–L)|de novo synthesis|synthesized ''de novo''}} to be used as primers for ''in vitro'' {{gli|reverse transcription}} reactions during {{gli|rtPCR}} techniques, where short chains of 12 to 18 thymine bases readily complement the {{gli|poly(A) tails}} of {{gli|mature}} {{gli|messenger RNAs}}, allowing the selective amplification and preparation of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cDNA}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|library}} from a pool of coding transcripts.<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|oligogene}} <dd></dd>

{{term|oligomer}}{{anchor|oligomers|oligomeric}} <dd>Any {{gli|polymeric}} molecule consisting of a relatively short series of connected {{gli|monomers}} or {{gli|subunits}}; e.g. an {{gli|oligonucleotide}} is a short series of nucleotides.</dd>

{{term|oligonucleotide}}{{anchor|oligonucleotides|oligo|oligos}} {{ghat|Also abbreviated '''oligo'''.}} <dd>A relatively short chain of {{gli|nucleic acid}} {{gli|residues}}. In the laboratory, oligonucleotides are commonly used as {{gli|primers}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridization probes}} to detect the presence of larger {{gli|mRNA}} molecules or assembled into two-dimensional microarrays for {{gli|glossary=Glossary of cellular and molecular biology (0–L)|high-throughput}} {{gli|sequencing}} analysis.</dd>

{{term|oligosaccharide}}{{anchor|oligosaccharides}} <dd>A {{gli|polymeric}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|carbohydrate}} molecule consisting of a relatively short chain of connected {{gli|monosaccharides}}. Oligosaccharides have important functions in processes such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell signaling}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell adhesion}}. Longer chains are called {{gli|polysaccharides}}.</dd>

{{term|omics}} <dd>A suffix used to describe any of the diverse fields of study that conduct rigorous, systematic analyses of any of the "omes", e.g. the {{gli|genome}}, {{gli|transcriptome}}, {{gli|proteome}}, {{gli|metabolome}}, etc.,<ref name="Oxford B&MB"/> each of which represents the totality of a specific class of biological content that has been or could hypothetically be isolated from an individual cell, population of cells, organism, species, or some other particular context. Thus {{gli|genomics}} is the field of study which analyzes the totality of genes in a genome, {{gli|proteomics}} studies the complete set of all of the proteins in a proteome, etc. The term may also be used to refer to all of these fields collectively.</dd>

{{term|oncogene}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} that has the potential to cause cancer. In tumor cells, such genes are often {{gli|mutated}} and/or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}} at abnormally high levels.</dd>

{{term|one gene–one polypeptide}} {{ghat|Also '''one gene–one protein''' or '''one gene–one enzyme'''.}} <dd>The hypothesis that there exists a large class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} in which each particular gene directs the synthesis of one particular {{gli|polypeptide}} or {{gli|protein}}.<ref name="DoG7"/> Historically it was thought that all genes and proteins might follow this rule by definition, but it is now known that many proteins are composites of different polypeptides and therefore the product of multiple genes, and also that some genes do not encode polypeptides at all but instead produce {{gli|ncRNA|non-coding RNAs}}, which are never translated.</dd>

{{term|opal}} {{ghat|Also '''umber'''.}} <dd>One of three {{gli|stop codons}} used in the {{gli|standard genetic code}}; in {{gli|RNA}}, it is specified by the nucleotide triplet {{font|UGA|font=courier|size=big}}. The other two stop codons are named {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amber}} and {{gli|ochre}}.</dd>

{{term|open chromatin}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|euchromatin}}''.</dd>

{{term|open reading frame (ORF)}}{{anchor|open reading frames|ORF|ORFs}} <dd>The part of a {{gli|reading frame}} that has the ability to be {{gli|translated}} from DNA or RNA into protein; any continuous stretch of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codons}} that contains a {{gli|start codon}} and a {{gli|stop codon}}.</dd>

{{term|operator}}{{anchor|operators}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation|regulatory}} sequence within an {{gli|operon}}, typically located between the {{gli|promoter}} sequence and the {{gli|structural genes}} of the operon, to which an uninhibited {{gli|repressor}} protein can bind, thereby physically obstructing {{gli|RNA polymerase}} from initiating the {{gli|transcription}} of adjacent cistrons.<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></dd>

{{term|operon}}{{anchor|operons}} <dd>A functional unit of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} consisting of a cluster of neighboring {{gli|structural genes}} which are collectively under the control of a single {{gli|promoter}}, along with one or more adjacent {{gli|regulatory}} sequences such as {{gli|operators}}. The set of genes is {{gli|transcribed}} together, resulting in a single {{gli|polycistronic}} {{gli|mRNA|messenger RNA}} molecule encoding multiple distinct polypeptides which may then be {{gli|translated}} together or undergo {{gli|RNA splicing|splicing}} to create multiple mRNAs which are translated independently; the result is that the genes contained in the operon are either expressed together or not at all. Regulatory proteins, including {{gli|repressors}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|activators}}, usually bind specifically to the regulatory sequences of a given operon; by some definitions, the genes that code for these regulatory proteins are also considered part of the operon.</dd>

{{term|opsonin}}{{anchor|opsonins}} <dd>Any substance, especially certain blood-serum proteins such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|immunoglobulins}}, that in binding to the surface of foreign cells or particulate matter increases the susceptibility of the foreign material to {{gli|phagocytosis}} by {{gli|phagocytes}}.<ref name="Lackie"/> Opsonins work by linking foreign particles to specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell-surface receptor|receptors}} on the surface of phagocytic cells in a process known as ''opsonization''.<ref name="Oxford B&MB"/></dd>

{{term|organelle}}{{anchor|organelles}} <dd>A spatially distinct compartment or subunit within a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell}} which has a specialized function. Organelles occur in both prokaryotic and eukaryotic cells. In the latter they are often separated from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}} by being enclosed with their own {{gli|membrane}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipid bilayer|bilayer}} (whence the term {{gli|membrane-bound organelles}}), though organelles may also be functionally specific areas or structures without a surrounding membrane; some cellular structures which exist partially or entirely outside of the cell membrane, such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cilia}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|flagella}}, are also referred to as organelles. There are numerous types of organelles with a wide variety of functions, including the various compartments of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endomembrane system}} (e.g. the {{gli|nuclear envelope}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endoplasmic reticulum}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|Golgi apparatus}}), {{gli|mitochondria}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chloroplasts}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lysosomes}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endosomes}}, and {{gli|vacuoles}}, among others. Many organelles are unique to particular cell types or species.</dd>

{{term|origin of replication (ORI)}}{{anchor|replication origin|origin}} {{ghat|Also '''replication origin''' or simply '''origin'''.}} <dd>A particular location within a DNA molecule at which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} is initiated. Origins are usually defined by the presence of a particular replicator sequence or by specific chromatin patterns.</dd>

{{term|osmosis}}{{anchor|osmotic}} <dd></dd>

{{term|osmotic shock}} {{ghat|Also '''osmotic stress'''.}} <dd>Physiological dysfunction caused by a sudden change in the concentration of dissolved solutes in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extracellular}} environment surrounding a cell, which provokes the rapid movement of water across the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}} by {{gli|osmosis}}, either into or out of the cell. In a severely {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hypertonic}} environment, where extracellular solute concentrations are extremely high, osmotic pressure may force large quantities of water to move out of the cell ({{gli|plasmolysis}}), leading to its desiccation; this may also have the effect of inhibiting transport of solutes into the cell, thus denying it the substrates necessary to sustain normal cellular activities. In a severely {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hypotonic}} environment, where extracellular solute concentrations are much lower than intracellular concentrations, water is forced to move into the cell ({{gli|turgescence}}), causing it to swell in size and potentially burst, or triggering {{gli|glossary=Glossary of cellular and molecular biology (0–L)|apoptosis}}.</dd>

{{term|outron}} <dd>A sequence near the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5'-end}} of a {{gli|primary transcript|primary mRNA transcript}} that is removed by a special form of {{gli|RNA splicing|splicing}} during {{gli|post-transcriptional modification|post-transcriptional processing}}. Outrons are located entirely outside of the transcript's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exon|coding sequences}}, unlike {{gli|glossary=Glossary of cellular and molecular biology (0–L)|introns}}.</dd>

{{term|overexpression}} <dd>An abnormally high level of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} which results in an excessive number of copies of one or more {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene products}}. Overexpression produces a pronounced gene-related {{gli|phenotype}}.<ref name="Overexpression - Oxford">{{cite web|title=overexpression|url=https://en.oxforddictionaries.com/definition/overexpression|archive-url=https://web.archive.org/web/20180210181025/https://en.oxforddictionaries.com/definition/overexpression|archive-date=February 10, 2018|website=Oxford Living Dictionary|publisher=Oxford University Press|access-date=18 May 2017|date=2017|quote=The production of abnormally large amounts of a substance which is coded for by a particular gene or group of genes; the appearance in the phenotype to an abnormally high degree of a character or effect attributed to a particular gene.}}</ref><ref name="Overexpress - NCI">{{cite web|title=overexpress|url=https://www.cancer.gov/publications/dictionaries/cancer-terms?cdrid=45812|website=NCI Dictionary of Cancer Terms|publisher=National Cancer Institute at the National Institutes of Health|access-date=18 May 2017|quote=overexpress In biology, to make too many copies of a protein or other substance. Overexpression of certain proteins or other substances may play a role in cancer development.|date = 2011-02-02}}</ref></dd>

{{term|oxidative phosphorylation}} {{ghat|Also '''electron transport-linked phosphorylation''' or '''terminal oxidation'''.}} <dd>The process by which cells use chemical energy obtained by the oxidation of nutrients to power the production of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenosine triphosphate}} (ATP). Oxidative phosphorylation couples two related processes: in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|electron transport chain}}, a series of enzyme-catalyzed redox reactions transfers electrons from energetic donors such as {{gli|NADH}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|FADH}} through various intermediates and ultimately to a terminal electron acceptor such as molecular oxygen ({{chem|O|2}}); the energy liberated by these reactions is simultaneously used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chemiosmosis}} to move protons ({{chem|H|+}}) across a membrane and against their concentration gradient, generating an electrochemical potential which powers {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP synthase}}, an enzyme that catalyzes the {{gli|phosphorylation}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ADP}} into ATP. In eukaryotes, both of these processes are carried out by proteins embedded in the membranes of {{gli|mitochondria}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chloroplasts}}; in prokaryotes, they occur in the cell membrane.</dd>

{{term|oxidative stress}} <dd></dd>

{{term|oxygen cascade}} <dd>The flow of oxygen from environmental sources (e.g. the air in the atmosphere) to the {{gli|mitochondria}} of a cell, where oxygen atoms participate in biochemical reactions that result in the oxidation of energy-rich substrates such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|carbohydrates}} in a process known as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|aerobic respiration}}.</dd>

==P== {{glossary}} {{term|p53}} {{ghat|Also '''Tumor protein P53 (TP53)''', '''transformation-related protein 53 (TRP53)''', and '''cellular tumor antigen p53'''.}} <dd>A class of regulatory proteins encoded by the ''TP53'' gene in vertebrates which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA binding|bind DNA}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation|regulate gene expression}} in order to protect the genome from mutation and block progression through the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell cycle}} if DNA damage does occur.<ref name="Alberts et al.">{{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> It is mutated in more than 50% of human cancers, indicating it plays a crucial role in preventing cancer formation.</dd>

{{term|pachynema}} {{ghat|Also '''pachytene stage'''.}} <dd>In {{gli|meiosis}}, the third of five substages of {{gli|prophase|prophase I}}, following {{gli|zygonema}} and preceding {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diplonema}}. During pachynema, the {{gli|synaptonemal complex}} facilitates {{gli|glossary=Glossary of cellular and molecular biology (0–L)|crossing over}} between the synapsed {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous chromosomes}}, and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centrosomes}} begin to move apart from each other.<ref name="DoG7"/></dd>

{{term|palindromic sequence}}{{anchor|palindromic sequences|palindrome|palindromes}} {{ghat|Also '''palindrome'''.}} <dd>A {{gli|nucleic acid sequence}} of a double-stranded {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}} molecule in which the unidirectional sequence (e.g. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5'}} to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3'}}) of {{gli|nucleobases}} on one strand is identical to the sequence in the same direction (e.g. 5' to 3') on the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} strand. In other words, a sequence is said to be palindromic if it is equal to its own {{gli|reverse complement}}. Palindromic {{gli|motifs}} are common {{gli|recognition sequence|recognition sites}} for {{gli|restriction enzymes}}.</dd>

{{term|paracellular transport}} <dd>The transfer of substances across an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epithelium}} by passing through the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extracellular}} space between cells, in contrast to {{gli|transcellular transport}}, where substances travel through cells by crossing the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|intracellular}} cytoplasm.</dd>

{{term|paracrine}} <dd>Describing or relating to a class of agonist {{gli|glossary=Glossary of cellular and molecular biology (0–L)|signaling}} molecules produced and secreted by regulatory cells into the extracellular environment and then transported by passive diffusion to target cells other than those which produced them. The term may refer to the molecules themselves, sometimes called ''paramones'', to the cells that produce them, or to signaling pathways which rely on them.<ref name="Oxford B&MB"/> Compare ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|autocrine}}'', ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|endocrine}}'', and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|juxtacrine}}''.</dd>

{{term|paratope}}{{anchor|paratopes}} {{ghat|Also '''antigen-binding site'''.}} <dd>An idiotope, i.e. the specific site or region within an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antibody}} that recognizes and binds to a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antigen}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epitope}}.<ref>{{cite book |vauthors = Lefranc MP |chapter=Paratope |date=2013 |title=Encyclopedia of Systems Biology |pages=1632–1633 |place=New York, NY |publisher=Springer |language=en |doi=10.1007/978-1-4419-9863-7_673 |isbn=978-1-4419-9863-7 |veditors = Dubitzky W, Wolkenhauer O, Cho KH, Yokota H}}</ref> The uniqueness of a paratope allows it to bind to only one epitope with very high affinity. At the end of each arm of the Y-shaped antibody is an identical paratope, and each paratope comprises a total of six {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementarity-determining regions}} (three from each of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|light chain|light}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|heavy chains}}) which protrude from a series of antiparallel {{gli|glossary=Glossary of cellular and molecular biology (0–L)|beta sheets}} in the antibody's higher structure.<ref>{{cite book | vauthors = Punt J, Stranford SA, Jones PP, Owen JA |title=Kuby immunology |date=2019 |isbn=978-1-4641-8978-4 |edition=Eighth |location=New York |oclc=1002672752}}</ref> The term is also sometimes used to refer to the specific site on a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligand}} molecule which defines the ligand's specificity for other molecules such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell-surface receptors}}.<ref name="Lackie"/></dd>

{{term|parent cell}}{{anchor|parent cells}} <dd>The original or ancestral cell from which a given set of descendant cells, known as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|daughter cells}}, have divided by {{gli|mitosis}} or {{gli|meiosis}}.</dd>

{{term|passenger}} <dd>A DNA fragment of interest designed to be {{gli|splicing|spliced}} into a 'vehicle' such as a {{gli|plasmid}} {{gli|vector}} and then {{gli|molecular cloning|cloned}}.<ref name="DoG7"/></dd>

{{term|passive transport}} <dd>The movement of a solute across a {{gli|membrane}} by traveling down an electrochemical or concentration gradient, using only the energy stored in the gradient and not any energy from external sources.<ref name="Lackie"/> Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|active transport}}''.</dd>

{{term|Pasteur effect}} {{ghat|Also '''Pasteur-Meyerhof effect'''.}} <dd>A phenomenon observed in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|facultatively anaerobic}} cells, including animal tissues and many microorganisms such as yeast, whereby the presence of oxygen in the environment inhibits the cell's use of ethanol {{gli|glossary=Glossary of cellular and molecular biology (0–L)|fermentation}} pathways to generate energy, and drives the cell to instead make use of the available oxygen in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|aerobic respiration}};<ref>{{cite journal | vauthors = Lagunas R |date=1981-01-01 |title=Is Saccharomyces cerevisiae a typical facultative anaerobe? |journal=Trends in Biochemical Sciences |language=en |volume=6 |pages=201–203 |doi=10.1016/0968-0004(81)90073-6 |issn=0968-0004}}</ref> or more generally the observed decrease in the rate of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycolysis}} or of lactate production in cells exposed to oxygenated air.<ref name="Oxford B&MB"/></dd>

{{term|PCR}} <dd>See ''{{gli|polymerase chain reaction}}''.</dd>

{{term|PCR product}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|amplicon}}''.</dd>

{{term|pentose}}{{anchor|pentose sugar|pentose sugars}} <dd>Any {{gli|monosaccharide}} containing five carbon atoms. The compounds {{gli|ribose}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}} are both pentose sugars, which, in the form of cyclic five-membered rings, serve as the central structural components of the {{gli|ribonucleotides}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonucleotides}} that make up {{gli|RNA}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}}, respectively.</dd>

{{term|peptidase}} <dd>See ''{{gli|protease}}''.</dd>

{{term|peptide}}{{anchor|peptides}} <dd>A short chain of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} {{gli|monomers}} linked by covalent {{gli|peptide bonds}}. Peptides are the fundamental building blocks of longer {{gli|polypeptide}} chains and hence of {{gli|proteins}}.</dd>

{{term|peptide bond}}{{anchor|peptide bonds}} <dd>A covalent chemical bond between the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|carboxyl group}} of one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino group}} of an adjacent amino acid, formed by a dehydration reaction catalyzed by {{gli|peptidyl transferase}}, an enzyme within the {{gli|ribosome}}, during {{gli|translation}}. A linear chain of amino acids linked by peptide bonds may be called a {{gli|peptide}} or {{gli|polypeptide}}.</dd>

{{term|peptidoglycan}} {{ghat|Also '''murein'''.}} <dd>A glycoconjugate complex of interwoven peptides and polysaccharides that is a primary constituent of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell wall}} in all bacteria and archaea, consisting of strands of glycosaminoglycans {{gli|glossary=Glossary of cellular and molecular biology (0–L)|crosslinked}} by short {{gli|polypeptide|oligopeptides}} (usually 4–10 residues<ref name="Lackie"/>) to form a rigid lattice of indefinite size.<ref name="Oxford B&MB"/> The proportion of the cell wall that is peptidoglycan varies widely by strain and is often used to aid strain identification: the higher peptidoglycan content of the cell walls of Gram-positive bacteria causes them to {{gli|histology|stain}} a darker color than Gram-negative bacteria.</dd>

{{term|pericentriolar material (PCM)}}{{anchor|pericentriolar material}} <dd></dd>

{{term|perinuclear space}} <dd>The space between the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inner nuclear membrane|inner}} and {{gli|outer nuclear membrane|outer membranes}} of the {{gli|nuclear envelope}}.</dd>

{{term|peripheral membrane protein}}{{anchor|peripheral membrane proteins}} {{ghat|Also '''extrinsic membrane protein'''.}} <dd>Any of a class of {{gli|membrane proteins}} which attach only temporarily to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}}, either by penetrating the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipid bilayer}} or by attaching to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|integral membrane protein|other proteins}} which are permanently embedded within the membrane.<ref>{{cite journal | vauthors = Goñi FM | title = Non-permanent proteins in membranes: when proteins come as visitors (Review) | journal = Molecular Membrane Biology | volume = 19 | issue = 4 | pages = 237–245 | year = 2002 | pmid = 12512770 | doi = 10.1080/0968768021000035078 | s2cid = 20892603 }}</ref> The ability to reversibly interact with membranes makes peripheral membrane proteins important in many different roles, commonly as regulatory {{gli|subunits}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|channel proteins}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell surface receptors}}. Their {{gli|glossary=Glossary of cellular and molecular biology (0–L)|domains}} often undergo rearrangement, dissociation, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conformational changes}} when they interact with the membrane, resulting in the activation of their biological activity.<ref>{{cite journal | vauthors = Johnson JE, Cornell RB | title = Amphitropic proteins: regulation by reversible membrane interactions (review) | journal = Molecular Membrane Biology | volume = 16 | issue = 3 | pages = 217–235 | year = 1999 | pmid = 10503244 | doi = 10.1080/096876899294544 | doi-access = free }}</ref> In {{gli|protein purification}}, peripheral membrane proteins are typically more water-soluble and much easier to isolate from the membrane than {{gli|glossary=Glossary of cellular and molecular biology (0–L)|integral membrane proteins}}.</dd>

{{term|periplasmic space}} {{ghat|Also '''periplasm'''.}} <dd>In a bacterial cell, the space between the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}} and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell wall}}.<ref name="Oxford B&MB"/></dd>

{{term|peroxisome}}{{anchor|peroxisomes}} <dd>A small {{gli|membrane-bound organelle}} found in many eukaryotic cells which specializes in carrying out oxidative reactions with various enzyme peroxidases and catalase, generally to mitigate damage from reactive oxygen species but also as a participant in various {{gli|metabolic pathways}} such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|beta-oxidation}} of fatty acids.<ref name="SwissBioPics">{{cite web |author1=SwissBioPics |title=Eukaryota cell |url=https://www.swissbiopics.org/name/Eukaryota_cell |website=www.swissbiopics.org |publisher=Swiss Institute of Bioinformatics (SIB)}}</ref></dd>

{{term|persistence}} <dd>1. The tendency of a {{gli|motility|moving cell}} to continue moving in the same direction as previously; that is, even in isotropic environments, there inevitably still exists an inherent bias by which, from instant to instant, cells are more likely not to change direction than to change direction. Averaged over long periods of time, however, this bias is less obvious and cell movements are better described as a {{gli|random walk}}.<ref name="Lackie"/></dd> <dd>2. The ability of some viruses to remain present and viable in cells, organisms, or populations for very long periods of time by any of a variety of strategies, including retroviral integration and immune suppression, often in a latent form which replicates very slowly or not at all.<ref name="Lackie"/></dd>

{{term|Petri dish}} <dd>A shallow, transparent plastic or glass dish, usually circular and covered with a lid, which is widely used in biology laboratories to hold solid or liquid {{gli|glossary=Glossary of cellular and molecular biology (0–L)|growth media}} for the purpose of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell culture|culturing cells}}. They are particularly useful for {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adherent cultures}}, where they provide a flat, sterile surface conducive to colony formation from which scientists can easily isolate and identify individual colonies.</dd>

{{term|phagemid}}{{anchor|phagemids}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|bacteriophage}} with a genome encoding a mobile {{gli|plasmid}} that can be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excised}} by co-infection of the host cell with a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|helper phage}}. Phagemids are useful as vectors for library production.<ref name="Lackie"/></dd>

{{term|phagocyte}}{{anchor|phagocytes}} <dd>Any cell capable of {{gli|phagocytosis}}, especially any of various cell types of the immune system which engulf and ingest harmful foreign molecules, bacteria, and dead or dying cells, including {{gli|neutrophils}} and {{gli|macrophages}}.<ref name="Lackie"/></dd>

{{term|phagocytosis}} <dd>The process by which foreign cells, molecules, and small particulate matter are engulfed and ingested via {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endocytosis}} by specialized cells known as {{gli|phagocytes}} (a class which includes {{gli|macrophages}} and neutrophils).<ref name="Alberts et al."/></dd>

{{term|phagosome}} <dd>A large, intracellular, membrane-bound {{gli|vesicle}} formed as a result of {{gli|phagocytosis}} and containing whatever previously extracellular material was engulfed during that process.<ref name="Alberts et al."/></dd>

{{term|pharmacogenomics}} <dd>The study of the role played by the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} in the body's response to pharmaceutical drugs, combining the fields of pharmacology and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomics}}.</dd>

{{term|phenome}} <dd>The complete set of {{gli|phenotypes}} that are or can be expressed by a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, cell, tissue, organism, or species; the sum of all of its manifest chemical, morphological, and behavioral characteristics or traits.</dd>

{{term|phenomic lag}} <dd>A delay in the {{gli|phenotypic}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} of a genetic {{gli|mutation}} owing to the time required for the manifestation of changes in the affected biochemical pathways.<ref name="Rieger"/></dd>

{{term|phenotype}}{{anchor|phenotypes|phenotypic|phenotypic trait|phenotypic traits}} <dd>The composite of the observable morphological, physiological, and behavioral {{gli|traits}} of an organism that result from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} of the organism's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genotype}} as well as the influence of environmental factors and the interactions between the two.</dd>

{{term|phenotypic switching}} <dd>A type of phenotypic plasticity in which a cell rapidly undergoes major changes to its morphology and/or function, usually via {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epigenetic}} modifications, allowing it to quickly switch back and forth between disparate phenotypes in response to changes in the local microenvironment.</dd>

{{term|phosphatase}}{{anchor|phosphatases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} that catalyze the hydrolytic cleavage of a phosphoric acid monoester into a {{gli|phosphate}} ion and an alcohol, e.g. the removal of a phosphate group from a {{gli|nucleotide}} via the breaking of the {{gli|phosphodiester bond|ester bond}} connecting the phosphate to a {{gli|ribose}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}} sugar or to another phosphate, a process termed {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dephosphorylation}}. The {{gli|phosphorylation|opposite process}} is performed by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinases}}.</dd>

{{term|phosphate}}{{anchor|phosphates|phosphate group|phosphate groups}} <dd>Any chemical species or functional group derived from phosphoric acid ({{chem|H|3|P|O|4}}) by the removal of one or more protons ({{chem|H|+}}); the completely ionized form, {{chem|[PO|4|]|3−}}, consists of a single, central phosphorus atom covalently bonded to four oxygen atoms via three single bonds and one double bond. Phosphates are abundant and ubiquitous in biological systems, where they occur either as free anions in solution, known as '''inorganic phosphates''' and symbolized '''Pi''', or bonded to organic molecules via ester bonds. The huge diversity of organophosphate compounds includes all {{gli|nucleotides}}, whose phosphate groups are linked by {{gli|phosphodiester bonds}} to form the {{gli|phosphate backbone|structural backbones}} of {{gli|nucleic acid|long nucleotide chains}} such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} and {{gli|RNA}}, and the high-energy diphosphate and triphosphate substituents of individual nucleotides such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ADP}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP}} serve as essential energy carriers in all cells. {{gli|phospholipid|Phospholipids}} are major components of most {{gli|membranes}}. Enzymes known as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinases}} and {{gli|phosphatases}} catalyze the {{gli|phosphorylation|addition}} and {{gli|dephosphorylation|removal}} of phosphate groups to and from these and other biomolecules.</dd>

{{term|phosphate backbone}}{{anchor|phosphodiester backbone|phosphate–sugar backbone|sugar–phosphate backbone|backbone}} {{ghat|Also '''phosphodiester backbone''', '''sugar–phosphate backbone''', and '''phosphate–sugar backbone'''.}} <dd>The linear chain of alternating {{gli|phosphate}} and sugar compounds that results from the linking of consecutive {{gli|nucleotides}} in the same {{gli|strand}} of a {{gli|nucleic acid}} molecule, and which serves as the structural framework of the nucleic acid. Each individual strand is held together by a repeating series of {{gli|phosphodiester bonds}} connecting each {{gli|phosphate group}} to the {{gli|ribose}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}} sugars of two adjacent nucleotides. These bonds are created by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligases}} and broken by {{gli|nucleases}}.</dd> thumb|right|350px|A defining element of nucleic acid structure is the linear chain of alternating {{gli|pentose|sugars}} (orange) and {{gli|phosphates}} (yellow) known as the '''{{gli|phosphate backbone}}''', which acts as a scaffold to which {{gli|nucleobases}} are attached. The phosphorus atom of each phosphate group forms two {{gli|phosphodiester bond|ester bonds}} to specific carbon atoms within the pentose sugars—{{gli|ribose}} in RNA and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}} in DNA—of two adjacent nucleosides.

{{term|phosphodiester bond}}{{anchor|phosphodiester bonds|phosphate bond|phosphate bonds}} <dd>A pair of ester bonds linking a {{gli|phosphate}} molecule with the two {{gli|pentose}} rings of consecutive {{gli|nucleosides}} on the same strand of a {{gli|nucleic acid}}. Each phosphate forms a covalent bond with the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3' carbon}} of one pentose and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5' carbon}} of the adjacent pentose; the repeated series of such bonds that holds together the long chain of nucleotides comprising {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} and {{gli|RNA}} molecules is known as the {{gli|phosphate backbone|phosphate or phosphodiester backbone}}.</dd>

{{term|phospholipid}}{{anchor|phospholipids}} <dd>Any of a subclass of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipids}} consisting of a central alcohol (usually glycerol) covalently bonded to three functional groups: a negatively charged {{gli|phosphate}} group, and two long {{gli|glossary=Glossary of cellular and molecular biology (0–L)|fatty acid}} chains. This arrangement results in a highly amphipathic molecule which in aqueous solutions tends to aggregate with similar molecules in a lamellar or micellar conformation with the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hydrophilic}} phosphate "heads" oriented outward, exposing them to the solution, and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hydrophobic}} fatty acid "tails" oriented inward, minimizing their interactions with water and other polar compounds. Phospholipids are the major structural {{gli|membrane lipid}} in almost all biological {{gli|membranes}} except the membranes of some plant cells and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chloroplasts}}, where {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycolipids}} dominate instead.<ref name="Lackie"/></dd>

{{term|phospholipid bilayer}} <dd>See ''{{gli|lipid bilayer}}''.</dd>

{{term|phosphorylation}}{{anchor|phosphorylate|phosphorylates|phosphorylating|phosphorylated}} <dd>The attachment of a {{gli|phosphate}} ion, {{chem|P|O|4|3-}}, to another molecule or ion or to a {{gli|protein}} by covalent bonding. Phosphorylation and the inverse reaction, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dephosphorylation}}, are essential steps in numerous biochemical pathways, including in the production of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenosine triphosphate}} (ATP) (as in {{gli|oxidative phosphorylation}}); in the metabolism of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glucose}} and the synthesis of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycogen}}; and in the {{gli|post-translational modification}} of amino acid residues in many proteins. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzyme|Enzymes}} which catalyze phosphorylation reactions are known as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinases}}; those that catalyze dephosphorylation are known as {{gli|phosphatases}}.</dd>

{{term|pinocytosis}}{{anchor|pinocytotic|pinocytic}} <dd>A form of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endocytosis}} in which liquid and suspended solids from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extracellular}} environment are captured in inward {{gli|glossary=Glossary of cellular and molecular biology (0–L)|invaginations}} of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}} which then "bud off" into enclosed {{gli|vesicles}} in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}. The contents of these vesicles are then passed to organelles such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endosomes}} by fusion of the vesicular and organellar membranes. Pinocytosis is the predominant form of endocytosis occurring in most cells, such that the term is often used interchangeably with endocytosis as a whole.<ref name="Oxford B&MB"/> </dd>

{{term|piRNA}} <dd>See ''{{gli|piRNA|Piwi-interacting RNA}}''.</dd>

{{term|pitch}} <dd>The number of {{gli|base pairs}} contained within a single complete turn of the {{gli|DNA}} {{gli|double helix}},<ref name="DoG7"/> used as a measure of the "tightness" or density of the helix's spiral.</dd>

{{term|Piwi-interacting RNA (piRNA)}} <dd></dd>

{{term|plasma membrane}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}}''.</dd>

{{term|plasmid}}{{anchor|plasmids}} <dd>Any small {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecule that is physically separated from the larger body of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomic DNA|chromosomal DNA}} and can {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replicate}} independently. Plasmids are typically small (less than 100 {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kilobase|kbp}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|circular}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecules in {{gli|prokaryotes}} such as bacteria, though they are also sometimes present in archaea and {{gli|eukaryotes}}.</dd>

{{term|plasmid partitioning}} <dd>The process by which {{gli|plasmids}} which have been {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replicated}} inside a {{gli|parent cell}} are distributed equally between {{gli|glossary=Glossary of cellular and molecular biology (0–L)|daughter cells}} during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}}.<ref name="Lackie">{{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|plasmid-mediated resistance}} <dd>The development of resistance to toxins or antibiotics which is enabled by the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|horizontal transfer}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antibiotic resistance gene|resistance genes}} encoded within small, independently replicating DNA molecules known as {{gli|plasmids}}. This process occurs naturally via mechanisms such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|bacterial conjugation}}, but is also a common aspect of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering}} methods such as {{gli|molecular cloning}}.</dd>

{{term|plasmolysis}} <dd>The temporary shrinkage of the {{gli|protoplasm}} of a plant or bacterial cell away from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell wall}}, caused by loss of water from the cell.<ref name="Oxford B&MB"/></dd>

{{term|plastid}}{{anchor|plastids}} <dd>Any of a class of {{gli|membrane-bound organelles}} found in the cells of some eukaryotes such as plants and algae which are hypothesized to have evolved from endosymbiotic cyanobacteria; examples include {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chloroplasts}}, chromoplasts, and leucoplasts. Plastids retain their own {{gli|glossary=Glossary of cellular and molecular biology (0–L)|circular DNA|circular chromosomes}} which replicate independently of the host cell's genome. Many contain photosynthetic pigments which allow them to perform {{gli|photosynthesis}}, while others have been retained for their ability to synthesize unique chemical compounds.</dd>

{{term|pleomorphism}} <dd>1. Variability in the size, shape, or {{gli|staining}} of cells and/or their nuclei, particularly as observed in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|histology}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytopathology}}, where morphological variation is frequently {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biomarker|an indicator}} of a cellular abnormality such as disease or {{gli|tumorigenesis|tumor formation}}.</dd> <dd>2. In microbiology, the ability of some microorganisms such as certain bacteria and viruses to alter their morphology, metabolism, or mode of reproduction in response to changes in their environment.</dd>

{{term|plithotaxis}} <dd>The tendency of cells within a {{gli|monolayer}} to migrate in the direction of the local highest tension or maximal principal stress, exerting minimal shear stress on neighboring cells and thereby propagating the tension across many intercellular junctions and causing the cells to exhibit a sort of collective migration.<ref>Tambe, D.T., Hardin, C., Angelini, T.E., Rajendran, K., Park, C.Y., Serra-Picamal, X., Zhou, E.H., Zaman, M.H., Butler, J.P., Weitz, D.A., Fredberg JJ, Trepat, X, Collective cell guidance by cooperative intercellular forces. ''Nature Materials'',10(6), 469–475, 2011, {{doi|10.1038/nmat3025}}.</ref></dd>

{{term|ploidy}}{{anchor|ploidy level}} <dd>The number of complete sets of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} in a cell, and hence the number of possible {{gli|glossary=Glossary of genetics and evolutionary biology|alleles}} present within the cell at any given {{gli|glossary=Glossary of cellular and molecular biology (0–L)|autosome|autosomal}} locus.</dd> thumb|right|350px|A cell's '''{{gli|ploidy}}''' level is defined by the number of copies it has of each specific chromosome: if the cell has two copies of each of three distinct chromosomes, it is said to be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} (2N).

{{term|pluripotency}}{{anchor|pluripotent}} <dd></dd>

{{term|plus-strand}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding strand}}''.</dd>

{{term|point mutation}}{{anchor|point mutations}} <dd>A {{gli|mutation}} by which a single {{gli|nucleotide}} base is changed, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|insertion|inserted}}, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deletion|deleted}} from a {{gli|sequence}} of DNA or RNA.</dd>

{{term|poly(A) tail}} <dd>A {{gli|post-transcriptional modification}} consisting of a chain of repeated {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenosine}} residues, 40–250 nucleotides in length, attached to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3' end}} of nearly all mature eukaryotic {{gli|messenger RNA}} transcripts (those of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|histones}} being a notable exception).<ref name="Oxford B&MB"/></dd>

{{term|polyadenylation}}{{anchor|polyadenylated}} <dd>The addition of a series of multiple {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenosine}} {{gli|ribonucleotides}}, known as a {{gli|poly(A) tail}}, to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3'-end}} of a {{gli|primary transcript|primary RNA transcript}}. A class of {{gli|post-transcriptional modification}}, polyadenylation serves different purposes for different classes of RNA and in different cell types and organisms. For eukaryotic {{gli|mRNA|messenger RNAs}}, the addition of a poly(A) tail is an important step in the processing of the raw transcript into a mature mRNA ready for export to the cytoplasm; primary transcripts are first cleaved 10–30 nucleotides downstream of a highly conserved {{font|AAUAAA|font=courier|size=big}} sequence, then the poly(A) tail is generated from the chaining of multiple {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP}} molecules through the action of polynucleotide adenylyltransferase.<ref name="Oxford B&MB"/>For {{gli|ncRNA|non-coding RNAs}} and in many bacteria, polyadenylation has the opposite function, instead promoting the RNA's degradation.</dd>

{{term|polyclonal}} <dd>Describing cells, proteins, or molecules descended or derived from more than one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|clone}} (i.e. from more than one genome or genetic lineage) or made in response to more than one unique stimulus. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antibody|Antibodies}} are often described as polyclonal if they have been produced or raised against multiple distinct {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antigens}} or multiple variants of the same antigen, such that they can recognize more than one unique {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epitope}}.<ref name="MacLean"/> Contrast ''{{gli|monoclonal}}''.</dd>

{{term|polylinker}} <dd>See ''{{gli|multiple cloning site}}''.</dd>

{{term|polymer}}{{anchor|polymers|polymeric}} <dd>A {{gli|macromolecule}} composed of multiple repeating {{gli|subunits}} or {{gli|monomers}}; a chain or aggregation of many individual molecules of the same compound or class of compound.<ref name="MacLean"/> The formation of polymers is known as {{gli|polymerization}} and generally only occurs when {{gli|nucleation}} sites are present and the concentration of monomers is sufficiently high.<ref name="Lackie"/> Many of the major classes of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biomolecules}} are polymers, including {{gli|nucleic acids}} and {{gli|polypeptides}}.</dd>

{{term|polymerase}}{{anchor|polymerases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} which catalyze the synthesis of {{gli|polymeric}} molecules, especially {{gli|nucleic acid}} polymers, typically by encouraging the {{gli|base pairing}} of free {{gli|nucleotides}} with those of an existing {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} {{gli|template}} strand. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerases}} and {{gli|RNA polymerases}} are essential for {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} and {{gli|transcription}}, respectively.</dd>

{{term|polymerase chain reaction (PCR)}}{{anchor|polymerase chain reaction}} <dd>Any of a wide variety of molecular biology methods involving the rapid production of millions or billions of copies of a specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA sequence}}, allowing scientists to selectively {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amplification|amplify}} fragments of a very small sample to a quantity large enough to study in detail. In its simplest form, PCR generally involves the incubation of a target DNA sample of known or unknown sequence with a reaction mixture consisting of {{gli|oligonucleotide}} {{gli|primers}}, a heat-stable {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerase}}, and free {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonucleotide triphosphates}} (dNTPs), all of which are supplied in excess. This mixture is then alternately heated and cooled to pre-determined temperatures for pre-determined lengths of time according to a specified pattern which is repeated for many cycles, typically in a {{gli|thermal cycler}} which automatically controls the required temperature variations. In each cycle, the most basic of which includes a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|denaturation}} phase, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|annealing}} phase, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|elongation}} phase, the copies synthesized in the previous cycle are used as {{gli|template strand|templates}} for synthesis in the next cycle, causing a chain reaction that results in the exponential growth of the total number of copies in the reaction mixture. Amplification by PCR has become a standard technique in virtually all molecular biology laboratories.</dd> thumb|right|450px|A diagram of the exponential amplification of a specific DNA sequence via the '''{{gli|polymerase chain reaction}}''' (PCR)

{{term|polymerization}} <dd>The formation of a {{gli|polymer}} from its constituent {{gli|monomers}}; the chemical reaction or series of reactions by which monomeric {{gli|subunits}} are covalently linked together into a polymeric chain or branching aggregate; e.g. the polymerization of a {{gli|nucleic acid}} chain by linking consecutive {{gli|nucleotides}}, a reaction catalyzed by a {{gli|polymerase}} enzyme.</dd>

{{term|polymorphism}} <dd>1. In genetics, the regular and simultaneous existence of two or more discontinuous {{gli|glossary=Glossary of cellular and molecular biology (0–L)|alleles}} or genotypes in the same population where the frequency of each allele is greater than can be explained by recurrent mutation alone, typically occurring in more than 1 percent of the population's individuals.<ref name="Lackie"/> An example is the different human blood types (A, B, AB, and O).<ref name="Oxford B&MB"/></dd> <dd>2. In chemistry, the existence of the same substance in two or more different crystalline forms.<ref name="Oxford B&MB"/></dd>

{{term|polypeptide}}{{anchor|polypeptides}} <dd>A long, continuous, and unbranched {{gli|polymeric}} chain of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} {{gli|monomers}} linked by covalent {{gli|peptide bonds}}, typically longer than a {{gli|peptide}}. {{gli|protein|Proteins}} generally consist of one or more polypeptides {{gli|protein folding|folded}} or arranged in a biologically functional way.</dd>

{{term|polyploid}}{{anchor|polyploidy|polyploids}} <dd>(of a cell or organism) Having more than two homologous copies of each {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}}; i.e. any {{gli|ploidy}} level that is greater than {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}}. Polyploidy may occur as a normal condition of chromosomes in certain cells or even entire organisms, or it may result from errors in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}} or mutations causing the duplication of the entire chromosome set.</dd>

{{term|polyribosome}} <dd>See ''{{gli|polysome}}''.</dd>

{{term|polysaccharide}}{{anchor|polysaccharides}} <dd>A linear or branched {{gli|polymeric}} chain of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|carbohydrate}} {{gli|monomers}} ({{gli|monosaccharides}}). Examples include {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycogen}} and cellulose.<ref name="Alberts et al."/></dd>

{{term|polysome}} {{ghat|Also '''polyribosome''' or '''ergosome'''.}} <dd>A functional unit of {{gli|protein synthesis}} consisting of multiple {{gli|ribosomes}} attached along the length of the same {{gli|messenger RNA}} transcript.<ref name="Lackie"/></dd>

{{term|polysomy}} <dd>The condition of a cell or organism having at least one more copy of a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} than is normal for its {{gli|ploidy}} level, e.g. a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} organism with three copies of a given chromosome is said to show {{gli|trisomy}}. Every polysomy is a type of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|aneuploidy}}.</dd>

{{term|polytene chromosome}} <dd></dd>

{{term|position effect}}{{anchor|position effects}} <dd>Any effect on the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} or functionality of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or sequence that is a consequence of its location or position within a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} or other DNA molecule. A sequence's precise location relative to other sequences and structures tends to strongly influence its activity and other properties, because different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|loci}} on the same molecule can have substantially different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic backgrounds}} and physical/chemical environments, which may also change over time. For example, the {{gli|transcription}} of a gene located very close to a {{gli|nucleosome}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromere}}, or {{gli|telomere}} is often {{gli|repressed}} or entirely prevented because the proteins that make up these structures block access to the DNA by {{gli|transcription factors}}, while the same gene is transcribed at a much higher rate when located in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|euchromatin}}. Proximity to {{gli|promoters}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enhancers}}, and other {{gli|regulatory elements}}, as well as to regions of frequent {{gli|transposition}} by {{gli|mobile genetic element|mobile elements}}, can also directly affect expression; being located near the end of a chromosomal arm or to common {{gli|glossary=Glossary of cellular and molecular biology (0–L)|crossover}} points may affect when {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replication}} occurs and the likelihood of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination|recombination}}. Position effects are a major focus of research in the field of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epigenetics}}.</dd>

{{term|positional cloning}} {{ghat|Also '''map-based cloning'''.}} <dd>A strategy for identifying and {{gli|molecular cloning|cloning}} a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|candidate gene}} based on knowledge of its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|locus}} or position alone and with little or no information about its {{gli|products}} or function, in contrast to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|functional cloning}}. This method usually begins by comparing the genomes of individuals expressing a {{gli|phenotype}} of unknown provenance (often a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic disorder|hereditary disease}}) and identifying {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic markers}} shared between them. Regions defined by markers flanking one or more {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes of interest}} are cloned, and the genes located between the markers can then be identified by any of a variety of means, e.g. by {{gli|sequencing}} the region and looking for {{gli|open reading frames}}, by comparing the sequence and expression patterns of the region in {{gli|mutant}} and {{gli|wild-type}} individuals, or by testing the ability of the putative gene to {{gli|rescue}} a mutant phenotype.<ref name="DoG7"/></dd>

{{term|positive (+) sense strand}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding strand}}''.</dd>

{{term|positive control}} {{ghat|Also '''positive regulation'''.}} <dd>The initiation, activation, or enhancement of some biological process by the presence of a specific molecular entity (e.g. an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|activator}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inducer}}), in the absence of which the process cannot proceed or is otherwise diminished.<ref name="Oxford B&MB"/> In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation}}, for example, the binding of an activating molecule such as a {{gli|transcription factor}} to a {{gli|promoter}} may recruit {{gli|RNA polymerase}} to a coding sequence, thereby causing it to be {{gli|transcribed}}. Contrast ''{{gli|negative control}}''.</dd>

{{term|positive supercoiling}} <dd>The {{gli|supercoiling}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecule in the same direction as the turn of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double helix}} itself (e.g. a right-handed coiling of a helix with a right-handed turn).<ref name="Rieger"/> Contrast ''{{gli|negative supercoiling}}''.</dd>

{{term|post-transcriptional modification}}{{anchor|post-transcriptional modifications|post-transcriptional}} <dd></dd>

{{term|post-translational modification}}{{anchor|post-translational modifications|post-translational}} <dd></dd>

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

{{term|precursor cell}}{{anchor|precursor cells}} {{ghat|Also '''blast cell'''.}} <dd>A partially differentiated or intermediate {{gli|stem cell}} with the ability to further differentiate into only one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell type}}; i.e. a {{gli|unipotent}} stem cell that is the immediate parent cell from which fully differentiated cell types divide. The term "precursor cell" is sometimes used interchangeably with {{gli|progenitor cell}}, though this term may also be considered technically distinct.</dd>

{{term|Pribnow box}} <dd></dd>

{{term|primary structure}} <dd></dd>

{{term|primary transcript}}{{anchor|primary transcripts}} <dd>The unprocessed, single-stranded {{gli|RNA}} molecule produced by the {{gli|transcription}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} sequence as it exists before {{gli|post-transcriptional modifications}} such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|alternative splicing}} convert it into a mature RNA product such as an {{gli|mRNA}}, {{gli|tRNA}}, or {{gli|rRNA}}. A ''precursor mRNA'' or ''pre-mRNA'', for example, is a primary transcript which, after processing, becomes a mature mRNA ready for {{gli|translation}}.</dd>

{{term|primase}} <dd>Any of a class of enzymes that catalyze the synthesis of short, ~10-base {{gli|RNA}} {{gli|oligonucleotides}}, which by complementing the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lagging strand}} during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} are used as {{gli|primers}} by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerase}} to initiate the synthesis of {{gli|Okazaki fragments}}.<ref name="Lackie"/></dd>

{{term|primer}}{{anchor|primers}} <dd>A short, {{gli|single-stranded}} {{gli|oligonucleotide}}, typically 5–100 bases in length, which "primes" or initiates nucleic acid synthesis by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridization|hybridizing}} to a complementary sequence on a {{gli|template strand}} and thereby providing an existing {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3'-end}} from which a {{gli|polymerase}} can {{gli|glossary=Glossary of cellular and molecular biology (0–L)|elongation|extend}} the new strand. Natural systems exclusively use {{gli|RNA}} primers to initiate {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} and some forms of prokaryotic {{gli|transcription}}, whereas the ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|in vitro}}'' syntheses performed in many laboratory techniques such as {{gli|PCR}} often use {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} primers. In modern laboratories, primers are carefully designed, often in "forward" and "reverse" pairs, to complement specific and unique sequences in target DNA molecules, with consideration given to their {{gli|melting}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|annealing}} temperatures, and then purchased from commercial suppliers which create oligonucleotides on demand by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|artificial gene synthesis|''de novo'' synthesis}}.</dd>

{{term|primer dimer (PD)}}{{anchor|primer dimer|primer dimers}} <dd></dd>

{{term|primer walking}} <dd></dd>

{{term|priming}} <dd>The initiation of {{gli|nucleic acid}} synthesis by the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridization}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|annealing}} of one or more {{gli|primers}} to a complementary sequence within a {{gli|template strand}}.</dd>

{{term|pro-enzyme}} <dd>See ''{{gli|zymogen}}''.</dd>

{{term|pro-protein}}{{anchor|pro-proteins}} {{ghat|Also '''pro-peptide'''.}} <dd>An inactive precursor of a {{gli|protein}} or {{gli|polypeptide}} that is converted into the active form by some {{gli|post-translational modification}}, such as by cleaving a specific peptide sequence from the precursor or by attaching other molecules to specific amino acid residues. The names of protein precursors are often prefixed with ''pro-'', as in proinsulin. Enzyme precursors may be called {{gli|zymogen|pro-enzymes or zymogens}}.</dd>

{{term|probe}}{{anchor|probes|probing|probed}} <dd>Any reagent used to make a single measurement in a biochemical assay such as a gene expression experiment. Molecules which have a specific affinity for one or more other molecules may be used to probe for the presence of those other molecules in samples of unknown composition. Probes are often {{gli|glossary=Glossary of cellular and molecular biology (0–L)|labelled}} or otherwise used as {{gli|reporters}} to indicate whether or not a specific chemical reaction is taking place. See also ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridization probe}}''.</dd>

{{term|probe-set}} <dd>A collection of two or more {{gli|probes}} designed to measure a single molecular species, such as a collection of {{gli|oligonucleotides}} designed to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridize}} to various parts of the {{gli|mRNA}} transcripts generated from a single gene.</dd>

{{term|process molecular gene concept}} <dd>An alternative definition of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} which emphasizes the contribution of non-DNA factors to the process by which the information encoded in a {{gli|DNA sequence}} results in the synthesis of a {{gli|polypeptide}}.</dd>

{{term|prometaphase}} <dd>The second stage of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}} in {{gli|mitosis}}, following {{gli|prophase}} and preceding {{gli|metaphase}}, during which the {{gli|nuclear membrane}} disintegrates, the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} inside form {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinetochores}} around their {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromeres}}, {{gli|microtubules}} emerging from the poles of the {{gli|mitotic spindle}} reach the nuclear space and attach to the kinetochores, and {{gli|motor proteins}} associated with the microtubules begin to push the chromosomes toward the center of the cell.</dd>

{{term|promoter}}{{anchor|promoters}} <dd>A sequence or region of DNA, usually 100–1,000 base pairs long, which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation|regulates}} the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression|expression}} of one or more associated {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} by containing binding sites for {{gli|transcription factors}} which recruit {{gli|RNA polymerase}} to the sequence and initiate {{gli|transcription}}. Promoters are typically located immediately {{gli|upstream}} of the genes they regulate, near to and often including the {{gli|transcription start site}}.</dd>

{{term|promotion}} <dd>See ''{{gli|upregulation}}''.</dd>

{{term|prophase}} <dd>The first stage of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}} in both {{gli|mitosis}} and {{gli|meiosis}}, occurring after {{gli|glossary=Glossary of cellular and molecular biology (0–L)|interphase}} and before {{gli|prometaphase}}, during which the DNA of the chromosomes is {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA condensation|condensed}} into {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatin}}, the {{gli|nucleolus}} disintegrates, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centrosomes}} move to opposite ends of the cell, and the {{gli|mitotic spindle}} forms.</dd>

{{term|protease}}{{anchor|proteases}} {{ghat|Also '''peptidase'''.}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} which catalyze {{gli|proteolysis}}, i.e. the decomposition of proteins into smaller polypeptides or individual amino acids, by cleaving {{gli|peptide bonds}} via hydrolysis. Proteases are ubiquitous components of numerous {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biological pathways}}, and therefore it is often necessary to inhibit them in order for laboratory techniques involving protein activity to be effective.</dd>

{{term|proteasome}}{{anchor|proteasomes}} {{ghat|Also '''ingensin''', '''macropain''', '''prosome''', '''multicatalytic proteinase''', and '''multicatalytic endopeptidase complex'''.}} <dd>A large {{gli|protein complex|complex}} of {{gli|protease}} enzymes that selectively degrades intracellular proteins which have been {{gli|protein tag|tagged}} for degradation by {{gli|ubiquitination}}. Proteasomes play important roles in the timing and onset of cellular processes through the signal-mediated {{gli|proteolysis}} of certain enzymes and regulatory proteins; they also contribute to the stress response by removing abnormal proteins and to the immune response by generating {{gli|antigenic}} peptides.<ref name="Oxford B&MB"/></dd>

{{term|protein}}{{anchor|proteins}} <dd>A {{gli|polymeric}} {{gli|macromolecule}} composed of one or more {{gli|peptide|long chains}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} linked by {{gli|peptide bonds}}. Proteins are the three-dimensional structures created when these chains {{gli|protein folding|fold}} into specific higher-order arrangements following {{gli|translation}}, and it is this folded structure which determines a protein's chemical activity and hence its biological function. Ubiquitous and fundamental in all living organisms, proteins are the primary means by which the activities of life are performed, participating in the vast majority of the biochemical reactions that occur inside and outside of cells. They are often classified according to the type(s) of reaction(s) they facilitate or catalyze, by the chemical substrate(s) they act upon, or by their functional role in cellular activity; e.g. as {{gli|structural proteins}}, {{gli|motor proteins}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}}, {{gli|transcription factors}}, or links within {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biochemical pathways}}.</dd>

{{term|protein complex}} <dd>An assembly or aggregate of multiple {{gli|proteins}} held together by intermolecular forces, especially one with a particular biological function. Complexes may include many of the same protein or all different proteins. Numerous cellular activities, including {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}, {{gli|transcription}}, and {{gli|translation}}, rely on protein complexes.<ref name="Alberts et al."/></dd>

{{term|protein folding}} <dd>The physical process by which the linear chains of amino acids (i.e. {{gli|polypeptides}}) synthesized during {{gli|translation}} are changed from random coils into stable, orderly, three-dimensional shapes (i.e. {{gli|proteins}}) by assuming a higher-order structure or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conformation}} which permits the protein to be biologically functional, known as its {{gli|native state}}. Folding is the consequence of amino acid residues participating in intermolecular electrostatic interactions with each other and with their surroundings, including other molecules, and so is strongly influenced by the particularities of the local chemical environment. The time it takes to properly fold a protein can vary greatly, but the process often begins while chain synthesis is still ongoing. Some chains may have {{gli|motifs}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|domains}} which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|intrinsically disordered protein|lack intrinsic order}} and remain unfolded across a wide range of chemical conditions. Having the correct three-dimensional structure is essential for proper protein function, and misfolded proteins are generally biologically inactive, though mutant folds can occasionally modify functionality in useful ways.</dd>

{{term|protein kinase}}{{anchor|protein kinases}} <dd>Any of a class of enzymes which {{gli|phosphorylate}} proteins by catalyzing the transfer of a {{gli|phosphate}} from {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP}} to an amino acid residue and often causing a functionally relevant {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conformational change}} as a result. The great majority of protein kinases phosphorylate the hydroxyl side chains of either serine, threonine, or tyrosine, though other types also exist.<ref name="Lackie"/> Separate classes of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinases}} phosphorylate non-protein molecules such as lipids and carbohydrates.</dd>

{{term|protein purification}} <dd>Any process by which one or more specific {{gli|proteins}} are isolated from a heterogeneous mixture such as whole cells or tissues, usually by first separating the protein and non-protein components of the mixture and then by separating the {{gli|protein of interest|protein(s) of interest}} from all other non-target proteins. Purification is often necessary in order to remove contaminants that might interfere with the study of the protein of interest's structure, function, and interactions with other molecules. A wide variety of isolation techniques exist, most of which exploit differences in the size, solubility, binding affinity, enzymatic activity, or other physico-chemical properties of the separated components. Some techniques tend to destroy {{gli|supersecondary structure|higher-order structure}} and thus are only useful for identification of the protein's {{gli|peptide sequence|primary amino acid sequence}}, while others are designed to preserve the structure and function of the protein's {{gli|native state}}.</dd>

{{term|protein sorting}} {{ghat|Also '''protein targeting'''.}} <dd>The set of biological mechanisms by which {{gli|proteins}} are directed and transported to appropriate destinations within or outside of the cell. Proteins must often be routed into the interior of {{gli|organelles}}, embedded {{gli|membrane protein|within a membrane}}, or {{gli|secreted}} into the extracellular environment in order to serve their functions, and information contained in the protein itself instructs this delivery process.<ref name="MCB9">{{Cite book |last=Lodish, Berk, Kaiser, Krieger, Bretscher, Ploegh, Martin, Yaffe, Amon |title=Molecular Cell Biology |publisher=W.H. Freeman and Company |year=2021 |isbn=978-1-319-20852-3 |edition=9th |location=New York, NY}}</ref> In eukaryotic cells, an expansive network of organelles and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biochemical pathway|pathways}} is specialized to facilitate protein sorting, including the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endoplasmic reticulum}} and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|Golgi apparatus}}.</dd>

{{term|protein tag}}{{anchor|protein tags|protein tagging}} <dd></dd>

{{term|protein targeting}} <dd>See ''{{gli|protein sorting}}''.</dd>

{{term|protein-coding gene}}{{anchor|protein-coding genes}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} containing a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding sequence}} which can be transcribed and translated to produce a {{gli|protein}}, as opposed to an {{gli|RNA gene}}, which produces {{gli|non-coding RNA}} transcripts that are not translated into proteins but instead have functions in and of themselves.</dd>

{{term|proteinogenic amino acid}} <dd>Any of the 20 canonical {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} which are encoded by the {{gli|standard genetic code}} and incorporated into {{gli|peptides}} and ultimately {{gli|proteins}} during {{gli|translation}}. The term may also be inclusive of an additional two amino acids encoded by non-standard {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic code|codes}} which can be incorporated by special translation mechanisms.</dd>

{{term|protein–protein interaction (PPI)}}{{anchor|protein–protein interaction|protein–protein interactions|protein-protein interaction|protein-protein interactions}} <dd></dd>

{{term|proteoglycan}}{{anchor|proteoglycans}} <dd>Any heavily {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycosylated}} {{gli|protein}}, i.e. a core polypeptide with one or more covalently attached glycosaminoglycan chains. Proteoglycans are therefore considered a subclass of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycoproteins}} in which the carbohydrate units are long, linear {{gli|polysaccharide}} polymers containing amino sugars<ref>{{cite web |title=Nomenclature of glycoproteins, glycopeptides and peptidoglycans, Recommendations 1985 |url=https://www.qmul.ac.uk/sbcs/iupac/misc/glycp.html |website=www.qmul.ac.uk |access-date=16 March 2021}}</ref> and generally bearing a net negative charge under physiological conditions due to the presence of sulfates and uronic acid groups. They are a major component of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extracellular matrix}} between animal cells, where they form large hydrated complexes commonly employed in connective tissues such as cartilage.</dd>

{{term|proteolysis}}{{anchor|proteolytic|proteolytically}} <dd>The decomposition of {{gli|proteins}} into their component {{gli|polypeptides}} or individual {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cleaving}} the {{gli|peptide bonds}} linking the amino acids together via hydrolysis. Proteolysis is an important reaction used not only for degrading and inactivating proteins but sometimes also to activate them by removing amino acid residues which inhibit their activity.<ref name="Lackie"/> It is usually catalyzed by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} known as {{gli|proteases}}.</dd>

{{term|proteome}} <dd>The entire set of {{gli|proteins}} that is or can be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}} by a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, cell, tissue, or species at a particular time (such as during a single lifespan or during a specific developmental stage) or under particular conditions (such as when compromised by a certain disease).</dd>

{{term|proteomics}} <dd>The study of the {{gli|proteome}} of a particular genome, cell, or organism, i.e. the sum total of all of the {{gli|proteins}} produced from it by {{gli|translation}}. Proteomics technologies allow scientists to {{gli|protein purification|purify}} and identify proteins and {{gli|polypeptides}} and determine which ones are most and least abundant at a given time or under a given experimental condition.</dd>

{{term|protomer}}{{anchor|protomers}} <dd>Any molecular subunit from which a larger {{gli|polymeric}} {{gli|macromolecule}} is built, including those subunits which are not strictly {{gli|monomers}} and can themselves be divided into subunits. For example, a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|heterodimer}} of tubulin proteins is the protomer for {{gli|microtubule}} assembly.<ref name="Lackie"/></dd>

{{term|proton motive force}} <dd>See ''{{gli|chemiosmotic coupling}}''.</dd>

{{term|protoplasm}} <dd>The biological contents enclosed within a {{gli|membrane-bound}} space, variously referring to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}, or the cytoplasm and {{gli|nucleoplasm}} considered collectively, and sometimes exclusive of {{gli|vacuoles}}.</dd>

{{term|protoplast}} <dd>A plant, fungal, or bacterial cell which has had its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell wall}} removed by mechanical, chemical, or enzymatic means; or the complete contents (the {{gli|protoplasm}}) of an intact cell excluding the cell wall.</dd>

{{term|pulsatile secretion}} <dd>The {{gli|secretion}} of substances from a cell, organelle, or tissue in a regular, rhythmic, pulse-like pattern. Many {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell signaling|intercellular signaling}} molecules such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hormones}} and {{gli|neurotransmitters}} are released in this manner in order to maintain {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homeostasis}} or to sensitize target cells by stimulating their production of {{gli|surface receptors}}.</dd>

{{term|purine}}{{anchor|purines}} {{ghat|Abbreviated in shorthand with the letter '''''{{font|R|font=courier|size=big}}'''''.}} <dd>A double-ringed heterocyclic organic compound which, along with {{gli|pyrimidine}}, is one of two molecules from which all {{gli|nitrogenous bases}} (including the {{gli|nucleobases}} used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} and {{gli|RNA}}) are derived. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine|Adenine}} ({{font|A|font=courier|size=big}}) and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine}} ({{font|G|font=courier|size=big}}) are classified as purines. The letter {{font|R|font=courier|size=big}} is sometimes used to indicate a generic purine; e.g. in a nucleotide sequence read, {{font|R|font=courier|size=big}} may be used to indicate that either purine nucleobase, {{font|A|font=courier|size=big}} or {{font|G|font=courier|size=big}}, can be substituted at the indicated position.</dd>

{{term|putative gene}} <dd>A specific {{gli|nucleotide sequence}} suspected to be a functional {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} based on the identification of its {{gli|open reading frame}}. The gene is said to be "putative" in the sense that no function has yet been described for its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene product|products}}.</dd>

{{term|pyknosis}} {{ghat|Also '''pycnosis''' or '''karyopyknosis'''.}} <dd>The irreversible condensation of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatin}} inside the {{gli|nucleus}} as the cell undergoes {{gli|necrosis}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|apoptosis}}, resulting in a compact mass which {{gli|stains}} strongly and is conspicuous under a microscope.<ref name="DoG7"/> It is followed by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|karyorrhexis}}.</dd>

{{term|pyrimidine}}{{anchor|pyrimidines}} {{ghat|Abbreviated in shorthand with the letter '''''{{font|Y|font=courier|size=big}}'''''.}} <dd>A single-ringed heterocyclic organic compound which, along with {{gli|purine}}, is one of two molecules from which all {{gli|nitrogenous bases}} (including the {{gli|nucleobases}} used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} and {{gli|RNA}}) are derived. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine|Cytosine}} ({{font|C|font=courier|size=big}}), {{gli|thymine}} ({{font|T|font=courier|size=big}}), and {{gli|uracil}} ({{font|U|font=courier|size=big}}) are classified as pyrimidines. The letter {{font|Y|font=courier|size=big}} is sometimes used to indicate a generic pyrimidine; e.g. in a nucleotide sequence read, {{font|Y|font=courier|size=big}} may be used to indicate that either pyrimidine nucleobase – {{font|C|font=courier|size=big}}, {{font|T|font=courier|size=big}}, or {{font|U|font=courier|size=big}} – can be substituted at the indicated position.</dd>

{{term|pyrimidine dimer}} <dd>A type of {{gli|molecular lesion}} caused by photochemical damage to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}}, whereby exposure to ultraviolet (UV) radiation induces the formation of covalent bonds between {{gli|pyrimidine}} {{gli|bases}} occupying adjacent positions in the same polynucleotide {{gli|strand}}, which in turn may cause local conformational changes in {{gli|secondary structure}} and prevent {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pairing}} with the opposite strand. In DNA, the dimerization reaction occurs between neighboring {{gli|thymine}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine}} residues ({{font|T|font=courier|size=big}}−{{font|T|font=courier|size=big}}, {{font|C|font=courier|size=big}}−{{font|C|font=courier|size=big}}, or {{font|T|font=courier|size=big}}−{{font|C|font=courier|size=big}}); it can also occur between cytosine and {{gli|uracil}} residues in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsRNA|double-stranded RNA}}. Pyrimidine dimers are usually quickly corrected by {{gli|nucleotide excision repair}}, but uncorrected lesions can inhibit or arrest polymerase activity during {{gli|transcription}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replication}}.</dd>

{{term|pyruvic acid}}{{anchor|pyruvate}} <dd></dd>

==Q== {{glossary}} {{term|quantitative PCR (qPCR)}}{{anchor|quantitative PCR}} {{ghat|Also '''real-time PCR (rtPCR)'''.}} <dd></dd>

{{term|quiescent culture}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell culture}} in which there is little or no active cell growth or replication but in which the cells nonetheless continue to survive, as observed with some {{gli|glossary=Glossary of cellular and molecular biology (0–L)|confluent}} cultures.<ref name="MacLean"/></dd>

==R== {{glossary}} {{term|random walk}} <dd>A popular description of the path followed by a {{gli|motility|locomotive cell}} or particle when there is no bias in movement, i.e. when the direction of movement at any given instant is not influenced by the direction of movement in the preceding instant. The essential randomness of cell movement in a uniform environment is only apparent over long periods of time, however; in the short term, cells can and do exhibit {{gli|persistence|a tendency to continue moving in the same direction}}.<ref name="Lackie"/></dd>

{{term|rDNA}} <dd>1. An abbreviation of {{gli|recombinant DNA}}.</dd> <dd>2. An abbreviation of {{gli|ribosomal DNA}}.</dd>

{{term|reading frame}}{{anchor|reading frames}} <dd>A way of dividing the {{gli|nucleotide sequence}} in a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}} molecule into a series of consecutive, non-overlapping groups of three nucleotides, known as {{gli|triplets}}, which is how the sequence is interpreted or "read" by {{gli|ribosomes}} during {{gli|translation}}. In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding DNA}}, each triplet is referred to as a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} and corresponds to a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} to be added to the nascent peptide chain during translation. In general, only one reading frame (the so-called {{gli|open reading frame}}) in a given sequence encodes a functional protein, though there are exceptions. A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|frameshift mutation}} results in a shift in the normal reading frame which affects all downstream codons and usually results in a completely different and senseless amino acid sequence.</dd>

{{term|readthrough}} <dd>The continued {{gli|transcription}} or {{gli|translation}} of a DNA or mRNA sequence, respectively, beyond encoded termination signals such as a {{gli|terminator}} sequence or a {{gli|stop codon}}.<ref name="Lackie"/></dd>

{{term|real-time PCR (rtPCR)}} <dd>See ''{{gli|quantitative PCR}}''.</dd>

{{term|reassociation kinetics}} <dd>The measurement and manipulation of the rate of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|annealing|reannealing}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} strands of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}}, generally by heating and denaturing a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsDNA|double-stranded}} molecule into {{gli|ssDNA|single strands}} and then observing their rehybridization at a cooler temperature. Because the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pair}} {{font|G|font=courier|size=big}}+{{font|C|font=courier|size=big}} requires more energy to anneal than the base pair {{font|A|font=courier|size=big}}+{{font|T|font=courier|size=big}}, the rate of reannealing between two strands depends partly on their {{gli|nucleotide sequence}}, and it is therefore possible to predict or estimate the sequence of the duplex molecule by the time it takes to fully hybridize. Reassociation kinetics is studied with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|C0t analysis|C0t analysis}}: fragments reannealing at low C0t values tend to have highly {{gli|repetitive sequences}}, while higher C0t values imply more unique sequences.<ref name="DoG7"/></dd>

{{term|receptor}}{{anchor|receptors}} <dd>A {{gli|protein}} which initiates a cellular response to an external stimulus or {{gli|signal transduction|propagates a molecular signal}} by binding a specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligand}}, often a dedicated signaling molecule. Numerous types of receptors exist which serve an enormous variety of functions. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell surface receptor|Cell-surface receptors}}, such as those that bind acetylcholine and insulin, are embedded within the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}} with their {{gli|glossary=Glossary of cellular and molecular biology (0–L)|binding sites}} exposed to the extracellular space; ''intracellular receptors'', including many {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hormone}} receptors, are located in the cytoplasm, where they bind ligands that have diffused across the membrane and into the cell.<ref name="Alberts et al."/></dd>

{{term|receptor-mediated endocytosis}} <dd></dd>

{{term|reciprocal translocation}} <dd>A type of {{gli|translocation|chromosomal translocation}} by which there is a reciprocal exchange of chromosome segments between two or more non-{{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous chromosomes|homologous}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}. When the exchange of material is evenly balanced, reciprocal translocations are usually harmless.</dd> thumb|right|350px|A '''{{gli|reciprocal translocation}}''' between chromosome 4 and chromosome 20

{{term|recognition site}}{{anchor|recognition sites}} <dd>A specific {{gli|motif}} or {{gli|sequence}}, either of nucleotides in a nucleic acid molecule or of amino acids in a protein, that is "recognized" or identified by another protein in order to direct the protein's activity to a specific molecule or location. Recognition motifs may consist of a simple consecutive sequence within a single molecule or may involve multiple non-consecutive motifs, e.g. amino acids in separate parts of the same polypeptide which are brought into juxtaposition by the {{gli|quaternary structure}} created during {{gli|protein folding}}. Recognition sites often help to distinguish the nucleic acid or protein bearing the motif from other similar molecules and thereby identify it as a valid target for some biochemical activity, or to specify a locus or subregion within the larger macromolecule at which the activity is to occur. In this sense recognition sites are critical for properly localizing proteins to their biochemical targets. A protein's recognition site is often but not necessarily the same as its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|binding site}} or {{gli|target site}}.<ref name="Oxford B&MB"/></dd>

{{term|recombinant DNA (rDNA)}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecule in which laboratory methods of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination}} have brought together genetic material from multiple sources, thereby creating a {{gli|nucleic acid sequence|sequence}} that would not otherwise be found in a naturally occurring genome. Because DNA molecules from all organisms share the same basic chemical structure and properties, DNA sequences from any species, or even sequences created ''de novo'' by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|artificial gene synthesis}}, may be incorporated into recombinant DNA molecules. Recombinant DNA technology is widely used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering}}.</dd>

{{term|recombinase}}{{anchor|recombinases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} united by their ability to catalyze some form of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination}}, by any of a variety of mechanisms, e.g. by {{gli|site-specific recombination}}.</dd>

{{term|recombination}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination}}'', ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous recombination}}'', and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomal crossover}}''.</dd>

{{term|recombinator}} <dd>Any {{gli|nucleotide sequence}} that increases the likelihood of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous recombination}} in nearby regions of the genome, e.g. the Chi sequence in certain species of bacteria.<ref name="DoG7"/></dd>

{{term|recon}} <dd>The smallest unit of a DNA molecule capable of undergoing {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous recombination}}, i.e. a pair of consecutive nucleotides, adjacent to each other in ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|cis}}''.<ref name="DoG7"/></dd>

{{term|regulon}} <dd>A group of non-contiguous {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} which are {{gli|regulated}} as a unit, generally by virtue of having their {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} controlled by the same regulatory element or set of elements, e.g. the same {{gli|repressor}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|activator}}. The term is most commonly used with prokaryotes, where a regulon may consist of genes from multiple {{gli|operons}}.</dd>

{{term|repeat}}{{anchor|repeats|repeated sequence|repeated sequences|repeated|repeating}} <dd>Any pattern of {{gli|nucleobases}} within a {{gli|nucleic acid sequence}} which occurs in multiple copies in the same nucleic acid molecule such as a chromosome or within a genome. Repeated sequences are classified according to their length, structure, location, mode of replication, or evolutionary origin. They may be any length, but are often short {{gli|motifs}} of less than 100 bases; they may be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|direct repeat|direct}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inverted repeat|inverted}}, and may occur in {{gli|tandem repeat|tandem arrays}} with the copies immediately adjacent to each other or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|interspersed repeat|interspersed}} with non-repeated sequences. Significant fractions of most eukaryotic genomes consist of {{gli|repetitive DNA}}, much of it retroviral in origin, though repeats may also result from errors in normal cellular processes, as with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|duplications}} during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}}. Because so many genetic mechanisms depend on the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA binding|binding}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementarity|complementing}} of locally unique sequences, sequences containing or adjacent to repeats are particularly prone to errors in replication and transcription by {{gli|strand slippage}}, or to forming problematic {{gli|secondary structures}}, and thus repeats are often unstable in the sense that the number of copies tends to expand or diminish stochastically with each round of replication, causing great {{gli|glossary=Glossary of cellular and molecular biology (0–L)|copy number variation|variation in copy number}} even between different cells in the same organism. When repeats occur within {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation|regulatory elements}}, these properties often result in aberrant expression and lead to disease. Repeats are also essential for normal genome function in other contexts, as with {{gli|telomeres}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromeres}}, which consist largely of repetitive sequences.</dd>

{{term|repetitive DNA}} {{ghat|Also '''repetitious DNA'''.}} <dd>A region or fragment of DNA consisting largely or entirely of {{gli|repeated}} {{gli|nucleotide sequences}}.</dd>

{{term|replacement mutation}} <dd>See ''{{gli|nonsynonymous mutation}}''.</dd>

{{term|replication}} <dd>1. The process by which certain biological molecules, notably the {{gli|nucleic acids}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} and {{gli|RNA}}, produce copies of themselves.</dd> <dd>2. A technique used to estimate technical and biological variation in experiments for statistical analysis of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA microarray|microarray}} data. Replicates may be ''technical replicates'', such as dye swaps or repeated array {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridizations}}, or ''biological replicates'', biological samples from separate experiments which are used to test the effects of the same experimental treatment.</dd>

{{term|replication eye}} {{ghat|Also '''replication bubble'''.}} <dd>The eye-shaped structure that forms when a pair of {{gli|replication forks}}, each growing away from the {{gli|origin of replication|origin}}, separates the strands of the double helix during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}.</dd>

{{term|replication fork}}{{anchor|replication forks}} {{ghat|Also '''Y fork'''.}} <dd>The point at which the paired strands of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecule are separated by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|helicase}} during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}, breaking the hydrogen bonds between the complementary strands and thereby forming a structure with two branching single strands of DNA. Once unpaired, these strands serve as {{gli|template strand|templates}} from which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerase}} synthesizes the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|leading strand}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lagging strand}}. As replication proceeds, helicase moves along the DNA and continues to separate the strands, causing the replication fork to move as well.<ref name="Lackie"/> A pair of replication forks forms when helicases work in opposite directions from a single {{gli|origin of replication}}, creating a {{gli|replication eye}}.</dd>

{{term|replication rate}} <dd>The speed at which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonucleotides}} are incorporated into an elongating chain by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerases}} during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}; or more generally the speed at which any chromosome, genome, cell, or organism makes a complete, independently functional copy of itself.</dd>

{{term|replicator}} <dd>1. Any fragment or region of DNA that contains a {{gli|replication origin}}.<ref name="DoG7"/></dd> <dd>2. Any molecule or structure capable of {{gli|replication|copying itself}}; namely, {{gli|nucleic acids}}, but also crystals of many minerals, e.g. kaolinite.</dd>

{{term|replicon}} <dd>Any molecule or region of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}} that {{gli|replication|replicates}} from a single {{gli|origin of replication}}.</dd>

{{term|replisome}} <dd>The entire complex of molecular machinery that carries out the process of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}, including all proteins, nucleic acids, and other molecules which participate at an active {{gli|replication fork}}.</dd>

{{term|reporter}} <dd>In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering}}, a gene which when properly expressed encodes a gene product that is easily detected or visualized with biochemical assays (e.g. green fluorescent protein, β-galactosidase, chloramphenicol O-acetyltransferase, etc.), allowing researchers to use its expression in order to study the functions and properties of associated {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation|regulatory sequences}}. Reporters are commonly {{gli|molecular cloning|cloned}} into plasmid {{gli|vectors}} in proximity to putative {{gli|promoters}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enhancers}}, or {{gli|response elements}}, which are then mutated in order to precisely identify the specific {{gli|recognition motifs}} within these sequences that are necessary for expression. In the broadest sense, reporters may also include things like molecular {{gli|protein tag|tags}}, fluorescent {{gli|glossary=Glossary of cellular and molecular biology (0–L)|labels}}, and {{gli|probe|hybridization probes}} which render their conjugated molecules conspicuous or able to be purified; or they may be used similarly to {{gli|selectable markers}}, to distinguish cells that express a given product from those that do not, so that researchers can easily identify mutants of interest or verify the success of an experimental treatment or laboratory procedure.<ref name="Oxford B&MB"/></dd>

{{term|repression}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|downregulation}}''.</dd>

{{term|repressor}}{{anchor|repressors}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding protein}} that inhibits the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} of one or more {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} by binding to the {{gli|operator}} and blocking the attachment of {{gli|RNA polymerase}} to the {{gli|promoter}}, thus preventing {{gli|transcription}}. This process is known as {{gli|negative control|negative gene regulation}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|downregulation|repression}}.</dd>

{{term|rescue}}{{anchor|rescued|rescues}} <dd>The restoration of a defective cell or tissue to a healthy or normal condition,<ref name="DoG7"/> or the {{gli|reversion}} or recovery of a mutant gene to its normal functionality, especially in the context of experimental genetics, where an experiment (e.g. a drug, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cross}}, or gene transfer) resulting in such a restoration is said to ''rescue'' the normal {{gli|phenotype}}.</dd>

{{term|residue}}{{anchor|residues}} <dd>An individual {{gli|monomer}} or {{gli|subunit}} of a larger {{gli|polymeric}} {{gli|macromolecule}}; e.g. a {{gli|nucleic acid}} is composed of {{gli|nucleotide}} residues, and a {{gli|peptide}} or {{gli|protein}} is composed of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} residues.<ref name="DoG7"/></dd>

{{term|response element}}{{anchor|response elements}} <dd>Any short sequence of DNA that serves some function related to the activity of a protein or other biomolecule, especially a sequence within a {{gli|promoter}} region that is able to bind specific {{gli|transcription factors}} in order to {{gli|regulate}} {{gli|transcription}} of specific genes.</dd>

{{term|restitution}} <dd>The spontaneous rejoining of an experimentally broken {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} which restores the original configuration.</dd>

{{term|restitution nucleus}} <dd>A {{gli|nucleus}} containing twice the expected number of chromosomes owing to an error in cell division, especially an unreduced, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} product of {{gli|meiosis}} resulting from the failure of the first or second meiotic division.</dd>

{{term|restriction cloning}} <dd>The use of {{gli|restriction sites}} and the {{gli|restriction enzyme|nucleases}} that specifically recognize them in order to {{gli|molecular cloning|clone}} {{gli|recombinant DNA}} molecules.</dd>

{{term|restriction enzyme}}{{anchor|restriction endonuclease|restriction exonuclease|restrictase|restriction enzymes}} {{ghat|Also '''restriction endonuclease''', '''restriction exonuclease''', or '''restrictase'''.}} <dd>An {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endonuclease}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exonuclease}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzyme}} that recognizes and cleaves a nucleic acid molecule into {{gli|restriction fragment|fragments}} at or near specific recognition sequences known as {{gli|restriction sites}} by breaking the {{gli|phosphodiester bonds}} of the nucleic acid {{gli|backbone}}. Restriction enzymes are naturally occurring in many organisms, but are also routinely used for artificial modification of DNA in laboratory techniques such as {{gli|restriction cloning}}.</dd>

{{term|restriction fragment}}{{anchor|restriction fragments}} <dd>Any DNA fragment that results from the cutting of a DNA strand by a {{gli|restriction enzyme}} at one or more {{gli|restriction sites}}.</dd>

{{term|restriction fragment length polymorphism (RFLP)}} <dd>Variability within a population of organisms observed in the size of the {{gli|restriction fragments}} produced when {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomic DNA}} (or any particular DNA molecule) is {{gli|restriction digest|digested}} by one or more {{gli|restriction endonucleases}}. This variability results from a corresponding {{gli|polymorphism}} in the locations of restriction sites within the molecule(s) due to slight differences in nucleotide sequence between individuals. RFLP is frequently exploited in the laboratory to construct physical {{gli|restriction map|maps of the genome}}, to identify the specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|locus}} occupied by a particular gene, and to detect genetic differences between closely related individuals or determine that different samples originated from the same individual. Analysis of restriction fragments can also reveal the presence of a mutation that may itself cause disease or be closely linked to one that does.<ref name="Oxford B&MB"/></dd>

{{term|restriction map}}{{anchor|restriction maps}} <dd>A diagram of known {{gli|restriction sites}} within a known DNA sequence, such as a {{gli|plasmid}} {{gli|vector}}, obtained by systematically exposing the sequence to various {{gli|restriction enzymes}} and then comparing the lengths of the resulting {{gli|restriction fragment|fragments}}, a technique known as {{gli|restriction mapping}}. See also ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene map}}''.</dd>

{{term|restriction mapping}} <dd>The use of type II {{gli|restriction endonucleases}} to cleave DNA molecules at specific {{gli|restriction sites}} in order to produce characteristic patterns of {{gli|restriction fragment|fragments}} which can be resolved by size using {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gel electrophoresis}}. {{gli|restriction digest|Digesting}} DNA molecules such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomic DNA}} or {{gli|plasmids}} with one or multiple restriction enzymes makes it possible to deduce from the sizes of the resulting fragments the order or arrangement of the restriction sites within the molecule and the distances between them, and thus to construct reliable {{gli|restriction map|maps}} with restriction sites effectively serving as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic markers}}.<ref name="Oxford B&MB"/></dd>

{{term|restriction site}}{{anchor|restriction sites}} {{ghat|Also '''restriction recognition site'''.}} <dd>A short, specific {{gli|sequence}} of nucleotides (typically 4 to 8 bases in length) that is reliably recognized by a particular {{gli|restriction enzyme}}. Because restriction enzymes usually bind as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homodimers}}, restriction sites are generally {{gli|palindromic sequences}} spanning both strands of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsDNA|double-stranded DNA}} molecule. Restriction {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endonucleases}} cleave the {{gli|phosphate backbone}} between two nucleotides within the recognized sequence itself, while other types of restriction enzymes make their cuts at one end of the sequence or at a nearby sequence.</dd> [[File:制限酵素 EcoR1 原理図.svg|thumb|right|350px|A double-stranded DNA molecule containing the sequence {{font|GAATTC|font=courier|size=big}} and its palindromic complement functions as a '''{{gli|restriction site}}''' for the bacterial enzyme ''EcoRI'', which recognizes and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cut|cuts or "digests"}} it in the manner shown here, by breaking phosphodiester bonds in the backbones of both strands and leaving behind {{gli|sticky end|single-stranded overhangs}} at the ends of each of the now separate molecules.]]

{{term|retrogene}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or other DNA sequence which has arisen in a genome by the stable {{gli|glossary=Glossary of cellular and molecular biology (0–L)|integration}} of the genetic material of a retrovirus, e.g. by the {{gli|reverse transcription}} of viral RNA and the subsequent {{gli|glossary=Glossary of cellular and molecular biology (0–L)|insertion}} of the resulting DNA fragments into the host cell's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomic DNA}}.<ref name="Oxford B&MB"/> These endogenous viral elements can then be replicated along with the host's own genes and thus persist indefinitely in the host genome, and in many cases retain the ability to produce functional viral proteins from this latent state, by which they may continue to copy, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excise}}, and/or {{gli|transpose}} themselves.</dd>

{{term|reverse genetics}} <dd>An experimental approach in {{gli|molecular genetics}} in which a researcher starts with a known {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} and attempts to determine its function or its effect on phenotype by any of a variety of laboratory techniques, commonly by deliberately mutating the gene's DNA sequence or by {{gli|repressing}} or {{gli|silencing}} its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} and then {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic screening|screening}} the mutant organisms for changes in phenotype. When the gene of interest is the only one in the genome whose expression has been manipulated, any observed phenotypic changes are assumed to be influenced by it. This is the opposite of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|forward genetics}}, in which a known phenotype is linked to one or more unknown genes.</dd>

{{term|reverse transcriptase (RT)}}{{anchor|reverse transcriptase}} <dd>An enzyme capable of synthesizing a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cDNA|complementary DNA}} molecule from an {{gli|RNA}} template, a process termed {{gli|reverse transcription}}.</dd>

{{term|reverse transcription}} <dd>The synthesis of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecule from an {{gli|RNA}} {{gli|template}}, i.e. the opposite of ordinary {{gli|transcription}}. This process, mediated by an enzyme known as a {{gli|reverse transcriptase}}, is used by many viruses to {{gli|replicate}} their genomes, as well as by {{gli|retrotransposons}} and in some eukaryotic cell types.</dd>

{{term|Rho factor}} <dd>A protein {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cofactor}} involved in signaling the termination of {{gli|transcription}} in bacteria. Rho factor recognizes and binds a C-rich/G-poor {{gli|terminator}} sequence in the nascent RNA transcript, which promotes dissociation of the transcriptional complex and release of the transcript from the DNA template. Other mechanisms of termination which do not require Rho factor also exist.</dd>

{{term|ribonuclease (RNase)}}{{anchor|ribonucleases}} <dd>Any of a class of {{gli|nuclease}} enzymes which catalyze the hydrolytic cleavage of {{gli|phosphodiester bonds}} in {{gli|RNA}} molecules, thus severing polymeric {{gli|strands}} of {{gli|ribonucleotides}} into smaller components. Compare ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonuclease}}''.</dd>

{{term|ribonucleic acid (RNA)}} <dd>A polymeric {{gli|nucleic acid}} molecule composed of a series of {{gli|ribonucleotides}} which incorporate a set of four {{gli|nucleobase|nucleobases}}: {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine}} ({{font|A|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine}} ({{font|G|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine}} ({{font|C|font=courier|size=big}}), and {{gli|uracil}} ({{font|U|font=courier|size=big}}). Unlike {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}}, RNA is more often found as a {{gli|ssRNA|single strand}} folded onto itself, rather than a paired double strand. Various types of RNA molecules serve in a wide variety of essential biological roles, including {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic code|coding}}, {{gli|translation|decoding}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation|regulating}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressing}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}}, as well as functioning as signaling molecules and, in certain viral genomes, as the primary genetic material itself.</dd>

{{term|ribonucleoprotein (RNP)}} <dd>A {{gli|nucleoprotein}} that is a complex of one or more {{gli|RNA}} molecules and one or more {{gli|proteins}}. Examples include {{gli|ribosomes}} and the enzyme ribonuclease P.</dd>

{{term|ribonucleotide}}{{anchor|ribonucleotides}} <dd>A {{gli|nucleotide}} containing {{gli|ribose}} as its pentose sugar component, and the monomeric subunit of {{gli|ribonucleic acid}} (RNA) molecules. Ribonucleotides canonically incorporate any of four {{gli|nitrogenous bases}}: {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine}} ({{font|A|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine}} ({{font|G|font=courier|size=big}}), {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine}} ({{font|C|font=courier|size=big}}), and {{gli|uracil}} ({{font|U|font=courier|size=big}}). Compare ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonucleotide}}''.</dd>

{{term|ribonucleotide reductase (RNR)}} {{ghat|Also '''ribonucleoside diphosphate reductase'''.}} <dd>An enzyme which catalyzes the formation of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribonucleotides}} via the reductive dehydroxylation of {{gli|ribonucleotides}}, specifically by removing the 2' hydroxyl group from the {{gli|ribose}} ring of {{gli|nucleoside diphosphate|ribonucleoside diphosphates}} (rNDPs). RNR plays a critical role in regulating the overall rate of DNA synthesis such that the ratio of DNA to cell mass is kept constant during cell division and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA repair}}.</dd>

{{term|ribose}} <dd>A {{gli|monosaccharide}} sugar which, as D-ribose in its {{gli|pentose}} ring form, is one of three primary components of the {{gli|ribonucleotides}} from which {{gli|RNA|ribonucleic acid}} (RNA) molecules are built. Ribose differs from its structural analog {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}} only at the 2' carbon, where ribose has an attached hydroxyl group that deoxyribose lacks.</dd>

{{term|ribosomal DNA (rDNA)}} <dd>A DNA sequence that codes for {{gli|ribosomal RNA}} (rRNA). In many eukaryotic genomes, rDNA occupies large, highly conserved regions of multiple chromosomes and is rich in both {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} and {{gli|repeats}}.</dd>

{{term|ribosomal RNA (rRNA)}}{{anchor|ribosomal RNA}} <dd>A type of {{gli|non-coding RNA}} which is the primary constituent of {{gli|ribosomes}}, binding to ribosomal proteins to form the {{gli|small ribosomal subunit|small}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|large ribosomal subunit|large subunits}}. It is ribosomal RNA which enables ribosomes to perform protein synthesis by working as a {{gli|ribozyme}} that catalyzes the set of reactions comprising {{gli|translation}}. Ribosomal RNA is transcribed from the corresponding {{gli|ribosomal DNA}} (rDNA) and is the most abundant class of RNA in most cells, bearing responsibility for the translation of all encoded proteins despite never being translated itself.</dd>

{{term|ribosome}}{{anchor|ribosomes|ribosomal}} <dd>A macromolecular complex made of both RNA and protein which serves as the site of protein synthesis by {{gli|translation}}. Ribosomes have two {{gli|subunits}}, each of which consists of one or more strands of {{gli|rRNA|ribosomal RNA}} bound to various ribosomal proteins: the {{gli|small subunit}}, which reads the messages encoded in {{gli|messenger RNA}} molecules, and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|large subunit}}, which links {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} in sequence to form a {{gli|polypeptide}} chain. Ribosomes are essential and ubiquitous in all cell types and are used by all known forms of life.</dd>

{{term|riboswitch}} <dd>A {{gli|regulatory}} sequence within a {{gli|messenger RNA}} {{gli|transcript}} that can bind a small effector molecule, preventing or disrupting {{gli|translation}} and thereby acting as a switch that regulates the mRNA's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}}.</dd>

{{term|ribozyme}}{{anchor|ribozymes}} <dd>An {{gli|RNA}} molecule with enzymatic activity,<ref name="Alberts et al."/> i.e. one that is capable of catalyzing one or more specific biochemical reactions, similar to {{gli|protein}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}}. Ribozymes function in numerous capacities, including in {{gli|ribosomes}} as part of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|large subunit}} {{gli|ribosomal RNA}}.</dd>

{{term|RNA}} <dd>See ''{{gli|RNA|ribonucleic acid}}''.</dd>

{{term|RNA gene}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} that codes for any of the various types of {{gli|ncRNA|non-coding RNA}} (e.g. {{gli|rRNA}} and {{gli|tRNA}}).<ref name="DoG7"/></dd>

{{term|RNA interference (RNAi)}} <dd></dd>

{{term|RNA polymerase}}{{anchor|RNA polymerase}} {{ghat|Often abbreviated '''RNAP''' or '''RNApol'''.}} <dd>Any of a class of {{gli|polymerase}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} that synthesize {{gli|RNA}} molecules from a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} template. RNA polymerases are essential for {{gli|transcription}} and are found in all living organisms and many viruses. They build long single-stranded polymers called {{gli|primary transcript|transcripts}} by adding {{gli|ribonucleotides}} one at a time in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5'}}-to-{{gli|glossary=Glossary of cellular and molecular biology (0–L)|3'}} direction, relying on the {{gli|template strand|template}} provided by the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} strand to transcribe the nucleotide sequence faithfully. Unlike {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerases}}, RNA polymerases notably do not require oligonucleotide {{gli|primers}} to initiate synthesis; i.e. they are capable of synthesizing RNA molecules ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|de novo synthesis|de novo}}''.</dd>

{{term|RNA splicing}} <dd></dd>

{{term|RNA-induced silencing complex (RISC)}} <dd>A {{gli|ribonucleoprotein}} complex which works to {{gli|silence}} endogenous and exogenous genes by participating in various {{gli|RNAi|RNA interference}} pathways at the transcriptional and translational levels. RISC can bind both {{gli|ssRNA|single-stranded}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsRNA|double-stranded RNA}} fragments and then cleave them or use them as guides to target complementary mRNAs for degradation.</dd>

{{term|RNase}} <dd>See ''{{gli|ribonuclease}}''.</dd>

{{term|Robertsonian translocation (ROB)}} <dd>A type of {{gli|translocation|chromosomal translocation}} by which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-strand breaks}} at or near the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromeres}} of two {{gli|glossary=Glossary of cellular and molecular biology (0–L)|acrocentric}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} cause a reciprocal exchange of segments that gives rise to one large {{gli|metacentric}} chromosome (composed of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|long arms}}) and one extremely small chromosome (composed of the {{gli|short arms}}), the latter of which is often subsequently lost from the cell with little effect because it contains very few genes. The resulting {{gli|glossary=Glossary of cellular and molecular biology (0–L)|karyotype}} shows one fewer than the expected total number of chromosomes, because two previously distinct chromosomes have essentially fused together. {{gli|glossary=Glossary of cellular and molecular biology (0–L)|carrier|Carriers}} of Robertsonian translocations are generally not associated with any phenotypic abnormalities, but do have an increased risk of generating meiotically unbalanced {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gametes}}.</dd>

{{term|rolling circle replication (RCR)}} <dd></dd>

{{term|rough endoplasmic reticulum (RER)}}{{anchor|rough endoplasmic reticulum|rough ER}} <dd>A type of membrane in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endoplasmic reticulum}} with numerous {{gli|ribosomes}} conspicuously attached to its surface, in contrast to the {{gli|smooth endoplasmic reticulum|"smooth" endoplasmic reticulum}} which lacks ribosomes. The rough ER serves as the site of protein synthesis for the majority of the cell's secreted and transmembrane proteins, as well as the site of synthesis of membrane lipids. It may be continuous with the smooth ER or exist separately.<ref name="Lackie"/></dd>

{{term|rRNA}} <dd>See ''{{gli|rRNA|ribosomal RNA}}''.</dd>

{{term|rtPCR}} <dd>1. An abbreviation of ''real-time polymerase chain reaction'', synonymous with {{gli|quantitative PCR}}.</dd> <dd>2. An abbreviation of {{gli|reverse transcription PCR|reverse transcription polymerase chain reaction}}.</dd>

==S== {{glossary}} {{term|S phase}} {{ghat|Also '''synthesis phase''' or '''synthetic phase'''.}} <dd>The phase of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell cycle}} during which {{gli|nuclear DNA}} is {{gli|replicated}}, occurring after the {{gli|G1}} phase and before the {{gli|G2}} phase.<ref name="MacLean"/></dd>

{{term|salvage pathway}} <dd>Any {{gli|metabolic pathway}} that utilizes compounds formed in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|catabolism}} for the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anabolism}} or biosynthesis of new compounds, e.g. by recycling building block monomers such as free {{gli|purine}} and {{gli|pyrimidine}} {{gli|nitrogenous base|bases}} to make new {{gli|nucleotides}}.<ref name="Oxford B&MB"/></dd>

{{term|samesense mutation}} <dd>See ''{{gli|synonymous mutation}}''.</dd>

{{term|Sanger sequencing}} <dd>A method of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA sequencing}} based on the ''in vitro'' {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replication}} of a DNA {{gli|template strand|template}} sequence, during which fluorochrome-labeled, chain-terminating dideoxynucleotides are randomly incorporated in the elongating strand; the resulting fragments are then sorted by size with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|electrophoresis}}, and the particular fluorochrome terminating each of the size-sorted fragments is detected by laser chromatography, thus revealing the {{gli|sequence}} of the original DNA template through the order of the fluorochrome labels as one reads from small-sized fragments to large-sized fragments. Though Sanger sequencing has been replaced in some contexts by {{gli|next-generation sequencing|next-generation}} methods, it remains widely used for its ability to produce relatively long sequence reads (500+ {{gli|nucleotides}}) and its very low error rate.</dd> thumb|right|450px|An outline of the '''{{gli|Sanger sequencing}}''' method

{{term|saturation hybridization}} <dd>An ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|in vitro}}'' nucleic acid {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridization}} reaction in which one polynucleotide component (either {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}}) is supplied in great excess relative to the other, causing all complementary sequences in the other polynucleotide to pair with the excess sequences and form hybrid {{gli|glossary=Glossary of cellular and molecular biology (0–L)|duplex}} molecules.<ref name="DoG7"/></dd>

{{term|scRNA}} <dd>See ''{{gli|scRNA|small conditional RNA}}''.</dd>

{{term|second messenger}}{{anchor|second messengers}} {{ghat|Also '''secondary messenger'''.}} <dd>A molecule or compound (often a protein) that is caused to accumulate in an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|effector}} cell by the action of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hormone}}, growth factor, or other agonist and thereby brings about the action of that agonist on the cell. Second messengers are therefore critical mediators of a diverse variety of {{gli|signal transduction}} pathways, including the synthesis of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cyclic AMP}} by adenylate cyclase and of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cyclic GMP}} by guanylate cyclase, the opening of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ion channels}}, and the {{gli|phosphorylation}} of proteins by serine/threonine-specific or tyrosine-specific {{gli|protein kinases}}.<ref name="Oxford B&MB"/></dd>

{{term|secondary structure}} <dd>The arrangement or {{gli|folding}} of a {{gli|polypeptide}}'s {{gli|primary structure}} into higher-order, locally organized structures, primarily via hydrogen bonding between non-adjacent amino acid residues, in particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|alpha helices|α helices}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|beta sheets|β sheets}}; or the arrangement of double-stranded {{gli|nucleic acid}} chains into the shape of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double helix}} stabilized by hydrogen bonds between the complementary bases.<ref name="Oxford B&MB"/></dd>

{{term|second-generation sequencing}} <dd>See ''{{gli|massively parallel sequencing}}''.</dd>

{{term|selectable marker}}{{anchor|selectable markers}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or other genetic material whose {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} in cultured cells confers a selective advantage in the culture environment, causing cells expressing the gene to have one or more traits suitable for artificial selection. Selectable markers are widely used in the laboratory as a type of {{gli|reporter}}, usually to indicate the success of a procedure meant to introduce exogenous DNA into a host cell such as {{gli|transfection}} or {{gli|transformation}}. A common example is an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antibiotic resistance gene}} which is transformed into competent bacterial cells cultured on a medium containing the particular antibiotic, such that only those cells which have successfully taken up and expressed the gene are able to survive and grow into colonies.</dd>

{{term|selfish genetic element}} {{ghat|Also '''selfish DNA''' or '''parasitic DNA'''.}} <dd>Any genetic material (e.g. a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or any other DNA sequence) which can enhance its own {{gli|replication}} and/or transmission into subsequent generations at the expense of other genes in the genome, even if doing so has no positive effect or even a net negative effect on the {{gli|glossary=Glossary of genetics and evolutionary biology|fitness}} of the genome as a whole. Selfish elements usually work by producing self-acting {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene products}} which repeatedly {{gli|glossary=Glossary of cellular and molecular biology (0–L)|copy and paste}} their own {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding sequences}} into other parts of the genome, independently of normal {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} (as with {{gli|transposable elements}}); by facilitating the uneven swapping of chromosome segments during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic recombination}} events (as with {{gli|unequal crossing over}}); or by disrupting the normally equal redistribution of replicated material during {{gli|mitosis}} or {{gli|meiosis}} such that the probability that the selfish element is present in a given {{gli|glossary=Glossary of cellular and molecular biology (0–L)|daughter cell}} is greater than the normal 50 percent (as with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene drives}}).</dd>

{{term|semiconservative replication}} <dd>The standard mode of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} that occurs in all living cells, in which each of the two parental {{gli|strands}} of the original {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsDNA|double-stranded DNA}} molecule are used as {{gli|template strands}}, with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA polymerases}} replicating each strand separately and simultaneously in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antiparallel}} directions. The result is that each of the two double-stranded daughter molecules is composed of one of the original parental strands and one newly synthesized complementary strand, such that each daughter molecule conserves the precise sequence of information (indeed the very same atoms) from one-half of the original molecule. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|conservative replication}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|dispersive replication}}''.</dd> thumb|right|400px|Three different modes of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}}. In '''{{gli|semiconservative replication}}''', each of the two daughter molecules is built from one of the original parental strands and one newly synthesized strand. In '''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|conservative replication}}''', the original parent molecule remains intact while the replicated molecule is composed of two newly synthesized strands. In '''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|dispersive replication}}''', each of the daughter molecules is an uneven mix of old and new, with some segments consisting of the two parental strands and others consisting of two newly synthesized strands. Only semiconservative replication occurs naturally.

{{term|sense}} <dd>A distinction made between the individual {{gli|strands}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecule in order to easily and specifically identify each strand. The two {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} strands are distinguished as ''sense'' and ''antisense'' or, equivalently, the ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding strand}}'' and the ''{{gli|template strand}}''. It is the antisense/template strand which is actually used as the template for {{gli|transcription}}; the sense/coding strand merely resembles the sequence of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codons}} on the RNA transcript, which makes it possible to determine from the DNA sequence alone the expected amino acid sequence of any protein {{gli|translated}} from the RNA transcript. Which strand is which is relative only to a particular RNA transcript and not to the entire DNA molecule; that is, either strand can function as the sense/coding or antisense/template strand.</dd>

{{term|sense codon}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} that specifies an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}}, as opposed to a {{gli|stop codon}}, which does not specify any particular amino acid but instead signals the end of translation.</dd>

{{term|sequence}} <dd>See ''{{gli|nucleic acid sequence}}''.</dd>

{{term|sequence logo}}{{anchor|sequence logos}} <dd>In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|bioinformatics}}, a graphical representation of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conserved sequence|conservation}} of {{gli|nucleobases}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} at each position within a nucleic acid or protein {{gli|sequence}}. Sequence logos are created by {{gli|sequence alignment|aligning}} many sequences and used to depict {{gli|glossary=Glossary of cellular and molecular biology (0–L)|consensus sequences}} as well as the degree of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|variability}} within the pool of aligned sequences.</dd> 400px|thumb|right|A '''{{gli|sequence logo}}''' depicts the statistical frequency with which each nucleobase (or amino acid) occurs within a given {{gli|sequence}}. Each position in the sequence is represented by a vertical stack of letters; the total height of the stack indicates the degree of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|consensus sequence|consensus}} at that position between all of the aligned sequences, and the height of each individual letter in the stack indicates the proportion of the aligned sequences having that nucleobase at that position. A single very large letter filling most of the stack indicates that most or all of the aligned sequences have that particular nucleobase at that position.

{{term|sequence-tagged site (STS)}} <dd>Any DNA {{gli|nucleotide sequence|sequence}} that occurs exactly once within a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, and whose {{gli|glossary=Glossary of cellular and molecular biology (0–L)|locus|location}} and nucleotide sequence are known with confidence.</dd>

{{term|sequencing}} <dd>The determination of the order or {{gli|nucleic acid sequence|sequence}} of {{gli|nucleotides}} in a {{gli|nucleic acid}} molecule, or of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} in a {{gli|peptide}}, by any means. Sequences are usually written as a linear string of letters which conveniently summarizes much of the atomic-level structure of the molecule.</dd>

{{term|sex chromosome}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|allosome}}''.</dd>

{{term|sex linkage}}{{anchor|sex-linked}} <dd>The presence of a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or DNA sequence on a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|sex chromosome}} (in mammals either the {{gli|X chromosome}} or the {{gli|Y chromosome}}) rather than on an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|autosome}}; these genes are said to be ''sex-linked''. Expression of sex-linked genes varies by organism depending on the mechanism of sex determination and the types of sex chromosomes present, but the associated {{gli|phenotypes}} often exclusively appear in either the homogametic or heterogametic sex.<ref name="MacLean"/></dd>

{{term|Shine–Dalgarno sequence}} <dd>In many prokaryotic {{gli|mRNA|messenger RNAs}}, the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|consensus sequence}} {{font|AGGAGGU|font=courier|size=big}}, located 6–8 bases upstream of the translation {{gli|start codon}}, which functions as a binding site for the {{gli|ribosome}} by complementing a sequence in the {{gli|ribosomal RNA}}.<ref name="DoG7"/></dd>

{{term|short arm}}{{anchor|short arms}} {{ghat|Denoted in shorthand with the symbol '''''p'''''.}} <dd>In condensed {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} where the positioning of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromere}} creates two segments or "arms" of unequal length, the shorter of the two arms of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatid}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|long arm}}''.</dd>

{{term|short interspersed nuclear element (SINE)}} <dd></dd>

{{term|short tandem repeat (STR)}} <dd>See ''{{gli|microsatellite}}''.</dd>

{{term|shotgun sequencing}} <dd>A method of {{gli|whole genome sequencing|sequencing entire genomes}} in which {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomic DNA}} is randomly fragmented (e.g. by {{gli|sonication}} or {{gli|restriction digests}}), {{gli|molecular cloning|cloned}} into {{gli|plasmid}} vectors, and then {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA sequencing|sequenced}} using {{gli|primers}} that anneal to flanking sequences in the plasmids. Computer software is used to {{gli|sequence alignment|align the sequenced fragments}} via overlapping {{gli|glossary=Glossary of cellular and molecular biology (0–L)|contigs}},<ref name="Oxford B&MB"/> allowing scientists to deduce the relative genomic locations of each fragment and thereby assemble a complete genome.</dd>

{{term|signal peptidase}} <dd></dd>

{{term|signal peptide}}{{anchor|signal peptides}} {{ghat|Also '''leader peptide''', '''prepeptide''', and '''presequence'''.}} <dd>Any sequence of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}}, usually 15–30 residues in length, that functions as a molecular signal directing the {{gli|sorting}} and transport of the {{gli|polypeptide}} bearing it to a {{gli|subcellular localization|specific location}} within a cell or organelle. Signal peptides are commonly located close to either the {{gli|N-terminal}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|C-terminal}} ends of {{gli|nascent}} or recently synthesized polypeptides, especially those destined for secretion from the cell or integration into a membrane, and are typically cleaved off by a dedicated {{gli|signal peptidase}} when their polypeptide reaches the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endoplasmic reticulum}}.<ref name="Oxford B&MB"/> Similar but distinct varieties of protein targeting are accomplished with {{gli|nuclear localization signals}} and {{gli|post-translational}} {{gli|protein tags}}.</dd>

{{term|signal transduction}} <dd>The process by which a chemical, electrical, or mechanical signal is converted into a cellular response, or the transmission or propagation of such a signal through a cell as a series of molecular events known as a {{gli|signaling pathway}}. For example, the extracellular interaction of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hormone}}, growth factor, or some other chemical agonist with a specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell surface receptor}} can trigger a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|biochemical cascade|cascade}} of sequential biochemical reactions which propagate through the cell membrane and into the cytoplasm, provoking the synthesis of {{gli|second messengers}} and leading to amplification of the signal or activation of other pathways. Other modes of transduction involve agonists which diffuse across the membrane freely, eliciting intracellular changes without amplification, or rapid shifts in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell polarity}} which transmit electrical impulses, such as those that cause the axons of neural cells to release neurotransmitters at synapses.<ref name="Oxford B&MB"/></dd>

{{term|silencer}} <dd>A sequence or region of DNA that can be bound by a {{gli|repressor}}, thereby blocking the {{gli|transcription}} of a nearby gene.</dd>

{{term|silencing}}{{anchor|silence|silences|silenced}} <dd>The total or near-total loss of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} of a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or DNA sequence by any mechanism, natural or artificial, whether before, during, or after {{gli|transcription}} or {{gli|translation}}, which completely prevents the normal {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene product}} from being produced and thereby deprives the cell of its ordinary function. Gene silencing may occur via natural {{gli|glossary=Glossary of cellular and molecular biology (0–L)|regulatory}} mechanisms such as condensation of the relevant segment of DNA into a transcriptionally inactive, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|heterochromatic}} state, in which case the term is more or less equivalent to {{gli|repression}}; genes are also commonly silenced artificially for research purposes by using techniques such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|knockdown}} (e.g. by {{gli|RNA interference}}) or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|knockout}} (by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deletion|deleting}} the gene from the genome entirely). See also ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|downregulation}}''.</dd>

{{term|silent allele}} <dd>An {{gli|glossary=Glossary of cellular and molecular biology (0–L)|allele}} that does not produce a detectable {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene product|product}}.<ref name="DoG7"/> Compare ''{{gli|null allele}}''.</dd>

{{term|silent mutation}}{{anchor|silent mutations}} <dd>A type of {{gli|neutral mutation}} which does not have an observable effect on the organism's {{gli|phenotype}}. Though the term "silent mutation" is often used interchangeably with {{gli|synonymous mutation}}, synonymous mutations are not always silent, nor vice versa. {{gli|missense mutation|Missense mutations}} which result in a different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}} but one with similar functionality (e.g. leucine instead of isoleucine) are also often classified as silent, since such mutations usually do not significantly affect protein function.</dd>

{{term|simple sequence repeat (SSR)}} <dd>See ''{{gli|microsatellite}}''.</dd>

{{term|single-nucleotide polymorphism (SNP)}} <dd>Any {{gli|substitution}} of a single {{gli|nucleotide}} which occurs at a specific position within a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} and with measurable frequency within a population; for example, at a specific base position in a DNA sequence, the majority of the individuals in a population may have a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine}} ({{font|C|font=courier|size=big}}), while in a minority of individuals, the same position may be occupied by an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine}} ({{font|A|font=courier|size=big}}). SNPs are usually defined with respect to a "standard" reference genome; an individual human genome differs from the reference human genome at an average of 4 to 5 million positions, most of which consist of SNPs and short {{gli|glossary=Glossary of cellular and molecular biology (0–L)|indels}}. See also ''{{gli|polymorphism}}''.</dd>

{{term|single-strand break (SSB)}} <dd>The loss of continuity of the {{gli|phosphate backbone|phosphate-sugar backbone}} in one {{gli|strand}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA|DNA duplex}}.<ref name="Rieger"/> See also ''{{gli|nick}}''; contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-strand break}}''.</dd>

{{term|single-stranded}}{{anchor|single strand|single strands}} <dd>Composed of a single, unpaired {{gli|nucleic acid}} molecule, i.e. one linear {{gli|strand}} of {{gli|nucleotides}} sharing a single {{gli|phosphodiester backbone}}, as opposed to a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|duplex}} of two such strands joined by base pairing. See also ''{{gli|ssDNA|single-stranded DNA}}'' and ''{{gli|ssRNA|single-stranded RNA}}''.</dd>

{{term|single-stranded DNA (ssDNA)}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecule that consists of a single nucleotide polymer or {{gli|strand}}, as opposed to a pair of complementary strands held together by hydrogen bonds ({{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsDNA|double-stranded DNA}}). In most circumstances, DNA is more stable and more common in double-stranded form, but high temperatures, low concentrations of dissolved salts, and very high or low pH can cause double-stranded molecules to decompose into two single-stranded molecules in a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|denaturation}} process known as {{gli|melting}}; this reaction is exploited by naturally occurring enzymes such as those involved in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} as well as by laboratory techniques such as {{gli|polymerase chain reaction}}.</dd>

{{term|siRNA}} <dd>See ''{{gli|siRNA|small interfering RNA}}''.</dd>

{{term|sister chromatids}} <dd>A pair of identical copies ({{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatids}}) produced as the result of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}}, particularly when both copies are joined together by a common {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromere}}; the pair of sister chromatids is called a ''dyad''. The two sister chromatids are ultimately separated from each other into two different cells during {{gli|mitosis}} or {{gli|meiosis}}.</dd>

{{term|site-directed mutagenesis}} <dd></dd>

{{term|small conditional RNA (scRNA)}} <dd>A class of small {{gli|RNA}} molecules engineered so as to change conformation conditionally in response to cognate molecular inputs, often with the goal of controlling {{gli|signal transduction}} pathways ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|in vitro}}'' or ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|in vivo}}''.</dd>

{{term|small interfering RNA (siRNA)}}{{anchor|small interfering RNA|small interfering RNAs|siRNA|siRNAs}} <dd></dd>

{{term|small nuclear RNA (snRNA)}}{{anchor|small nuclear RNA|small nuclear RNAs|snRNA|snRNAs}} <dd>A class of small {{gli|non-coding RNA}} molecules, approximately 100–300 nucleotides in length and rich in {{gli|uridine}} residues, found in association with specific proteins as part of {{gli|ribonucleoprotein}} complexes known as snRNPs within {{gli|nuclear speckles}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|Cajal bodies}} of the eukaryotic nucleus.<ref name="Oxford B&MB"/> SnRNPs assemble into larger complexes known as {{gli|spliceosomes}} which play important roles in the {{gli|RNA splicing|splicing}} of {{gli|pre-mRNA}} transcripts ({{gli|glossary=Glossary of cellular and molecular biology (0–L)|hnRNAs}}) before they are exported to the cytoplasm.</dd>

{{term|small nucleolar RNA (snoRNA)}}{{anchor|small nucleolar RNA|small nucleolar RNAs|snoRNA|snoRNAs}} <dd>A class of small {{gli|non-coding RNA}} molecules whose primary function is to direct the {{gli|post-transcriptional modification}} of other RNAs, mainly {{gli|transfer RNAs}} (tRNA), {{gli|small nuclear RNAs}} (snRNA), and especially {{gli|ribosomal RNAs}} (rRNA) as a part of {{gli|ribosome}} synthesis in the {{gli|nucleolus}}. SnoRNAs contain {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antisense}} sequences that complement sequences within these target RNAs and guide {{gli|ribonucleoprotein}} complexes to them, which can then catalyze specific nucleoside modifications, typically {{gli|methylation}} or {{gli|pseudouridylation}}.</dd>

{{term|small temporal RNA (stRNA)}} <dd>A subclass of {{gli|miRNA|microRNAs}}, originally described in nematodes, which regulate the timing of developmental events by binding to complementary sequences in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3' UTR|3' untranslated regions}} of {{gli|mRNA|messenger RNAs}} and inhibiting their {{gli|translation}}. In contrast to {{gli|siRNA|siRNAs}}, which serve similar purposes, stRNAs bind to their target mRNAs after the initiation of translation and without affecting mRNA stability, which makes it possible for the target mRNAs to resume translation at a later time.</dd>

{{term|small ubiquitin-like modifier (SUMO)}}{{anchor|small ubiquitin-like modifier|SUMO|SUMO protein|SUMO proteins}} <dd>Any of a family of small {{gli|proteins}}, each approximately 100 amino acids, which are covalently {{gli|glossary=Glossary of cellular and molecular biology (0–L)|conjugated}} to and removed from charged residues of other proteins in a form of {{gli|post-translational modification}} known as {{gli|SUMOylation}}, thereby functioning as a {{gli|protein tag}} in a manner resembling {{gli|ubiquitin}}.</dd>

{{term|smooth endoplasmic reticulum (SER)}}{{anchor|smooth endoplasmic reticulum|smooth ER}} <dd>A type of membrane in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endoplasmic reticulum}} that lacks {{gli|ribosomes}} on its surface, thus visibly contrasting with the {{gli|rough endoplasmic reticulum|"rough" endoplasmic reticulum}}. Smooth ER tends to be tubular rather than sheet-like, and may form an extension of the rough ER or exist separately. It is especially abundant in cells concerned with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lipid}} metabolism.<ref name="Lackie"/></dd>

{{term|snoRNA}} <dd>See ''{{gli|small nucleolar RNA}}''.</dd>

{{term|snRNA}} <dd>See ''{{gli|small nuclear RNA}}''.</dd>

{{term|soluble RNA (sRNA)}} <dd>See ''{{gli|transfer RNA}}''.</dd>

{{term|somatic cell}}{{anchor|somatic cells}} {{ghat|Also '''vegetal cell''' or '''soma'''.}} <dd>Any biological cell forming the body of an organism, or, in multicellular organisms, any cell other than a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gamete}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|germ cell}}, or undifferentiated {{gli|stem cell}}. Somatic cells are theoretically distinct from cells of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|germ line}}, meaning the {{gli|mutations}} they have undergone can never be transmitted to the organism's descendants, though in practice exceptions do exist.</dd>

{{term|somatic cell nuclear transfer (SCNT)}} <dd></dd>

{{term|somatic crossover}} <dd>See ''{{gli|mitotic recombination}}''.</dd>

{{term|sonication}} {{ghat|Also '''ultrasonication'''.}} <dd>The application of sound energy at ultrasonic frequencies in order to agitate particles in a chemical or biological sample. Intense acoustic vibrations produce pressure waves and cavitations that propagate through a liquid medium, thereby converting the sound energy to mechanical energy which can disperse solutes, disrupt intermolecular interactions, and break covalent bonds. At various amplitudes sonication can be used to increase the permeability of cell or nuclear {{gli|membranes}}, a technique known as {{gli|sonoporation}}, or to {{gli|lysis|completely destroy them}} and release their contents for isolation and extraction. It has a wide variety of applications in industry and research, including creating nanoparticles such as {{gli|liposomes}}, shearing DNA and proteins into smaller fragments, degassing liquids, and ultrasonic cleaning.</dd>

{{term|Southern blotting}} <dd>A {{gli|molecular biology}} method used to detect a specific {{gli|sequence}} in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} samples. The method combines separation of DNA fragments by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gel electrophoresis}}, transfer of the DNA to a synthetic membrane, and subsequent identification of target fragments with radio-{{gli|glossary=Glossary of cellular and molecular biology (0–L)|labeled}} or fluorescent {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridization probes}}. Compare ''{{gli|northern blotting}}'', ''{{gli|western blotting}}'', and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|eastern blotting}}''.</dd>

{{term|spacer}} {{ghat|Also '''intergenic spacer (IGS)''' or '''non-transcribed spacer (NTS)'''.}} <dd>Any sequence or region of {{gli|ncDNA|non-coding DNA}} separating neighboring {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}}, whether {{gli|transcribed}} or not. The term is used in particular to refer to the non-coding regions between the many repeated copies of the {{gli|rRNA|ribosomal RNA}} genes.<ref name="Rieger"/> See also ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|intergenic region}}''.</dd>

{{term|spatially-restricted gene expression}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} of one or more genes only within a specific anatomical region or tissue, often in response to a {{gli|paracrine}} signal. The boundary between the jurisdictions of two spatially restricted genes may generate a sharp {{gli|phenotypic}} gradient there, as with striping patterns.</dd>

{{term|spindle apparatus}}{{anchor|spindle|mitotic spindle|meiotic spindle}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoskeletal}} structure that forms during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell division}} in eukaryotic cells, consisting of a network of long, flexible {{gli|microtubules}} extending from each pole of the {{gli|parent cell}} and attaching to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinetochores}} at the centromeres of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous chromosomes}} or {{gli|sister chromatids}} near the cell equator. As the microtubules shorten, they pull the chromosomes apart, separating them into different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|daughter cells}}. Proper formation of this apparatus is critical in both {{gli|mitosis}} and {{gli|meiosis}}, where it is respectively referred to as the ''mitotic spindle'' and ''meiotic spindle''.</dd>

{{term|spliceosome}}{{anchor|spliceosomes}} <dd>A large {{gli|ribonucleoprotein}} complex found primarily in the nucleus of eukaryotic cells, composed of multiple small nuclear ribonucleoproteins (snRNPs) which are themselves composed of {{gli|small nuclear RNAs}} (snRNAs) and a variety of snRNP-specific proteins. Spliceosomes assemble at the junctions between {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exons}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|introns}} within {{gli|primary transcript|primary RNA transcripts}} and catalyze the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excision|removal}} of the introns and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligation}} of the flanking exons in a form of post-transcriptional processing known as {{gli|RNA splicing}}.<ref>{{cite journal | vauthors = Will CL, Lührmann R | title = Spliceosome structure and function | journal = Cold Spring Harbor Perspectives in Biology | volume = 3 | issue = 7 | article-number = a003707 | date = July 2011 | pmid = 21441581 | pmc = 3119917 | doi = 10.1101/cshperspect.a003707 | bibcode = 2011CSHPB...303707W }}</ref></dd>

{{term|splicing}}{{anchor|splice|splices|spliced}} <dd>Any natural or artificial process involving the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excision}} of oligonucleotide sequences from nucleic acid molecules (either DNA or RNA) or of peptide sequences from proteins and the subsequent {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligation|re-ligation}} of the flanking fragments into a single continuous molecule lacking the excised sequence. {{gli|RNA splicing}} in particular is an important form of {{gli|post-transcriptional modification|post-transcriptional processing}} whereby {{gli|glossary=Glossary of cellular and molecular biology (0–L)|introns}} are removed from {{gli|primary mRNA}} transcripts and the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exons}} rejoined (and sometimes {{gli|glossary=Glossary of cellular and molecular biology (0–L)|alternative splicing|rearranged}}) to create mature mRNAs; a {{gli|protein splicing|similar process}} also occurs with the removal of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inteins}} and the rejoining of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exteins}} in the {{gli|post-translational modification}} of certain proteins. The term may also refer more generally to artificial techniques for creating {{gli|recombinant}} sequences in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering}}.<ref name="Oxford B&MB"/></dd>

{{term|sRNA}} <dd>See ''{{gli|tRNA|transfer RNA}}''.</dd>

{{term|ssDNA}} <dd>See ''{{gli|ssDNA|single-stranded DNA}}''.</dd>

{{term|ssRNA}} <dd>See ''{{gli|ssRNA|single-stranded RNA}}''.</dd>

{{term|standard genetic code}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic code}} used by the vast majority of living organisms for {{gli|translating}} {{gli|nucleic acid sequences}} into {{gli|proteins}}. In this system, of the 64 possible permutations of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon|three-letter codons}} that can be made from the four {{gli|nucleotides}}, 61 code for one of the 20 {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}}, and the remaining three code for {{gli|stop codon|stop signals}}. For example, the codon {{font|CAG|font=courier|size=big}} codes for the amino acid glutamine and the codon {{gli|ochre|{{font|UAA|font=courier|size=big}}}} is a stop codon. The standard genetic code is described as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|degeneracy|degenerate}} or redundant because some amino acids can be coded for by more than one different codon.</dd> thumb|right|400px|The '''{{gli|standard genetic code}}''' specifies a set of 20 different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} from triplet arrangements of the four different {{gli|RNA}} {{gli|nucleobases}} ({{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine|{{font|A|font=courier|size=big}}}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine|{{font|G|font=courier|size=big}}}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine|{{font|C|font=courier|size=big}}}}, and {{gli|uracil|{{font|U|font=courier|size=big}}}}). To read this chart, choose one of the four letters in the innermost ring and then move outward, adding two more letters to complete a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} triplet: a total of 64 unique codons can be made this way, 61 of which signal the addition of one of the 20 amino acids (identified by single-letter abbreviation as well as by full name and chemical structure) to a nascent {{gli|peptide}} chain, while the remaining three codons are {{gli|stop codons}} signalling the termination of translation. Also indicated are some of the chemical properties of the amino acids and the various ways in which they can be modified.

{{term|start codon}} <dd>The first {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} {{gli|translated}} by a {{gli|ribosome}} from a mature {{gli|messenger RNA}} transcript, used as a signal to initiate {{gli|peptide}} synthesis. In the {{gli|standard genetic code}}, the start codon always codes for the same {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}}, methionine, in eukaryotes and for a modified methionine in prokaryotes. The most common start codon is the triplet {{font|AUG|font=courier|size=big}}. Contrast ''{{gli|stop codon}}''.</dd>

{{term|statistical genetics}} <dd>A branch of genetics concerned with the development of statistical methods for drawing inferences from genetic data. The theories and methodologies of statistical genetics often support research in {{gli|quantitative genetics}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic epidemiology}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|bioinformatics}}.</dd>

{{term|stem cell}}{{anchor|stem cells}} <dd>Any biological cell which has not yet {{gli|glossary=Glossary of cellular and molecular biology (0–L)|differentiated}} into a specialized cell type and which can divide through {{gli|mitosis}} to produce more undifferentiated stem cells.</dd>

{{term|stem-loop}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|hairpin}}''.</dd>

{{term|sticky end}}{{anchor|sticky ends}} <dd>A term used to describe the end of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded DNA}} molecule where one {{gli|strand}} is longer than the other by one or more {{gli|nucleobases}}, creating a single-stranded "overhang" of unpaired bases, in contrast to a so-called {{gli|glossary=Glossary of cellular and molecular biology (0–L)|blunt end}}, where no such overhang exists because the terminal nucleobases on each strand are {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pairing|base-paired}} with each other. Blunt ends and sticky ends are relevant when {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligating}} linear DNA molecules, e.g. in {{gli|restriction cloning}}, because many {{gli|restriction enzymes}} cleave the phosphate backbone in a way that leaves behind terminal overhangs in the digested fragments. These sticky-ended molecules ligate much more readily with other sticky-ended molecules having {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} overhangs, allowing scientists to ensure that specific DNA fragments are ligated together in specific places.</dd>

{{term|stop codon}}{{anchor|stop codons}} {{ghat|Also '''termination codon'''.}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} that signals the termination of protein synthesis during {{gli|translation}} of a {{gli|mRNA|messenger RNA}} transcript. In the {{gli|standard genetic code}}, three different stop codons are used to dissociate {{gli|ribosomes}} from the growing amino acid chain, thereby ending translation: {{font|UAG|font=courier|size=big}} (nicknamed "amber"), {{font|UAA|font=courier|size=big}} ("ochre"), and {{font|UGA|font=courier|size=big}} ("opal"). Contrast ''{{gli|start codon}}''.</dd>

{{term|strand}}{{anchor|strands}} <dd>An individual chain of {{gli|nucleotides}} comprising a {{gli|nucleic acid}} polymer, existing either independently (in which case the nucleic acid molecule is said to be {{gli|single-stranded}}) or {{gli|base-paired}} in a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|duplex}} with another, separate strand (in which case it is said to be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double-stranded}}).</dd>

{{term|stringency}} <dd>The effect of conditions such as temperature and pH upon the degree of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementarity}} that is required for a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hybridization}} reaction to occur between two single-stranded nucleic acid molecules. In the most stringent conditions, only exact complements can successfully hybridize; as stringency decreases, an increasing number of {{gli|mispairing|mismatches}} can be tolerated between the two hybridizing strands.<ref name="Lewin"/></dd>

{{term|stRNA}} <dd>See ''{{gli|small temporal RNA}}''.</dd>

{{term|structural gene}}{{anchor|structural genes}} <dd>A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} that codes for any protein or RNA {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene product|product}} other than a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation|regulatory factor}}. Structural gene products include {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}}, {{gli|structural proteins}}, and certain {{gli|non-coding RNAs}}.</dd>

{{term|structural protein}}{{anchor|structural proteins}} <dd>Any {{gli|protein}} which contributes to the mechanical shape and structure of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cells}}, {{gli|organelles}}, or {{gli|tissues}} (e.g. collagen and actin), as distinguished from proteins which serve some other purpose, such as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}}. This distinction is not well-defined, however, as many proteins have both structural and non-structural roles.<ref name="MacLean"/></dd>

{{term|subcellular localization}} <dd>1. The subdivision of the interior of a cell into functionally distinct spaces or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell compartmentalization|compartments}} (e.g. {{gli|membrane-bound organelles}}) and the delegation of particular cellular functions and activities to these particular spaces.</dd> <dd>2. The determination by any of various laboratory methods (e.g. fluorescent {{gli|glossary=Glossary of cellular and molecular biology (0–L)|labelling}}) of the precise location(s) within a cell where a specific molecule has occupancy, or at which a specific activity occurs.</dd>

{{term|submetacentric}} <dd>(of a linear {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} or chromosome fragment) Having a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromere}} positioned close to but not exactly in the middle of the chromosome, resulting in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatid}} arms of slightly different lengths.<ref name="CoG"/> Compare ''{{gli|metacentric}}''.</dd>

{{term|substitution}} <dd>A type of {{gli|point mutation}} in which a single {{gli|nucleotide}} and its attached {{gli|nucleobase}} is replaced by a different nucleotide.</dd>

{{term|substrate}}{{anchor|substrates}} <dd>1. A chemical compound or molecule upon which a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzyme}} directly acts, often but not necessarily binding the molecule by forming one or more chemical bonds.<ref name="MacLean"/> See also ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligand}}''.</dd> <dd>2. The substance, biotic or abiotic, upon which an organism grows or lives, or by which it is supported; e.g. a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|growth medium}} used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell culture}}. See also ''{{gli|substratum}}''.</dd>

{{term|substratum}}{{anchor|substrata}} <dd>A solid surface to which a cell or organism adheres or by which it is supported, or over which it moves.<ref name="Lackie"/> See also ''{{gli|substrate}}''.</dd>

{{term|subunit}}{{anchor|subunits}} <dd>A single unit of a multi-unit compound or molecular aggregate; e.g. a {{gli|monomer}} from which a larger {{gli|polymer}} is composed (as with {{gli|nucleotides}} in {{gli|nucleic acids}}), or an individual {{gli|polypeptide}} chain in a multi-chain {{gli|protein}}, or an entire protein which participates alongside other proteins as part of a {{gli|protein complex}}.<ref name="MacLean"/><ref name="Alberts et al."/></dd>

{{term|SUMOylation}} {{ghat|Also '''sumoylation'''.}} <dd>A type of {{gli|post-translational modification}} in which a {{gli|SUMO protein}} is conjugated to a polar residue of another protein (usually a lysine) via a covalent {{gli|glossary=Glossary of cellular and molecular biology (0–L)|isopeptide bond}}. This effectively {{gli|protein tag|tags}} the second protein, making it distinguishable to other biomolecules and in many cases allowing it to participate in specific reactions or to interact with specific {{gli|protein complexes}}. SUMOlyation is closely related to {{gli|ubiquitination}}, relying on the same E1/E2/E3 enzymes to transfer SUMO to specific recognition motifs in the target protein, though detaching SUMO depends on SUMO-specific {{gli|proteases}}. It plays important roles in numerous cellular processes, including {{gli|protein sorting|protein localization}}, transcriptional regulation, stress-response pathways, and {{gli|cell cycle checkpoints}}, among others. SUMOlyation is also used in the laboratory as a molecular {{gli|label}} and to help solubilize proteins which are difficult to {{gli|protein purification|purify}}.</dd>

{{term|supercoiling}}{{anchor|supercoil|supercoils|supercoiled}} <dd></dd>

{{term|supersecondary structure}} <dd></dd>

{{term|suppression}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|downregulation}}''.</dd>

{{term|suspension culture}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell culture}} in which individual {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cells}} or aggregates of cells are suspended in a liquid {{gli|glossary=Glossary of cellular and molecular biology (0–L)|growth medium}}, and usually prevented from settling by continuous gentle agitation. Many prokaryotic and eukaryotic cell types readily proliferate in suspension cultures, but they are particularly useful for culturing non-adherent cell lines such as hematopoietic cells, plant cells, and insect cells. Compare ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|adherent culture}}''.</dd>

{{term|symporter}}{{anchor|symporters}} <dd>Any of a class of {{gli|transmembrane}} {{gli|transporter proteins}} which facilitate the transport of two or more different molecules across the membrane at the same time and in the same direction; e.g. glucose and sodium ions. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|antiporter}}'' and ''{{gli|uniporter}}''.</dd>

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

{{term|synaptonemal complex}} <dd>A complex of scaffolding proteins that mediates {{gli|synapsis}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous recombination}} between the chromatids of homologous chromosomes during {{gli|prophase I}} of {{gli|meiosis}}.</dd>

{{term|syncytium}} {{ghat|Also '''symplasm'''; pl. '''syncytia'''.}} <dd>A {{gli|multinucleate}} cell, i.e. a cell containing more than one {{gli|nucleus}} or, in the broadest sense, more than one {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genomic DNA|nuclear genome}} (a meaning which is equated with {{gli|polyploidy}}). Syncytia may form as a result of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell fusion}} between uninucleate cells, migration of a nucleus from one cell to another, or multiple nuclear divisions without accompanying {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytokinesis}} (forming a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coenocyte}}).<ref name="Rieger"/> The term may also refer to cells which are interconnected by specialized membranes with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gap junctions}} as in some neuromuscular cell types.</dd>

{{term|syndesis}} <dd>The {{gli|synapsis}} of chromosomes during {{gli|meiosis}}.<ref name="DoG7"/></dd>

{{term|synezis}} <dd>The aggregation of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}} into a dense knot that adheres to one side of the {{gli|nucleus}}, commonly observed during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|leptonema}} in certain organisms.<ref name="DoG7"/></dd>

{{term|synonymous mutation}}{{anchor|synonymous mutations}} {{ghat|Also '''synonymous substitution''' or '''samesense mutation'''.}} <dd>A type of {{gli|mutation}} in which the {{gli|substitution}} of one {{gli|nucleotide}} base for another results, after {{gli|transcription}} and {{gli|translation}}, in an amino acid sequence which is identical to the original unmutated sequence. This is possible because of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|degeneracy}} of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic code}}, which allows different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codons}} to code for the same amino acid. Though synonymous mutations are often considered {{gli|silent mutation|silent}}, this is not always the case; a synonymous mutation may affect the efficiency or accuracy of {{gli|transcription}}, {{gli|splicing}}, {{gli|translation}}, or any other process by which genes are {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}}, and thus become effectively non-silent. Contrast ''{{gli|nonsynonymous mutation}}''.</dd>

{{term|synthesis phase}} <dd>See ''{{gli|S phase}}''.</dd>

==T== {{glossary}} {{term|tandem repeat}}{{anchor|tandem repeats}} <dd>A pattern within a {{gli|nucleic acid sequence}} in which one or more {{gli|nucleobases}} are repeated and the repetitions are directly adjacent (i.e. tandem) to each other. An example is {{font|ATGACATGACATGAC|font=courier|size=big}}, in which the sequence {{font|ATGAC|font=courier|size=big}} is repeated three times.</dd>

{{term|target site}}{{anchor|target sites}} <dd>The site or locus upon another molecule at which a protein performs a particular biochemical activity; e.g. the nucleotide {{gli|motif}} at which a {{gli|restriction endonuclease}} cleaves a DNA molecule, often but not necessarily the same as the enzyme's {{gli|recognition site}} (i.e. a restriction enzyme may recognize one motif, known as a {{gli|restriction site}}, and cleave at another).<ref name="Oxford B&MB"/></dd>

{{term|TATA box}} {{ghat|Also '''Goldberg-Hogness box'''.}} <dd>A highly conserved {{gli|non-coding DNA}} sequence containing a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|consensus sequence|consensus}} of repeating {{font|{{gli|thymine|T}}|font=courier|size=big}} and {{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine|A}}|font=courier|size=big}} base pairs that is commonly found in {{gli|promoter|promoter regions}} of genes in archaea and eukaryotes. The TATA box often serves as the site of initiation of {{gli|transcription}} or as a binding site for {{gli|transcription factor|transcription factors}}.</dd>

{{term|taxis}} <dd>A directional response by a cell or a population of cells to a specific stimulus; a movement or other activity occurring in a non-random direction and dependent on the direction from which the stimulus originated.<ref name="Lackie"/> This contrasts with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinesis}}, a response without directional bias.</dd>

{{term|TCA}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|citric acid cycle}}''.</dd>

{{term|telestability}} <dd>The transmission of three‐dimensional structural stability from a stable part of a macromolecule to a distal part of the same molecule, especially an inherently less stable part.<ref name="Oxford B&MB"/> Instability may also be transmitted in this way, e.g. destabilization of the DNA {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double helix}} may occur at a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|locus}} that is relatively distant from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|binding site}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding protein}}.<ref name="DoG7"/></dd>

{{term|telocentric}} <dd>(of a linear {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} or chromosome fragment) Having a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centromere}} positioned at the terminal end of the chromosome (near or within the {{gli|telomere}}), resulting in only a single arm.<ref name="CoG"/> Compare ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|acrocentric}}''.</dd>

{{term|telomere}}{{anchor|telomeres}} <dd>A region of {{gli|repetitive DNA|repetitive}} {{gli|nucleic acid sequence|nucleotide sequences}} at each end of a linear {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} which protects the end of the chromosome from deterioration and from fusion with other chromosomes. Since each round of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replication}} results in the shortening of the chromosome, telomeres act as disposable buffers which are sacrificed to perpetual truncation instead of nearby genes; telomeres can also be lengthened by the enzyme telomerase.</dd>

{{term|telomeric silencing}} <dd>The {{gli|repression}} of {{gli|transcription}} of genes in regions adjacent to {{gli|telomeres}}. Telomeres also appear to reduce the accessibility of subtelomeric {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatin}} to modification by DNA {{gli|methyltransferases}}.<ref name="DoG7"/></dd>

{{term|telophase}} <dd>The final stage of cell division in both {{gli|mitosis}} and {{gli|meiosis}}, occurring after {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anaphase}} and before or simultaneously with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytokinesis}}, during which a nuclear membrane is synthesized around each set of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromatids}}, {{gli|nucleolus|nucleoli}} are reassembled, and the {{gli|mitotic spindle}} is disassembled. Following cytokinesis, the new daughter cells resume {{gli|glossary=Glossary of cellular and molecular biology (0–L)|interphase}}.</dd>

{{term|template strand}}{{anchor|template}} {{ghat|Also '''antisense strand''', '''negative (-) sense strand''', and '''noncoding strand'''.}} <dd>The {{gli|strand}} of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsDNA|double-stranded DNA}} molecule which is used as a template for RNA synthesis during {{gli|transcription}}. The sequence of the template strand is {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complementary}} to the resulting RNA transcript. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding strand}}''; see also ''{{gli|sense}}''.</dd>

{{term|terminalization}} <dd>In cytology, the progressive shift of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chiasmata}} from their original to more distal positions as {{gli|meiosis}} proceeds through {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diplonema}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diakinesis}}.<ref name="DoG7"/></dd>

{{term|termination codon}} <dd>See ''{{gli|stop codon}}''.</dd>

{{term|terminator}} <dd>A DNA {{gli|sequence}} or its RNA {{gli|glossary=Glossary of cellular and molecular biology (0–L)|complement}} which signals the termination of {{gli|transcription}} by triggering processes that ultimately arrest the activity of {{gli|RNA polymerase}} and/or cause the release of the {{gli|nascent}} RNA {{gli|transcript}} from the transcriptional complex. Terminator sequences are usually found near the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3'-ends}} of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding sequences}} of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}} or {{gli|operons}}. They generally function after being themselves transcribed into the nascent strand, whereupon the part of the strand containing the sequence either directly interacts with the transcriptional complex or a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cofactor}} or forms a {{gli|secondary structure}} such as a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hairpin loop}} which signals the recruitment of enzymes that promote its disassembly.<ref name="DoG7"/></dd>

{{term|tetramer}}{{anchor|tetramers|tetrameric}} <dd>A molecular aggregate consisting of four {{gli|subunits}}.<ref name="MacLean"/> The term is often used to refer to {{gli|protein complexes}} composed of four proteins, e.g. haemoglobin, or to individual proteins composed of four {{gli|polypeptides}}. Compare ''{{gli|monomer}}'', ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|dimer}}'', and ''{{gli|trimer}}''.</dd>

{{term|three-prime end}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|3'-end}}''.</dd>

{{term|three-prime untranslated region}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|3' untranslated region}}''.</dd>

{{term|term=thymidine|content=thymidine ({{font|T|font=courier|size=large}}, {{font|dT|font=courier|size=large}})}} {{ghat|Also '''deoxythymidine'''.}} <dd>One of the four standard {{gli|nucleosides}} used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecules, consisting of a {{gli|thymine}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base}} with its N9 nitrogen {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycosidic bond|bonded}} to the C1 carbon of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|deoxyribose}} sugar. The prefix ''deoxy-'' is commonly omitted, since there are no ribonucleoside analogs of thymidine used in RNA, where it is replaced with {{gli|uridine}} instead.</dd>

{{term|term=thymine|content=thymine ({{font|T|font=courier|size=large}})}} {{ghat|Also '''5-methyluracil'''.}} <dd>A {{gli|pyrimidine}} {{gli|nucleobase}} used as one of the four standard nucleobases in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecules. Thymine forms a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pair}} with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine}}. In {{gli|RNA}}, thymine is not used at all, and is instead replaced with {{gli|uracil}}.</dd>

{{term|thymine dimer}} <dd>See ''{{gli|pyrimidine dimer}}''.</dd>

{{term|tight junction}}{{anchor|tight junctions}} {{ghat|Also '''occluding junction''' or '''zonula occludens'''.}} <dd>A type of specialized intercellular junction characterized by very close contact between the plasma membranes of adjacent cells, which are held together by large {{gli|protein complex|multiprotein complexes}} that completely or nearly completely occlude the passage of water and solutes through the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|intercellular space}} between cells. Tight junctions occur in many vertebrate tissues, especially between the epithelial and endothelial cells that line the surfaces of most organs and vessels. These cells are completely encircled by tight junctions which create a gasket-like seal that separates each cell's plasma membrane into apical and basolateral domains and prevents the exchange of extracellular materials between them.<ref name="Oxford B&MB"/><ref name="Lackie"/></dd>

{{term|tissue}}{{anchor|tissues}} <dd>In a multicellular organism, a contiguous aggregation of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cells}} held together by a common {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extracellular matrix}} and specialized to perform a particular function. Some tissues are composed primarily of a single cell type; others are a heterogeneous mixture of many cell types.<ref name="MacLean"/> Tissues represent a level of multicellular organization between that of individual cells and that of organs, which may be composed of one or more distinct types of tissue.<ref name="Lackie"/></dd>

{{term|tissue culture}} <dd>The growth and maintenance, or "culturing", of multicellular {{gli|tissues}}, or of cells harvested from tissues, under carefully controlled conditions ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|in vitro}}'', in the strictest sense by taking a piece of explanted tissue directly from a living plant or animal and maintaining it outside of the body of the source organism. In common usage, the term may also refer to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell culture}} in general, especially when growing certain cell types which have been harvested from tissues but dispersed from their original tissue-specific organization into a population of more or less independently growing cells.<ref name="Lackie"/></dd>

{{term|tissue-specific gene expression}} <dd>Gene function and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression}} which is restricted to a particular {{gli|tissue}} or cell type. Tissue-specific expression is usually the result of an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enhancer}} which is activated only in the proper cell type.</dd>

{{term|tonicity}} <dd>A measure of the effective osmotic pressure gradient of one solution relative to another solution, used especially to describe the water potential that exists between two aqueous solutions separated by a semipermeable membrane (as with a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell}}, where the intracellular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosol}} is separated from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extracellular fluid}} by the {{gli|plasma membrane}}). Tonicity depends on the relative concentrations of solutes on either side of the membrane, which determine the direction and extent to which solvent molecules move across the membrane by {{gli|osmosis}}; it is affected only by those solutes which cannot cross the membrane, as those which can cross freely can achieve equilibrium without any net movement of solute. The extracellular environment is commonly described as {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hypotonic}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|hypertonic}}, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|isotonic}} with respect to the intracellular environment.</dd> thumb|right|350px|'''{{gli|tonicity|Tonicity}}''' describes the pressure to restore {{gli|osmosis|osmotic equilibrium}} between the inside and outside of cells by moving water across the cell membrane. Red blood cells tend to lose water and shrivel up in a severely '''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|hypertonic}}''' environment (left) or gain water and swell to bursting in a severely '''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|hypotonic}}''' environment (right), but the water potential is balanced in an '''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|isotonic}}''' environment (center).

{{term|tonoplast}} <dd>See ''{{gli|vacuole}}''.</dd>

{{term|topoisomerase}}{{anchor|topoisomerases}} <dd>Any of a class of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding protein|DNA-binding}} enzymes which catalyze changes in the topological state of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dsDNA|double-stranded DNA}} molecule by {{gli|nicking}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cutting}} the {{gli|sugar-phosphate backbone}} of one or both strands, relaxing the torsional stress inherent in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|double helix}} and unwinding or untangling the paired strands before {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ligation|re-ligating}} the nicks. This process is usually necessary prior to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replication}} and {{gli|transcription}}. Topoisomerases thereby convert DNA between its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|B-DNA|relaxed}} and {{gli|supercoiled}}, linked and unlinked, and knotted and unknotted forms without changing the sequence or overall chemical composition, such that the substrate and product molecules are structural isomers, differing only in their shape and their {{gli|twisting number|twisting}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|linking number|linking}}, and/or {{gli|writhing number|writhing numbers}}.</dd>

{{term|totipotency}}{{anchor|totipotent}} <dd>A state of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell potency}} in which a cell or nucleus fully retains the ability to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|differentiate}} into all of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell types}} represented in the adult organism, or to give rise to all of these cell types upon {{gli|transplantation}} into an appropriate cytoplasm (as in {{gli|nuclear transfer}}). Such cells or nuclei are said to be '''totipotent'''. The {{gli|zygote}} that serves as the progenitor cell for sexually reproducing multicellular organisms is the archetypal totipotent cell; almost all of the cells into which it ultimately differentiates are not totipotent, though some cells such as {{gli|stem cells}} remain totipotent or {{gli|pluripotent}} throughout the organism's life.<ref name="MacLean"/></dd>

{{term|tracer}}{{anchor|tracers}} <dd>A molecule or a specific atom within a molecule that has been chemically or radioactively {{gli|glossary=Glossary of cellular and molecular biology (0–L)|labelled}} so that it can be easily tracked or followed through a biochemical process or located in a cell or tissue.<ref name="Alberts et al."/></dd>

{{term|trailer sequence}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|3' untranslated region}}''.</dd>

{{term|''trans''}} <dd>On the opposite side; across from; {{gli|trans-acting|acting}} from a different molecule. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|cis}}''.</dd>

{{term|''trans''-acting}} <dd>Affecting a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or sequence on a different nucleic acid molecule or {{gli|strand}}. A {{gli|glossary=Glossary of cellular and molecular biology (0–L)|locus}} or sequence within a particular DNA molecule such as a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} is said to be ''trans''-acting if it or its {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene product|products}} influence or act upon other sequences located relatively far away or on an entirely different molecule or chromosome. For example, a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding protein}} acts "in ''trans''" if it binds to or interacts with a sequence located on any strand or molecule different from the one on which it is encoded. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|cis-acting}}''.</dd>

{{term|''trans''-splicing}} <dd>A form of {{gli|RNA splicing}} in which different RNA {{gli|transcripts}}, synthesized in separate {{gli|transcription}} events, are spliced together into a single, continuous transcript. This contrasts with the more conventional "''cis''-splicing", where segments of the same transcript are excised or re-arranged.<ref name="Lackie"/></dd>

{{term|transactivation}} <dd>An experimental approach to artificially control {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} by introducing a transactivator gene under the control of an inducible {{gli|promoter}} into a genome. The transactivator encodes a {{gli|transcription factor}} capable of {{gli|trans-acting|acting in ''trans''}} upon one or more other genes by recognizing and specifically binding their promoters; thus by inducing the transactivator gene, the expression of many other genes can be experimentally manipulated.<ref name="Lackie"/></dd>

{{term|transcribed spacer}} <dd>A {{gli|spacer}} sequence that is transcribed and thus included in the primary {{gli|rRNA|ribosomal RNA}} {{gli|transcript}} (as opposed to a {{gli|non-transcribed spacer}}) but subsequently {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excised}} and discarded during the maturation of functional RNAs of the {{gli|ribosome}}.<ref name="DoG7"/></dd>

{{term|transcript}}{{anchor|transcripts}} <dd>A product of {{gli|transcription}}; that is, any {{gli|RNA}} molecule which has been synthesized by {{gli|RNA polymerase}} using a complementary {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecule as a {{gli|template}}. When transcription is completed, transcripts separate from the DNA and become independent {{gli|primary transcripts}}. Particularly in eukaryotes, multiple {{gli|post-transcriptional modifications}} are usually necessary for raw transcripts to be converted into stable and persistent molecules, which are then described as ''mature'', though not all transcribed RNAs undergo maturation. Many transcripts are accidental, spurious, incomplete, or defective; others are able to perform their functions immediately and without modification, such as certain {{gli|ncRNA|non-coding RNAs}}.</dd>

{{term|transcript of unknown function (TUF)}} <dd>Any RNA {{gli|transcript}} whose function is unclear. Such transcripts may include functional {{gli|non-coding RNAs}} which have not yet been studied in detail as well as spurious transcripts without any definite function. The DNA sequences from which TUFs are transcribed are generally located in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|intergenic}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|intronic}} regions of the genome. See also ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|junk RNA}}''.</dd>

{{term|transcriptase}} <dd>See ''{{gli|RNA polymerase}}''.</dd>

{{term|transcription}}{{anchor|transcriptional|transcriptionally|transcribe|transcribes|transcribing|transcribed}} <dd>The first step in the process of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}}, in which an {{gli|RNA}} molecule, known as a {{gli|transcript}}, is synthesized by enzymes called {{gli|RNA polymerases}} using a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or other {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} sequence as a {{gli|template}}. Transcription is a critical and fundamental process in all living organisms and is necessary in order to make use of the information encoded within a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}. All classes of RNA must be transcribed before they can exert their effects upon a cell, though only {{gli|mRNA|messenger RNA}} (mRNA) proceeds to {{gli|translation}} to produce a functional {{gli|protein}}, whereas the many types of {{gli|non-coding RNA}} fulfill their duties without being translated. Transcription is also not always beneficial for a cell: when it occurs at the wrong time or at a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|junk DNA|functionless locus}}, or when {{gli|mobile genetic element|mobile elements}} or infectious pathogens utilize the host's transcription machinery, the resulting transcripts (not to mention the waste of valuable energy and resources) are often harmful to the host cell or genome.</dd> thumb|right|450px|A simplified diagram of '''{{gli|transcription}}'''. RNA polymerase (RNAP) synthesizes an RNA transcript (blue) in the 5'-to-3' direction, using one of the DNA strands as a {{gli|template strand|template}}, while a complex of multiple {{gli|transcription factors}} binds to a {{gli|promoter}} upstream of the gene.

{{term|transcription factor (TF)}}{{anchor|transcription factors}} <dd>Any {{gli|protein}} that controls the rate of {{gli|transcription}} of genetic information from {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} to {{gli|RNA}} by binding to a specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA sequence}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|activator|promoting}} or {{gli|repressor|blocking}} the recruitment of {{gli|RNA polymerase}} to nearby {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}}. Transcription factors can effectively turn "on" and "off" specific genes in order to make sure they are {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}} at the right times and in the right places; for this reason, they are a fundamental and ubiquitous mechanism of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene regulation}}.</dd>

{{term|transcription start site (TSS)}} {{ghat|Also '''transcription initiation site'''.}} <dd>The specific location within a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} at which {{gli|RNA polymerase}} begins {{gli|transcription}}, defined by the specific nucleotide or codon corresponding to the first ribonucleotide(s) to be assembled in the {{gli|nascent}} transcript (which is not necessarily the same as the {{gli|start codon|first codon}} to be {{gli|translated}}). This site is usually considered the beginning of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding sequence}} and is the reference point for numbering the individual nucleotides within a gene. Nucleotides {{gli|upstream}} of the start site are assigned negative numbers and those {{gli|glossary=Glossary of cellular and molecular biology (0–L)|downstream}} are assigned positive numbers, which are used to indicate the positions of nearby sequences or structures relative to the TSS. For example, the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding protein|binding site}} for RNA polymerase might be a short sequence immediately upstream of the TSS, from approximately -80 to -5, whereas an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|intron}} within the coding region might be defined as the sequence starting at nucleotide +207 and ending at nucleotide +793.</dd>

{{term|transcription unit}} <dd>The segment of DNA between the {{gli|transcription start site|initiation site}} and the termination site of {{gli|transcription}}, containing the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding sequences}} for one or more {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genes}}. All genes within a transcription unit are transcribed together into a single transcript during a single transcription event; the resulting {{gli|polycistronic}} RNA may subsequently be cleaved into separate RNAs, or may be {{gli|translated}} as a unit and then cleaved into separate polypeptides.<ref name="DoG7"/></dd>

{{term|transcriptional bursting}} <dd>The intermittent nature of {{gli|transcription}} and {{gli|translation}} mechanisms. Both processes occur in "bursts" or "pulses", with periods of gene activity separated by irregular intervals.</dd>

{{term|transcriptome}} <dd>The entire set of {{gli|RNA}} molecules (often referring to all types of RNA but sometimes exclusively to {{gli|mRNA|messenger RNA}}) that is or can be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}} by a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, cell, population of cells, or species at a particular time or under particular conditions. The transcriptome is distinct from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exome}} and the {{gli|translatome}}.</dd>

{{term|transcriptomics}} <dd>The study of the {{gli|transcriptome}} of a particular genome, cell, or organism, i.e. the sum total of all of the RNA {{gli|transcripts}} produced from it by {{gli|transcription}}. Transcriptomics technologies allow scientists to isolate and sequence transcriptomes, which can then be mapped to the genome to determine which genes are being expressed or which cellular processes are active and which are dormant at a given time.</dd>

{{term|transcytosis}} <dd>The transport of molecules across the interior of a cell, i.e. through the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytoplasm}}, especially a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell polarity|polarized cell}} such as an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epithelial cell}} with contrasting {{gli|glossary=Glossary of cellular and molecular biology (0–L)|apical}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|basal}} surfaces, thereby providing a spatially oriented transport system. Molecules undergoing transcytosis are usually contained within {{gli|vesicles}}.<ref name="Oxford B&MB"/></dd>

{{term|transductant}}{{anchor|transductants}} <dd>A cell which has undergone {{gli|transduction}} and been successfully transduced.</dd>

{{term|transduction}}{{anchor|transduce|transduces|transducing|transduced}} <dd>The transfer of genetic material between cells by a virus or viral vector, either naturally or artificially.</dd>

{{term|transfectant}}{{anchor|transfectants}} <dd>A cell which has undergone {{gli|transfection}} and been successfully transfected.</dd>

{{term|transfection}}{{anchor|transfections|transfect|transfects|transfecting|transfected}} <dd>The deliberate experimental introduction of exogenous {{gli|nucleic acids}} into a cell or embryo. In the broadest sense the term may refer to any such transfer and is sometimes used interchangeably with {{gli|transformation}}, though some applications restrict the usage of transfection to the introduction of naked or purified non-viral {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}} into cultured eukaryotic cells (especially animal cells) resulting in the subsequent incorporation of the foreign DNA into the host {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} or the non-hereditary modification of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} by the foreign RNA. As a contrast to both standard non-viral transformation and {{gli|transduction}}, transfection has also occasionally been used to refer to the uptake of purified viral nucleic acids by bacteria or plant cells without the aid of a viral vector.<ref name="DoG7"/></dd>

{{term|transfer RNA (tRNA)}}{{anchor|transfer RNA|transfer RNAs|tRNA|tRNAs}} {{ghat|Formerly referred to as '''soluble RNA (sRNA)'''.}} <dd>A special class of {{gli|RNA}} molecule, typically 76 to 90 {{gli|nucleotides}} in length, that serves as a physical adapter allowing {{gli|mRNA}} transcripts to be {{gli|translated}} into sequences of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acids}} during protein synthesis. Each tRNA contains a specific {{gli|glossary=Glossary of cellular and molecular biology (0–L)|anticodon}} triplet corresponding to an amino acid that is covalently attached to the tRNA's opposite end; as translation proceeds, tRNAs are recruited to the {{gli|ribosome}}, where each mRNA {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}} is paired with a tRNA containing the complementary anticodon. Depending on the organism, cells may employ as many as 41 distinct tRNAs with unique anticodons; because of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|degeneracy|codon degeneracy}} within the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic code}}, several tRNAs containing different anticodons carry the same amino acid.</dd>

{{term|transferase}}{{anchor|transferases}} <dd>Any of a class of {{gli|enzymes}} which {{gli|catalyze}} the chemical transfer of a functional group or substituent from one molecule to another.<ref name="MacLean"/> For example, acetyltransferases catalyze the movement of an acetyl group in a process known as {{gli|acetylation}}; methyltransferases catalyze the movement of one or more methyl groups in a process known as {{gli|methylation}}.</dd>

{{term|transfer-messenger RNA (tmRNA)}} <dd>A type of RNA molecule in some bacteria which has dual {{gli|tRNA}}-like and {{gli|mRNA}}-like properties, allowing it to simultaneously perform a number of different functions during {{gli|translation}}.</dd>

{{term|transformant}}{{anchor|transformants}} <dd>A cell or organism which has taken up extracellular DNA by {{gli|transformation}} and which can express genes encoded by it.</dd>

{{term|transformation}}{{anchor|transformations|transform|transforms|transforming|transformed}} <dd></dd>

{{term|transgene}}{{anchor|transgenes|transgenic}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} or other segment of genetic material that has been isolated from one organism and then transferred either naturally or by any of a variety of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic engineering}} techniques into another organism, especially one of a different species. Transgenes are usually introduced into the second organism's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|germ line}}. They are commonly used to study gene function or to confer an advantage not otherwise available in the unaltered organism.</dd>

{{term|transition}} <dd>A {{gli|point mutation}} in which a {{gli|purine}} nucleotide is substituted for another purine ({{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine|A}}|font=courier|size=big}} ↔ {{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine|G}}|font=courier|size=big}}) or a {{gli|pyrimidine}} nucleotide is substituted for another pyrimidine ({{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine|C}}|font=courier|size=big}} ↔ {{font|{{gli|thymine|T}}|font=courier|size=big}}). Contrast ''{{gli|transversion}}''.</dd>

{{term|translation}}{{anchor|translational|translationally|translate|translates|translated}} <dd>The second step in the process of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}}, in which the {{gli|mRNA|messenger RNA}} transcript produced during {{gli|transcription}} is read by a {{gli|ribosome}} to produce a functional {{gli|protein}}.</dd>

{{term|translatome}} <dd>The entire set of {{gli|mRNA|messenger RNA}} molecules that are {{gli|translated}} by a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, cell, tissue, or species at a particular time or under particular conditions. Like the {{gli|transcriptome}}, it is often used as a proxy for quantifying levels of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}}, though the transcriptome also includes many RNA molecules that are never translated.</dd>

{{term|translocation}}{{anchor|translocations}} <dd>A type of chromosomal abnormality caused by the structural rearrangement of large sections of one or more {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomes}}. There are two main types: {{gli|reciprocal translocation|reciprocal}} and {{gli|Robertsonian translocation|Robertsonian}}.</dd>

{{term|transmembrane protein}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|integral polytopic protein}}''.</dd>

{{term|transmission genetics}} <dd>The branch of genetics that studies the mechanisms involved in the transfer of genes from parents to offspring.<ref name="DoG7"/></dd>

{{term|transport protein}}{{anchor|transport proteins|transporter|transporters}} {{ghat|Also '''transporter'''.}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|integral polytopic protein|transmembrane protein}} which functions by permitting the movement of particular molecules, proteins, or other substances across a {{gli|membrane}}, either {{gli|glossary=Glossary of cellular and molecular biology (0–L)|active transport|actively}} or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|facilitated diffusion|passively}} and in either or both directions (by which they may be further subclassified into {{gli|uniporters}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antiporters}}, and {{gli|symporters}}).<ref name="Lackie"/> {{gli|glossary=Glossary of cellular and molecular biology (0–L)|channel protein|Channel proteins}} and {{gli|nuclear pores}} are examples of transport proteins.</dd>

{{term|transporter}} <dd>See ''{{gli|transport protein}}''.</dd>

{{term|transposable element (TE)}}{{anchor|transposable element|transposable elements|transposon|transposons}} {{ghat|Also '''transposon'''.}} <dd>Any of a diverse variety of {{gli|selfish}} {{gli|mobile genetic elements}} consisting of self-acting DNA sequences capable of {{gli|replicating}} themselves semi-autonomously and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inserting}} into random or specific sites within a host genome, a process known as {{gli|transposition}}. Transposons contain one or more genes which encode enzymes known as {{gli|transposases}} capable of recognizing sequences within a flanking pair of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|inverted repeats}}, such that the enzymes effectively catalyze their own replication, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excision}}, and/or re-insertion into other DNA molecules by any of various mechanisms.<ref name="Lackie"/></dd>

{{term|transposase}}{{anchor|transposases}} <dd>Any of a class of self-acting {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymes}} capable of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding protein|binding}} to the flanking sequences of the {{gli|transposable element}} which encodes them and catalyzing its movement to another part of the genome, typically by an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excision}}/{{gli|glossary=Glossary of cellular and molecular biology (0–L)|insertion sequence|insertion}} mechanism or a replicative mechanism, in a process known as {{gli|transposition}}.</dd>

{{term|transposition}}{{anchor|transpose|transposes|transposing|transposed}} <dd>The process by which a nucleic acid sequence known as a {{gli|transposable element}} changes its position within a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}}, either by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|excising}} and re-inserting itself at a different {{gli|glossary=Glossary of cellular and molecular biology (0–L)|locus}} (cut-and-paste) or by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|duplicating}} itself and inserting into another locus without moving the original element from its original locus (copy-paste). These reactions are catalyzed by an enzyme known as a {{gli|transposase}} which is encoded by a gene within the transposable element itself; thus the element's products are self-acting and can autonomously direct their own replication. Transposed sequences may re-insert at random loci or at sequence-specific targets, either on the same DNA molecule or on different molecules.</dd>

{{term|transvection}}{{anchor|transvect|transvects|transvecting|transvected}} <dd></dd>

{{term|transversion}} <dd>A {{gli|point mutation}} in which a {{gli|purine}} nucleotide is substituted for a {{gli|pyrimidine}} nucleotide, or vice versa (e.g. {{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine|A}}|font=courier|size=big}} ↔ {{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine|C}}|font=courier|size=big}} or {{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine|A}}|font=courier|size=big}} ↔ {{font|{{gli|thymine|T}}|font=courier|size=big}}). Contrast ''{{gli|transition}}''.</dd>

{{term|tricarboxylic acid cycle (TCA)}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|citric acid cycle}}''.</dd>

{{term|triglyceride}}{{anchor|triglycerides}} {{ghat|Also '''triacylglycerol''' and '''triacylglyceride'''.}} <dd>Any of a class of chemical compounds which are ester derivatives of glycerol, consisting of a glycerol backbone connected to any three {{gli|glossary=Glossary of cellular and molecular biology (0–L)|fatty acid}} substituents via ester bonds. Triglycerides are one of three major classes of esters formed by fatty acids in biological systems, along with {{gli|phospholipids}} and cholesteryl esters. They are the primary constituent of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adipose}} tissue in vertebrates.</dd>

{{term|trimer}}{{anchor|trimers|trimeric}} <dd>A molecular aggregate consisting of three {{gli|subunits}}.<ref name="UVM">{{cite web |title=Levels of Protein Organization: A 2014 Foundations of Medicine eLAB |url=https://comis.med.uvm.edu/VIC/coursefiles/MD540/MD540-Protein_Organization_10400_574581210/Protein-org/Protein_Organization_print.html |website=comis.med.uvm.edu |publisher=University of Vermont, Larner College of Medicine}}</ref> The term is often used to refer to {{gli|protein complexes}} composed of three proteins, e.g. many membrane porins, or to individual proteins composed of three {{gli|polypeptides}}. Compare ''{{gli|monomer}}'', ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|dimer}}'', and ''{{gli|tetramer}}''.</dd>

{{term|trinucleotide repeat}} <dd>Any sequence in which an individual nucleotide {{gli|triplet}} is {{gli|repeated}} many times {{gli|tandem repeat|in tandem}}, whether in a gene or non-coding sequence. At most {{gli|glossary=Glossary of cellular and molecular biology (0–L)|loci}} some degree of repetition is normal and harmless, but mutations which cause specific triplets (especially those of the form {{font|{{gli|glossary=Glossary of cellular and molecular biology (0–L)|cytosine|C}}n{{gli|glossary=Glossary of cellular and molecular biology (0–L)|guanine|G}}|font=courier|size=big}}) to increase in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|copy number}} above the normal range are highly unstable and responsible for a variety of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genetic disorders}}.</dd>

{{term|triplet}}{{anchor|triplets}} <dd>A unit of three successive {{gli|nucleotides}} in a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} or {{gli|RNA}} molecule.<ref name="DoG7"/> A triplet within a coding sequence that codes for a specific amino acid is known as a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|codon}}.</dd>

{{term|trisomy}} <dd>A type of {{gli|polysomy}} in which a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|diploid}} cell or organism has three copies of a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}} instead of the normal two.</dd>

{{term|tRNA}} <dd>See ''{{gli|tRNA|transfer RNA}}''.</dd>

{{term|tRNA-ligase}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|aminoacyl-tRNA synthetase}}''.</dd>

{{term|tropism}} {{ghat|Also '''tropic movement'''.}} <dd>The directional growth or movement of a cell or organism in response to a stimulus, e.g. light, heat, the pull of gravity, or the presence of a particular chemical, such that the response is dependent on the direction of the stimulus (as opposed to a non-directional {{gli|nastic movement|nastic response}}). ''Positive tropism'' is growth or movement toward the stimulus; ''negative tropism'' is away from the stimulus.<ref name="MacLean"/> See also ''{{gli|taxis}}'' and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|kinesis}}''.</dd>

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

{{term|turgor pressure}} {{ghat|Also '''turgidity'''.}} <dd>The force within a cell which pushes the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|plasma membrane}} against the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell wall}},<ref>{{cite encyclopedia|author-last=Pritchard |author-first=Jeremy |date=2001 |title=Turgor Pressure |encyclopedia=Encyclopedia of Life Sciences |doi=10.1038/npg.els.0001687 |isbn=978-0-470-01590-2 |publisher=American Cancer Society}}</ref> a type of hydrostatic pressure influenced by the {{gli|osmosis|osmotic flow}} of water into and out of the cell. Turgidity is observed in plants, fungi, bacteria, and some protists with cell walls, but generally not in animal cells.</dd>

{{term|turnover number}} <dd>In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzymology}}, a measure of the rate at which a particular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enzyme}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|catalyzes}} a particular biochemical reaction, usually expressed as the average number of {{gli|substrate}} molecules it is capable of converting into reaction products per unit time at a given concentration of enzyme.<ref>{{cite journal |last1=Smejkal |first1=Gary B. |last2=Kakumanu |first2=Srikanth |title=Enzymes and their turnover numbers |journal=Expert Review of Proteomics |date=3 July 2019 |volume=16 |issue=7 |pages=543–544 |doi=10.1080/14789450.2019.1630275|pmid=31220960 }}</ref></dd>

{{term|twisting number}} <dd></dd>

==U== {{glossary}} {{term|ubiquitin (Ub)}}{{anchor|ubiquitin}} <dd>A small {{gli|protein}} of 76 amino acids found in great quantities (ubiquitously) in all eukaryotic cells, employed chiefly as a {{gli|post-translational modification|post-translational}} {{gli|protein tag}}, by which its C-terminal glycine residue is covalently bonded to electrically charged {{gli|residues}} within other proteins or polypeptides, a process known as {{gli|ubiquitination}}. Ubiquitin tags have functions in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|heat-shock response}}, {{gli|protein sorting}}, {{gli|proteolysis}}, {{gli|membrane trafficking}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell signaling}}, regulation of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell cycle}}, {{gli|X-inactivation|X chromosome inactivation}}, and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|histone modification}}, among others.<ref name="Lackie"/></dd>

{{term|ubiquitination}}{{anchor|ubiquitinated}} {{ghat|Also '''ubiquitylation'''.}} <dd>The {{gli|glossary=Glossary of cellular and molecular biology (0–L)|labelling}} of a biomolecule (often another protein) by covalently attaching a {{gli|ubiquitin}} protein to it—generally via the formation of an amide bond between the ubiquitin's C-terminal glycine and positively charged side chains (often lysine or arginine residues) of the labelled molecule, an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ATP}}-dependent reaction catalyzed by ubiquitin-conjugating enzymes<ref name="Lackie"/>—thus making it identifiable to molecules capable of recognizing ubiquitin {{gli|glossary=Glossary of cellular and molecular biology (0–L)|epitopes}}. Ubiquitination is a widely used {{gli|post-translational modification}} by which proteins are {{gli|protein tag|tagged}}; the attachment of a single ubiquitin molecule (''monoubiquitination'') can variously activate or inhibit a protein's activity, while the attachment of a chain of multiple consecutively linked ubiquitin molecules (''polyubiquitination'') commonly targets the protein for degradation by {{gli|proteasomes}}.</dd>

{{term|umber}} <dd>See ''{{gli|opal}}''.</dd>

{{term|uncharged tRNA}}{{anchor|uncharged tRNA}} <dd>A {{gli|tRNA|transfer RNA}} without an attached {{gli|glossary=Glossary of cellular and molecular biology (0–L)|amino acid}}. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|charged tRNA}}''.</dd>

{{term|underwinding}} <dd>See ''{{gli|negative supercoiling}}''.</dd>

{{term|unequal crossing over}} <dd></dd>

{{term|uniporter}}{{anchor|uniporters}} <dd>A type of {{gli|transport protein}} which catalyzes the movement of a single, specific solute or chemical species across a lipid membrane in either direction.<ref name="Alberts et al."/> Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|antiporter}}'' and ''{{gli|symporter}}''.</dd> thumb|right|350px|Three classes of {{gli|membrane protein|membrane}} {{gli|transport proteins}} can be distinguished by function: '''{{gli|uniporters}}''', '''{{gli|symporters}}''', and '''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|antiporters}}'''.

{{term|unique DNA}} {{ghat|Also '''non-repetitive DNA'''.}} <dd>A class of DNA {{gli|nucleic acid sequence|sequences}} determined by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|C0t analysis|C0''t'' analysis}} to be present only once in the analyzed genome, as opposed to {{gli|repetitive DNA|repetitive sequences}}. Most structural genes and their introns are unique.<ref name="DoG7"/></dd>

{{term|unstable mutation}} <dd>A {{gli|mutation}} with a high frequency of {{gli|reversion}}.<ref name="DoG7"/></dd>

{{term|untranslated region (UTR)}}{{anchor|untranslated regions|UTR|UTRs}} <dd>Any {{gli|non-coding}} sequence which is transcribed along with a {{gli|protein-coding sequence}}, and thus included within a {{gli|messenger RNA}}, but which is not ultimately {{gli|translated}} during protein synthesis. A typical mRNA transcript includes two such regions: one immediately upstream of the coding sequence, known as the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5' untranslated region}} (5'-UTR), and one downstream of the coding sequence, known as the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|3' untranslated region}} (3'-UTR). These regions are not removed during {{gli|post-transcriptional modification|post-transcriptional processing}} (unlike {{gli|glossary=Glossary of cellular and molecular biology (0–L)|introns}}) and are usually considered distinct from the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5' cap}} and the {{gli|poly(A) tail|3' polyadenylated tail}} (both of which are later additions to a primary transcript and not themselves products of transcription). UTRs are a consequence of the fact that transcription usually begins considerably upstream of the {{gli|start codon}} of the coding sequence and terminates long after the {{gli|stop codon}} has been transcribed, whereas translation is more precise. They often include motifs with regulatory functions.</dd>

{{term|upregulation}} {{ghat|Also '''promotion'''.}} <dd>Any process, natural or artificial, which increases the level of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene expression}} of a certain {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}}. A gene which is observed to be expressed at relatively high levels (such as by detecting higher levels of its {{gli|mRNA}} transcripts) in one sample compared to another sample is said to be ''upregulated''. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|downregulation}}''.</dd>

{{term|upstream}} <dd>Towards or closer to the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|5'-end}} of a chain of {{gli|nucleotides}}, or the {{gli|N-terminus}} of a {{gli|peptide}} chain. Contrast ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|downstream}}''.</dd>

{{term|upstream activating sequence (UAS)}} {{ghat|Also '''upstream activator sequence''' and '''upstream activation sequence'''.}} <dd>A type of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cis-regulatory element|cis-acting regulatory element}} found in the DNA of yeast such as ''Saccharomyces cerevisiae'', usually a few hundred base pairs upstream of the {{gli|transcription initiation site}} within the {{gli|promoter}} of a protein-coding gene, which helps to increase the gene's expression by serving as a binding site for transcriptional {{gli|transactivators}}, analogous to the function of an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|enhancer}} in multicellular eukaryotes.<ref>{{cite journal|last1=Webster|first1=Nocholas|last2=Jin|first2=Jia Rui|last3=Green|first3=Stephen|last4=Hollis|first4=Melvyn|last5=Chambon|first5=Pierre|title=The Yeast UAS{{sub|G}} is a transcriptional enhancer in human hela cells in the presence of the GAL4 trans-activator|journal=Cell|date=29 January 1988|volume=52|issue=2|pages=169–178|doi=10.1016/0092-8674(88)90505-3|pmid=2830022|s2cid=26819676}}</ref><ref name="Oxford B&MB"/></dd>

{{term|term=uracil|content=uracil ({{font|U|font=courier|size=large}})}} <dd>A {{gli|pyrimidine}} {{gli|nucleobase}} used as one of the four standard nucleobases in {{gli|RNA}} molecules. Uracil forms a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base pair}} with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|adenine}}. In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}}, uracil is not used at all, and is instead replaced with {{gli|thymine}}.</dd>

{{term|term=uridine|content=uridine ({{font|U|font=courier|size=large}}, Urd)}} <dd>One of the four standard {{gli|nucleosides}} used in {{gli|RNA}} molecules, consisting of a {{gli|uracil}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|base}} with its N9 nitrogen {{gli|glossary=Glossary of cellular and molecular biology (0–L)|glycosidic bond|bonded}} to the C1 carbon of a {{gli|ribose}} sugar. In {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}}, uridine is replaced with {{gli|thymidine}}.</dd>

==V== {{glossary}} {{term|vacuole}}{{anchor|vacuoles}} <dd>Any of a class of {{gli|membrane-bound organelle|enclosed}}, fluid-filled {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell compartmentalization|compartments}} present in many eukaryotic cells as well as bacteria, often large and conspicuous under the microscope and serving any of a huge variety of functions, including acting as a resizable reservoir for the storage of water, {{gli|metabolic waste}}, toxins, or foreign material; maintaining cellular {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homeostasis}} and {{gli|turgor|hydrostatic pressure}}; supporting immune functions; housing symbiotic bacteria; and assisting in the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|autophagy|degradation and recycling}} of old cellular components.<ref name="MacLean"/></dd>

{{term|variable number tandem repeat (VNTR)}}{{anchor|variable number tandem repeat|VNTR}} <dd>Any of a class of {{gli|tandem repeats}} for which the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|copy number}} of the {{gli|repeat|repeated sequence}} at a particular locus tends to vary between individuals of the same species. VNTRs may occur throughout the genome, both within and outside of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|coding DNA}}, and if the copy number is stably inherited may be used in {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA fingerprinting}} to uniquely identify individuals or to determine their genealogical relatedness to other individuals.</dd>

{{term|variegation}} <dd>Variation or irregularity in a particular {{gli|phenotype}}, especially a conspicuous visible {{gli|trait}} such as color or pigmentation, occurring simultaneously in different parts of the same individual organism due to any of a variety of causes, such as {{gli|X-inactivation}}, {{gli|mitotic recombination}}, {{gli|transposable element}} activity, {{gli|position-effect variegation|position effects}}, or infection by pathogens.</dd>

{{term|vector}}{{anchor|vectors}} <dd>Any {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA}} molecule used as a vehicle to artificially transport foreign genetic material into another cell, where it can be {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA replication|replicated}} and/or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expressed}}. Vectors are typically engineered {{gli|recombinant DNA}} sequences consisting of an {{gli|glossary=Glossary of cellular and molecular biology (0–L)|insertion sequence|insert}} (often a {{gli|transgene}}) and a longer "backbone" sequence containing an {{gli|origin of replication}}, a {{gli|multiple cloning site}}, and a {{gli|selectable marker}}. Vectors are widely used in molecular biology laboratories to isolate, {{gli|molecular cloning|clone}}, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|expression vector|express}} the insert in the target cell.</dd>

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

{{term|vegetal cell}} <dd>See ''{{gli|somatic cell}}''.</dd>

{{term|vesicle}}{{anchor|vesicles|vesicular}} <dd>Any {{gli|membrane-bound}} space completely enclosed by its own {{gli|membrane}}, which is separate though usually derived from other membranes (often the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|cell membrane}}) either by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|budding}} or by mechanical disruption such as {{gli|sonication}}.<ref name="Lackie"/> The term is applied to many different structures but especially to the small, roughly spherical {{gli|glossary=Glossary of cellular and molecular biology (0–L)|compartments}} created during {{gli|glossary=Glossary of cellular and molecular biology (0–L)|endocytosis}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|exocytosis}}, as well as to {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lysosomes}} and various other small intracellular or extracellular organelles.<ref name="MacLean"/></dd>

==W== {{glossary}} {{term|Warburg effect}} <dd></dd>

{{term|western blotting}}{{anchor|western blot|western blots}} <dd>A {{gli|blotting}} method used for detecting and identifying specific {{gli|proteins}} in heterogeneous biological samples. The technique involves separating proteins by size with {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gel electrophoresis}} and then immobilizing them upon a nitrocellulose, nylon, or other synthetic membrane, after which they may be visualized by autoradiography or by {{gli|glossary=Glossary of cellular and molecular biology (0–L)|labelling}} with chemiluminescent, radioactive, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|ELISA|enzyme-linked}} {{gli|glossary=Glossary of cellular and molecular biology (0–L)|antibodies}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|lectins}}, or other specific binding agents.<ref name="Oxford B&MB"/> Compare ''{{gli|Southern blotting}}'', ''{{gli|northern blotting}}'', and ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|eastern blotting}}''.</dd> thumb|right|400px|An outline of the steps involved in a '''{{gli|western blot}}'''

{{term|whole genome sequencing (WGS)}} <dd>The process of {{gli|sequencing|determining}} the entirety or near-entirety of the DNA sequences comprising an organism's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} with a single procedure or experiment, generally inclusive of all {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosomal}} and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|extrachromosomal DNA|extrachromosomal}} (e.g. {{gli|mtDNA|mitochondrial}}) DNA.</dd>

{{term|wild type (WT)}}{{anchor|wild-type}} {{ghat|Denoted in shorthand with a '''''+''''' superscript.}} <dd>The {{gli|phenotype}} of the typical form of a species as it occurs in nature; a product of the standard "normal" {{gli|glossary=Glossary of cellular and molecular biology (0–L)|allele}} at a given {{gli|glossary=Glossary of cellular and molecular biology (0–L)|locus}}, as opposed to that produced by a non-standard {{gli|mutant}} allele.</dd>

{{term|wobble base pairing}} <dd></dd>

{{term|writhing number (W)}}{{anchor|writhing number}} {{ghat|Also '''writhe'''.}} <dd>An index of the {{gli|supercoiling|superhelical coiling}} of a DNA molecule. The writhing number does not have a precise quantitative definition but instead represents the degree of supercoiling. Together, the writhing number and the {{gli|twisting number}} determine the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|linking number}}.<ref name="Oxford B&MB"/></dd>

==X== {{glossary}} {{term|X chromosome}} <dd>One of two {{gli|glossary=Glossary of cellular and molecular biology (0–L)|allosome|sex chromosomes}} present in organisms which use the XY sex-determination system, and the only sex chromosome in the X0 system. The X chromosome is found in both males and females and typically contains much more {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}} content than its counterpart, the {{gli|Y chromosome}}.</dd>

{{term|X-inactivation}} <dd>The process by which one of the two copies of the {{gli|X chromosome}} is silenced by being irreversibly condensed into transcriptionally inactive {{gli|glossary=Glossary of cellular and molecular biology (0–L)|heterochromatin}} in the cells of female therian mammals. A form of {{gli|glossary=Glossary of cellular and molecular biology (0–L)|dosage compensation}}, X-inactivation prevents females from producing twice as many {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene products}} from genes on the X chromosome as males, who only have one copy of the X chromosome. Which X chromosome is inactivated is randomly determined in the early embryo, making it possible for cell lineages with different inactive Xs to exist in the same organism.</dd>

{{term|X-linked trait}} <dd>A {{gli|phenotypic trait}} whose expression is governed or influenced by one or more genes located on the {{gli|X chromosome}} (making it a {{gli|sex linkage|sex-linked}} trait).</dd>

==Y== {{glossary}} {{term|Y chromosome}} <dd>One of two {{gli|glossary=Glossary of cellular and molecular biology (0–L)|allosome|sex chromosomes}} present in organisms which use the XY sex-determination system. The Y chromosome is found only in males and is typically much smaller than its counterpart, the {{gli|X chromosome}}.</dd>

{{term|Y fork}} <dd>See ''{{gli|replication fork}}''.</dd>

{{term|yeast artificial chromosome (YAC)}} <dd></dd>

==Z== {{glossary}} {{term|Z-DNA}}{{anchor|Z-DNA}} <dd></dd>

{{term|zinc finger (ZF)}}{{anchor|zinc finger|zinc fingers}} <dd>A {{gli|supersecondary}} {{gli|polypeptide}} structural motif and {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding domain}} occurring in many {{gli|glossary=Glossary of cellular and molecular biology (0–L)|DNA-binding proteins}}, characterized by a series of non-adjacent amino acid {{gli|residues}} which {{gli|protein folding|fold}} into a three-dimensional arrangement capable of coordinating one or more zinc ions ({{chem|Zn|2+}}) between them, thus stabilizing the fold into a definite structure that can interact specifically with other biomolecules such as nucleic acids or other polypeptides. There are many distinct classes of zinc fingers using different ligands and spatial arrangements to achieve coordination; in perhaps the most common variant, a short {{gli|glossary=Glossary of cellular and molecular biology (0–L)|alpha helix}} is oriented antiparallel to a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|beta sheet|β sheet}}, with two histidine residues in the former and two cysteines in the latter forming the coordination complex. Zinc fingers bind DNA as the primary functional domain of many {{gli|transcription factors}}.</dd>

{{term|zonula adherens}} <dd>See ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|adherens junction}}''.</dd>

{{term|zonula occludens}} <dd>See ''{{gli|tight junction}}''.</dd>

{{term|zygonema}} {{ghat|Also '''zygotene stage'''.}} <dd>In {{gli|meiosis}}, the second of five substages of {{gli|prophase|prophase I}}, following {{gli|glossary=Glossary of cellular and molecular biology (0–L)|leptonema}} and preceding {{gli|pachynema}}. During zygonema, {{gli|synapsis}} occurs, physically binding {{gli|glossary=Glossary of cellular and molecular biology (0–L)|homologous chromosomes}} to each other, and the cell's {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centrosome}} divides into two daughter centrosomes, each containing a single {{gli|glossary=Glossary of cellular and molecular biology (0–L)|centriole}}.<ref name="DoG7"/></dd>

{{term|zygosity}} <dd>The degree to which multiple copies of a {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gene}}, {{gli|glossary=Glossary of cellular and molecular biology (0–L)|chromosome}}, or {{gli|glossary=Glossary of cellular and molecular biology (0–L)|genome}} have the same genetic sequence; e.g. in a diploid organism with two complete copies of its genome (one maternal and one paternal), the degree of similarity of the {{gli|glossary=Glossary of cellular and molecular biology (0–L)|alleles}} present in each copy. Individuals carrying two different alleles for a particular gene are said to be ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|heterozygous}}'' for that gene; individuals carrying two identical alleles are said to be ''{{gli|glossary=Glossary of cellular and molecular biology (0–L)|homozygous}}'' for that gene. Zygosity may also be considered collectively for a group of genes, or for the entire set of genes and genetic {{gli|glossary=Glossary of cellular and molecular biology (0–L)|loci}} comprising the genome.</dd>

{{term|zygote}}{{anchor|zygotes|zygotic}} <dd>A type of eukaryotic cell formed as the direct result of a fertilization event between two {{gli|glossary=Glossary of cellular and molecular biology (0–L)|gametes}}. In multicellular organisms, the zygote is the earliest developmental stage.</dd>

==References== {{reflist}}

==Further reading== *{{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 animal cell from SwissBioPics]

{{Glossaries of science and engineering}} {{MolBioGeneExp}} {{DEFAULTSORT:Genetics Glossary}} * Glossary Genetics Category:Molecular-biology-related lists Category:Wikipedia glossaries using description lists