Transition (genetics)

{{Short description|DNA mutation that exchanges two nucleotides}} {{Other uses|Transition (disambiguation){{!}}Transition}} [[Image:All transitions and transversions.svg|thumb|right|300px|Illustration of a transition: each of the 4 nucleotide changes between purines or between pyrimidines (in blue). The 8 other changes are transversions (in red).]]

'''Transition''', in genetics and molecular biology, refers to a point mutation that changes a purine nucleotide to another purine (A ↔ G) or a pyrimidine nucleotide to another pyrimidine (C ↔ T).<ref name=":0">{{Citation |last=Brown |first=Terence A. |title=Mutation, Repair and Recombination |date=2002 |work=Genomes. 2nd edition |url=https://www.ncbi.nlm.nih.gov/books/NBK21114/ |access-date=2026-04-23 |publisher=Wiley-Liss |language=en}}</ref> Transitions are common in genomes; approximately two out of three single nucleotide polymorphisms (SNPs) are transitions.<ref>{{cite journal |vauthors=Collins DW, Jukes TH |title=Rates of transition and transversion in coding sequences since the human-rodent divergence |journal=Genomics |volume=20 |issue=3 |pages=386–96 |date=April 1994 |pmid=8034311 |doi=10.1006/geno.1994.1192 }}</ref>

== Molecular basis == Transitions can be caused by several mechanisms, including spontaneous deamination and tautomerization. Deamination of cytosine produces uracil while deamination of 5-methylcytosine produces thymine. Rare tautomeric forms of DNA bases can also mispair during replication without major distortion of the double helix. If these base changes and mismatches escape DNA repair mechanisms, they can become fixed as transition mutations.<ref name=":0" />

== Transition bias == Although there are twice as many possible transversions, transitions occur more often in genomes,<ref>{{cite journal |vauthors=Ebersberger I, Metzler D, Schwarz C, Pääbo S |date=June 2002 |title=Genomewide comparison of DNA sequences between humans and chimpanzees |journal=Am. J. Hum. Genet. |volume=70 |issue=6 |pages=1490–7 |doi=10.1086/340787 |pmc=379137 |pmid=11992255 |bibcode=2002AmJHG..70.1490E }}</ref> a pattern known as transition/transversion bias. This bias results mainly from molecular structure and selection pressure. In coding regions, transitions are more likely than transversions to produce synonymous substitutions. Also, transitions are less likely than transversions to cause drastic amino acid changes because of the structure of the genetic code<ref>{{Cite journal |last1=Arlin |first1=Stoltzfus |last2=W. Norris |first2=Ryan |date=2016-03-01 |title=On the Causes of Evolutionary Transition:Transversion Bias |url=https://academic.oup.com/mbe/article/33/3/595/2579658 |journal=Molecular Biology and Evolution |language=en |volume=33 |issue=3 |doi=10.1093/molbev/m |doi-broken-date=23 April 2026 |issn=0737-4038 |archive-url=http://web.archive.org/web/20260212114544/https://academic.oup.com/mbe/article/33/3/595/2579658 |archive-date=2026-02-12}}</ref>. Moreover, some transition-generating base-pair mismatches may cause less distortion to the DNA double helix which may therefore escape repair mechanisms.<ref>{{Cite journal |last1=Zou |first1=Zhengting |last2=Zhang |first2=Jianzhi |date=2021-01-04 |title=Are Nonsynonymous Transversions Generally More Deleterious than Nonsynonymous Transitions? |journal=Molecular Biology and Evolution |volume=38 |issue=1 |pages=181–191 |doi=10.1093/molbev/msaa200 |issn=1537-1719 |pmc=7783172 |pmid=32805043}}</ref>

== CpG sites and methylation == In vertebrate genomes, spontaneous deamination of 5-methylcytosine creates T:G mismatches that become fixed as C→T transitions. As a result, methylated CpG dinucleotides are important hotspots for transition mutations.<ref>{{Cite book |last=Pfeifer |first=G. P. |title=DNA Methylation: Basic Mechanisms |date=2006 |chapter=Mutagenesis at methylated CpG sequences |series=Current Topics in Microbiology and Immunology |volume=301 |pages=259–281 |doi=10.1007/3-540-31390-7_10 |issn=0070-217X |pmid=16570852 |isbn=3-540-29114-8 }}</ref> This mechanism contributes to the genome-wide depletion of CpG dinucleotides over evolutionary time. However, CpG islands are an exception because they are often unmethylated and therefore less affected by this mutational process.<ref>{{Cite journal |last1=Deaton |first1=Aimée M. |last2=Bird |first2=Adrian |date=2011-05-15 |title=CpG islands and the regulation of transcription |url=http://genesdev.cshlp.org/content/25/10/1010 |journal=Genes & Development |language=en |volume=25 |issue=10 |pages=1010–1022 |doi=10.1101/gad.2037511 |issn=0890-9369 |pmid=21576262 |pmc=3093116 }}</ref> * Transversion

==References== {{reflist}}

==External links== {{Commons category|Transition (genetics)}} * [https://www.mun.ca/biology/scarr/Transitions_vs_Transversions.html Diagram at mun.ca]

Category:Mutation