Transition (genetics)
Encyclopedia
In genetics, a transition is a point mutation
that changes a purine
nucleotide to another purine (A
↔ G
) or a pyrimidine
nucleotide to another pyrimidine (C
↔ T
). Approximately two out of three single nucleotide polymorphism
s (SNPs) are transitions.
Transitions can be caused by oxidative deamination
and tautomerization. Although there are twice as many possible transversions, transitions appear more often in genomes, possibly due to the molecular mechanisms that generate them.
5-Methylcytosine
is more prone to transition than unmethylated cytosine
, due to spontaneous deamination
. This mechanism is important because it dictates the rarity of CpG islands.
Point mutation
A point mutation, or single base substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA. Often the term point mutation also includes insertions or deletions of a single base pair...
that changes a purine
Purine
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
nucleotide to another purine (A
Adenine
Adenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and protein synthesis, as a chemical component of DNA...
↔ G
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
) or a pyrimidine
Pyrimidine
Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
nucleotide to another pyrimidine (C
Cytosine
Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
↔ T
Thymine
Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...
). Approximately two out of three single nucleotide polymorphism
Single nucleotide polymorphism
A single-nucleotide polymorphism is a DNA sequence variation occurring when a single nucleotide — A, T, C or G — in the genome differs between members of a biological species or paired chromosomes in an individual...
s (SNPs) are transitions.
Transitions can be caused by oxidative deamination
Oxidative deamination
Oxidative deamination is a form of deamination that generates oxoacids in the liver.The presence of nitrous acid can cause transition mutations, by converting cytosine to uracil. Primarily occurs in the liver and kidneys.-In Urea cycle:...
and tautomerization. Although there are twice as many possible transversions, transitions appear more often in genomes, possibly due to the molecular mechanisms that generate them.
5-Methylcytosine
5-Methylcytosine
5-Methylcytosine is a methylated form of the DNA base cytosine that may be involved in the regulation of gene transcription. When cytosine is methylated, the DNA maintains the same sequence, but the expression of methylated genes can be altered .In the figure on the right, a methyl group, is...
is more prone to transition than unmethylated cytosine
Cytosine
Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
, due to spontaneous deamination
Deamination
Deamination is the removal of an amine group from a molecule. Enzymes which catalyse this reaction are called deaminases.In the human body, deamination takes place primarily in the liver, however glutamate is also deaminated in the kidneys. Deamination is the process by which amino acids are...
. This mechanism is important because it dictates the rarity of CpG islands.