= Nucleotide transition
A <nucleotide transition> is a <nucleotide substitution> between two <purine>-derived bases or between two <pyrimidine>-derived bases. In <DNA>, the possibilities are $A\leftrightarrow G$ and $C\leftrightarrow T$: <adenine> exchanges with <guanine>, or <cytosine> with <thymine>. The chemical class is preserved; the identity of the base changes.<A simple method for estimating evolutionary rates of base substitutions through comparative studies of nucleotide sequences by Motoo Kimura>[1980, Summary: transition/type-I differences]{ref}
From any specified <DNA> base, there is exactly one possible transition target and two <nucleotide transversion> targets. If all three targets were equally likely, transitions would comprise $1/3$ of changes. This is a counting baseline, not a biological law about <mutation rates>. The <Kimura two-parameter model> allows the rates for these two classes to differ.<A simple method for estimating evolutionary rates of base substitutions through comparative studies of nucleotide sequences by Motoo Kimura>[Two-substitution-class model]{ref}
The classification does not determine a <mutation>'s effect on a <protein>. A transition can preserve an <amino acid>, change it or introduce a <stop codon>, depending on the affected <codon>.
Back to article page
