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.^[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.^[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.