transversjon

Transversion, known in Norwegian as transversjon, is a type of nucleotide substitution in DNA in which a purine is replaced by a pyrimidine, or vice versa. The four possible base substitutions are A<->C, A<->T, G<->C, and G<->T, along with their reverse complements on the opposite strand. This contrasts with transitions, where purines replace purines (A<->G) or pyrimidines replace pyrimidines (C<->T). Transversions are generally less frequent than transitions in many genomes, contributing to characteristic mutational spectra and affecting estimates of mutation rates.

Mechanistically, transversions can arise from DNA damage such as oxidation, alkylation, or deamination events that favor purine–pyrimidine changes, as well as by errors made by DNA polymerases during replication. Their repair can involve base excision repair or mismatch repair pathways.

In coding regions, transversions are more likely than transitions to alter amino acid identity, potentially producing missense or nonsense mutations. They can also disrupt regulatory motifs if they occur in promoters or enhancers. The distribution of transversions across the genome is a component of mutational signatures used in studies of evolution, cancer genomics, and population genetics.

In practical analyses, the ratio of transitions to transversions (Ti/Tv) is a common quality-control metric in variant calling; deviations from expected ratios can indicate sequencing or mapping biases. Transversions thus provide information about mutational processes and evolutionary history, as well as practical benchmarks for genomics workflows.