A type of lesion in a DNA molecule that results in a mutation. There are two subtypes. In the case of transitions, one purine is substituted by the other or one pyrimidine by the other, and so the purine-pyrimidine axis is preserved. In the case of transversions, a purine is substituted by a pyrimidine or vice versa, and the purine-pyrimidine axis is reversed. See Chronology, 1959, Freese.
Subjects: Genetics and Genomics.