DNA substitution mutations are of two types. Transitions are interchanges of two-ring purines (A <-> G) or of one-ring pyrimidines (C <-> T): they therefore involve bases of similar shape. Transversions are interchanges of purine for pyrimidine bases, which therefore involve exchange of one-ring and two-ring structures.
Transversions are interchanges of purine for pyrimidine bases, which therefore involve exchange of one-ring and two-ring structures.
Although there are twice as many possible transversions, because of the molecular mechanisms by which they are generated, transition mutations are generated at higher frequency than transversions. As well, transitions are less likely to result in amino acid substitutions (due to “wobble”), and are therefore more likely to persist as “silent substitutions” in populations as single nucleotide polymorphisms (SNPs).
Induction of transition mutations by spontaneous tautomeric shifts
In the original double-stranded DNA molecule, A in the standard (amino) form pairs with T. During replication, the two strands separate. In the upper diagram, T pairs with A as usual, which replicates the wild-type sequence. In the lower diagram, A has undergone a tautomeric shift to the non-standard (imino) form A’, which pairs with C. In the next round of replication, the imino A’ shifts back to the amino A form, which pairs with T, which again reproduces the wild-type sequence. Replication of the other strand pairs C with G. By comparison with the original molecule, the result is a T -> C mutation. A tautomeric shift in one strand has produced a transition mutation in the complementary strand. If the mutation occurs in the germline, it will be transmitted to future generations.
IMPORTANT: Note that a tautomeric shift is not itself a mutation, but a transient change to an alternative form of the molecule.