In the next round of replication, the mismatched members of the base pair separate; the tautomer usually shifts back to its normal isomer. Replication of the 2 strands by normal base pairing results in a point mutation called a transition mutation, where a purine substitutes for a purine, or a pyrimidine substitutes for a pyrimidine. In this example, the wild type T - A pair has mutated to C - G.