The two-step process of a point mutation. (a) DNA polymerase incorporates an incorrect nucleotide (C instead of T), leading to a mismatch in one of the first-generation cells. If the mismatch is not corrected before this cell replicates, one of the second-generation cells will incur a mutation. (b) The parental DNA contains a damaged adenosine base (X). The damaged A more readily pairs with a C than a T, so replication over the damaged base results in a mismatched nucleotide in one of the first-generation cells. Further replication can result in a permanent mutation in a cell of the second generation.