Figure 1.

Activation-induced cytidine deaminase (AID) induces genetic changes in human DNA sequences. AID converts cytosine (C) to uracil (U) on the target DNA sequence, creating U-guanine (G) mismatch that is resolved by several pathways. (Left pathway) The general replication machinery interprets U as if it was thymine (T), generating C to T and G to adenine (A) mutations. (Middle pathway) Uracil-DNA glycosylase removes an U nucleotide, creating an abasic site. Short-patch base excision repair fills the gap with error-prone polymerases, which can insert any nucleotide in place of the U nucleotide, leading to both transition and transversion mutations at C-G pairs. (Right pathway) Mismatch repair recognizes U-G mismatch. U-bearing strand is excised by Exo1 and error-prone polymerases fill the gap, leading to mutations at A-T pairs as well as at neighboring C-G pairs.