Figure 2.

The base excision repair pathway. (A) Uracil spontaneously occurs in DNA due to the deamination of cytosine bases. If left uncorrected, the uracil-containing strand will give rise to a permanently mutated daughter strand upon replication. (B) A uracil-DNA glycosylase (UDG) cleaves the uracil base from the DNA backbone, leaving an abasic site in the DNA. (C) An AP-endonuclease (APE) cleaves the phosphodiester backbone, 5′ of an abasic site [AP refers to apyrimidinic, or lacking a pyrimidine; it can also refer to apurinic in other contexts]. (D) A type-B DNA polymerase (POLB), typically involved in short-patch DNA repair, resynthesises several bases complementary to the undamaged strand while displacing the cleaved strand beginning at the nick generated by AP-endonuclease. (E) A Flap-Endonuclease (FEN) cleaves off the displaced DNA strand after the B-type DNA polymerase has dissociated, leaving behind a nick in the DNA duplex. (F) A DNA ligase (LIG) seals the nick to return DNA to its canonical pre-damaged state.