Base excision repair. (a) In bacteria, a glycosylase excises a damaged nucleotide base, then an AP endonuclease nicks the backbone at the abasic site. Nick translation by Pol I excises the 5′ deoxyribose phosphate (5′-dRP) and some dNMPs, and synthesizes a new strand. Ligase seals the gap. (b) Eukaryotic BER, after the first two steps (similar to those in bacteria), can take either of two paths. In long patch repair (left), a DNA polymerase extends the DNA strand from the 3′ terminus, displacing the 5′ single-stranded DNA; this is followed by cleavage by a flap endonuclease and ligation. In short patch repair (right), only one nucleotide is inserted (by Pol β) prior to ligation.