▁repair ▁path way ▁( BER ). ▁Its ▁main ▁role ▁in ▁the ▁repair ▁of ▁dam aged ▁or ▁mismatch ed ▁nucle ot ides ▁in ▁DNA ▁is ▁to ▁create ▁a ▁nick ▁in ▁the ▁ph osph odi ester ▁back bone ▁of ▁the ▁AP ▁site ▁created ▁when ▁DNA ▁g ly cos yl ase ▁removes ▁the ▁dam aged ▁base . ▁ ▁There ▁are ▁four ▁types ▁of ▁AP ▁end on uc leases ▁that ▁have ▁been ▁class ified ▁according ▁to ▁their ▁mechanism ▁and ▁site ▁of ▁inc ision . ▁Class ▁I ▁AP ▁end on uc leases ▁() ▁cle ave ▁ 3 ′ ▁to ▁AP ▁sites ▁by ▁a ▁ β - ly ase ▁mechanism , ▁leaving ▁an ▁uns atur ated ▁al de hy de , ▁term ed ▁a ▁ 3 ′ -( 4 - h ydro xy - 5 - ph osph o - 2 - p enten al ) ▁resid ue , ▁and ▁a ▁ 5 ′ - ph osph ate . ▁Class ▁II ▁AP ▁end on uc leases ▁inc ise ▁DNA ▁ 5 ′ ▁to ▁AP ▁sites ▁by ▁a ▁hydro ly t ic ▁mechanism , ▁leaving ▁a ▁ 3 ′ - h ydro x yl ▁and ▁a ▁ 5 ′ - de ox y ri bose ▁ph osph ate ▁resid ue . ▁Class ▁III ▁and ▁class ▁IV ▁AP ▁end on uc leases ▁also ▁cle ave ▁DNA ▁at ▁the ▁ph osph ate ▁groups ▁ 3 ′ ▁and ▁ 5 ′ ▁to ▁the ▁bas eless ▁site , ▁but ▁they ▁generate ▁a ▁ 3 ′ - ph osph ate ▁and ▁a ▁ 5 ′