Practice question

After glycosylase action, apurinic-apyrimidinic site lacks base but retains sugar-phosphate backbone. AP endonuclease family represented by E. coli exonuclease III and Xth and human APE1 incises phosphodiester immediately 5' to AP lesion generating 3'-hydroxyl and 5'-deoxyribose phosphate termini. This nick provides entry for DNA polymerase performing repair synthesis and dRP lyase activity removing sugar remnant. Cleavage strictly 5' side of lesion, not 3', creates appropriate primer for replacement synthesis. Defects in APE1 lead to accumulation of breaks and hypersensitivity to oxidizing agents due to blocked BER progression.