②3”→5外切酶活性 The mispaired 3'-OH DNA polymerase I end of the growing DNA polymerase strand blocks further elongation.DNA active site polymerase slides back 3'→5'(proofreading to position the exonuclease mispaired base in the 3'→5'exonuclease active site active site. C removed 5'000 OH OH 3 Cis a rare tautomeric OH form of cytosine(C*) The mispaired that pairs with A and 日0a日日 nucleotide is removed is incorporated into the growing strand DNA polymerase slides C (unable to pair with A) forward and resumes its polymerization activity. Before the polymerase 0 moves on,the cytosine OH undergoes a tautomeric shift from C*to C.The new nucleotide is now 00Y000000001001010 ispaired. 单链C (unable to pair with A) C removed ② 3´ → 5´ 外切酶活性 单链