Discussion
Which of the following reactions is required for proofreading during DNA replication by DNA polymerase III?
Answer: B
Correct answer: 3’ to 5’ exonuclease activity
Proofreading means backing up over the base just added, so the enzyme must chew in the direction opposite to synthesis - 3’→5’. When a mispaired base distorts the geometry at the active site, the mismatched nucleotide is moved into the exonuclease site, excised, and replaced. This raises the fidelity of replication by roughly a hundredfold. (Pol I’s separate 5’→3’ exonuclease is for removing RNA primers, not proofreading.)
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