What is the mechanism by which proofreading corrects a misincorporated nucleotide?

• A. A separate repair enzyme fixes the mismatch after replication
• B. The polymerase detects a mismatch, transfers the mispaired terminus to its 3'→5' exonuclease site, removes it, and resynthesizes correctly
• C. Ligase seals the mismatch
• D. Exonuclease activity is not involved in proofreading

Answer: (B)

During DNA replication the polymerase has a built-in proofreading function. When a mispaired nucleotide is added, the 3' end of the nascent strand is shifted into the polymerase’s 3' to 5' exonuclease site, the incorrect nucleotide is removed, and the polymerase then resynthesizes the correct base before continuing. This immediate correction helps prevent the error from becoming permanent in the genome. This proofreading is distinct from a separate mismatch repair system that acts after replication, which is why another option describing a post-replication fix isn’t correct. Ligase seals nicks after synthesis but doesn’t correct a mispair, and exonuclease activity is indeed involved in proofreading, so saying it isn’t involved would be incorrect.