How does DNA proofreading by DNA polymerase contribute to replication fidelity?

• A. 3'→5' exonuclease activity removes misincorporated nucleotides during synthesis; post-replication mismatch repair further increases fidelity.
• B. 5'→3' exonuclease does proofreading, no additional repair.
• C. Polymerase cannot proofread; proofreading occurs only after cell division.
• D. Proofreading reduces fidelity.

Answer: (A)

DNA proofreading by the polymerase works by a 3'→5' exonuclease activity that can remove a nucleotide that has just been misincorporated. If the wrong base is added, the polymerase backtracks, excises the incorrect nucleotide, and once the site is clean, it inserts the correct one and continues synthesis. This immediate correction during synthesis drastically lowers the chance that an error becomes part of the genome. After the new DNA is made, mismatch repair systems scan for any remaining mispairs and fix them, using signals that distinguish the newly synthesized strand from the template strand to ensure the right base is corrected. This combination—intrinsic proofreading during replication plus post-replication mismatch repair—greatly enhances replication fidelity. The 5'→3' exonuclease activity is involved in other processes, not proofreading; proofreading happens during synthesis, not after cell division; and proofreading does not reduce fidelity.