Which activity provides proofreading during DNA synthesis?

• A. 5'→3' exonuclease activity of DNA polymerases.
• B. Mismatch repair during S-phase.
• C. 3'→5' exonuclease activity of DNA polymerases.
• D. Topoisomerase activity.

Answer: (C)

Proofreading during DNA synthesis is carried out by the 3'→5' exonuclease activity of DNA polymerases. As nucleotides are added in the 5'→3' direction, the polymerase checks the newly added base; if a mismatch is detected, its 3' end is resected by the exonuclease domain, removing the incorrect nucleotide and a few preceding ones if needed. This allows the polymerase to reinsert the correct base and continue synthesis with high fidelity. In contrast, the 5'→3' exonuclease activity is mainly used to remove RNA primers or perform nick translation rather than proofreading the nascent strand. Mismatch repair operates after replication to fix errors on the newly synthesized strand, not during synthesis itself. Topoisomerase relieves torsional strain from supercoiling and does not participate in proofreading.