DNA replication fidelity – Which enzyme is directly responsible for proofreading base pairing during DNA synthesis?

Introduction:
High-fidelity DNA replication depends on accurate base selection and post-incorporation error correction. The central proofreading step is carried out by the replicative DNA polymerases through exonuclease activity. This question asks you to identify the enzyme that performs proofreading directly at the replication fork.

Given Data / Assumptions:

• Replicative polymerases possess 3'→5' exonuclease activity for proofreading.
• Accessory enzymes have distinct functions (e.g., ligation, primer synthesis, end maintenance).
• Proofreading corrects mispaired nucleotides immediately after insertion.

Concept / Approach:
DNA polymerases (e.g., bacterial Pol III; eukaryotic Pol δ and Pol ε) monitor the geometry of nascent base pairs. Upon detecting a mismatch, the polymerase shifts the primer terminus to a 3'→5' exonuclease site, excises the incorrect nucleotide, and then resumes synthesis. This intrinsic activity greatly lowers the error rate before mismatch repair pathways act.

Step-by-Step Solution:

Verification / Alternative check:
Mutations disabling polymerase exonuclease increase mutation rates dramatically, demonstrating the essential role of polymerase-based proofreading in genomic fidelity.

Why Other Options Are Wrong:

• Telomerase: extends telomeres; does not proofread replication errors.
• Primase: synthesizes RNA primers; lacks proofreading.
• DNA ligase: seals nicks; does not check base pairing.
• Topoisomerase I: manages supercoils; unrelated to base selection.

Common Pitfalls:
Attributing proofreading to mismatch repair (a separate, downstream pathway) rather than to polymerase exonuclease activity at the fork.

Final Answer:
DNA polymerase