How does DNA polymerase ensure fidelity during replication?

• A. It has proofreading 3' to 5' exonuclease activity that removes misincorporated bases.
• B. It uses high-fidelity RNA primer.
• C. It only synthesizes in the 5' to 3' direction with no proofreading.
• D. It relies on ligase to fix errors.

Answer: (A)

Fidelity during DNA replication mainly comes from the proofreading ability of DNA polymerase in a 3' to 5' direction. When a mismatched nucleotide is added, the geometry at the active site becomes distorted, and the enzyme transfers the nascent strand to its exonuclease site where the incorrect base is removed. The primer is then moved back to the polymerizing site to continue synthesis with the correct nucleotide. This proofreading step, combined with the enzyme’s strict selectivity for proper Watson-Crick base pairing (an induced-fit check that favors correct pairs), greatly reduces errors. If any mistakes escape proofreading, mismatch repair after replication can catch and correct them, further boosting fidelity. The other options don’t provide a mechanism for correcting misincorporations: RNA primers initiate synthesis but don’t fix errors, synthesis direction alone isn’t enough for high fidelity, and ligase only seals nicks, not correct base errors.