Which statement describes DNA proofreading during repair?

Which statement describes DNA proofreading during repair?

• A. DNA polymerases proofread each nucleotide against the template and remove incorrect nucleotides
• B. DNA ligase proofreads the base pairing
• C. Helicase repairs mismatches
• D. Primase performs nucleotide excision repair

Answer: (A)

DNA proofreading is accomplished by the DNA polymerase itself through a 3' to 5' exonuclease editing activity. As the enzyme adds a nucleotide, it checks base pairing with the template. If a mismatch is detected, the polymerase pauses, removes the incorrect nucleotide via its exonuclease function, and then resynthesizes correctly. This direct editing step ensures mistakes are caught and corrected right away, preventing them from becoming permanent mutations. Other enzymes mentioned—ligase seals nicks after synthesis, helicase unwinds DNA, primase lays down RNA primers, and nucleotide excision repair removes bulky damage via a different pathway—do not perform this immediate proofreading of base pairing during synthesis.

DNA proofreading is accomplished by the DNA polymerase itself through a 3' to 5' exonuclease editing activity. As the enzyme adds a nucleotide, it checks base pairing with the template. If a mismatch is detected, the polymerase pauses, removes the incorrect nucleotide via its exonuclease function, and then resynthesizes correctly. This direct editing step ensures mistakes are caught and corrected right away, preventing them from becoming permanent mutations. Other enzymes mentioned—ligase seals nicks after synthesis, helicase unwinds DNA, primase lays down RNA primers, and nucleotide excision repair removes bulky damage via a different pathway—do not perform this immediate proofreading of base pairing during synthesis.