What is the primary action of nucleotide excision repair in response to DNA damage?

• A. Remove damaged DNA segment and synthesize new DNA (correct)
• B. Mismatch recognition
• C. Recombine with undamaged strand
• D. Deaminate cytosine

Answer: (A)

Nucleotide excision repair focuses on bulky, helix-distorting DNA damage. Its main action is to remove a short segment of the damaged strand containing the lesion and then synthesize new DNA to fill the gap. When the repair machinery detects a lesion like a thymine dimer from UV light or a bulky chemical adduct, it makes incisions on both sides of the damaged site, excises about 24–30 nucleotides, and uses the undamaged strand as a template to resynthesize the correct sequence, followed by sealing the backbone. This systematic removal-and-replication restores the original DNA sequence across the damaged region, which is essential to prevent replication blocks and mutations.

This is different from mismatch repair, which corrects mispaired bases that arise during replication; it’s not about recombining with an undamaged sister chromatid to fix breaks; and it’s not about merely removing a damaged base like removing a deaminated cytosine, which is typically handled by base excision repair.