Which repair mechanism removes bulky lesions and UV-induced thymine dimers?

• A. Base excision repair (BER)
• B. Mismatch repair (MMR)
• C. Direct reversal
• D. Nucleotide excision repair (NER)

Answer: (D)

Bulky, helix-distorting DNA damage like UV-induced thymine dimers needs a repair system that can recognize structural distortions and remove a chunk of DNA around the lesion. Nucleotide excision repair does exactly that: it detects the distortion, excises a short DNA segment containing the damaged site, and then DNA polymerase fills in the gap followed by ligase sealing. This contrasts with base excision repair, which targets small, specific base lesions one at a time; mismatch repair, which fixes replication errors like mispaired bases; and direct reversal, which can reverse certain lesions but does not broadly remove bulky distortions in human cells. Therefore, removing bulky lesions and UV-induced thymine dimers is handled by nucleotide excision repair.