Which repair pathway removes bulky DNA lesions such as thymine dimers?

• A. Mismatch repair
• B. Base-excision repair
• C. Nucleotide-excision repair
• D. Homologous recombination

Answer: (C)

Bulky, helix-distorting DNA lesions like thymine dimers are addressed by nucleotide-excision repair. When the DNA backbone is twisted or bent by such damage, specialized proteins recognize the distortion rather than a specific base change. They then make two cuts in the DNA strand flanking the damaged site and remove a short segment that includes the lesion. The gap is filled by DNA polymerase using the undamaged strand as a template, and DNA ligase seals the final nick. This pathway is designed to remove large, bulky damages that disrupt the DNA helix, which smaller repair methods can’t fix.

In contrast, mismatch repair fixes mispaired bases that arise during replication, usually small errors. Base-excision repair handles small, non-helix-distorting lesions like oxidized or deaminated bases, removing just the damaged base and replacing it. Homologous recombination repairs double-strand breaks using a sister chromatid as a template for accurate restoration.