Pyrimidine dimers are primarily repaired by which mechanism?

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

Answer: (D)

Pyrimidine dimers formed by UV damage distort the DNA helix, so the cell uses nucleotide excision repair to fix them. NER recognizes bulky, helix-distorting lesions, makes cuts on both sides of the damaged region, removes a short patch of DNA containing the dimer, and then a DNA polymerase fills in the gap followed by ligase sealing. This pathway is specifically equipped to handle UV-induced distortions, unlike base excision repair (which removes small, non-helix-distorting base lesions), mismatch repair (which fixes replication errors), or homologous recombination (which repairs double-strand breaks using a sister chromatid as a template).