UV light damages DNA by creating what type of lesions, addressed by nucleotide excision repair?

• A. Mismatches
• B. Abasic sites
• C. Pyrimidine dimers
• D. Double-strand breaks

Answer: (C)

UV light creates covalent bonds between adjacent pyrimidine bases in DNA, forming pyrimidine dimers (most commonly thymine dimers). This distorts the DNA helix and blocks replication and transcription. Nucleotide excision repair is specialized for removing these bulky, helix-distorting lesions: it makes cuts on both sides of the damaged segment, excises a short stretch of strand containing the dimer, and then fills the gap with new DNA and seals it. Other types of damage described—mismatches from replication errors, abasic sites from base loss, and double-strand breaks from other causes—are fixed by different repair pathways (mismatch repair, base excision repair, and end-joining or homologous recombination, respectively).