Pyrimidine dimers are caused by UV light and are primarily repaired by which mechanism?

• A. Oxidative damage; base excision repair.
• B. Double-strand breaks; non-homologous end joining.
• C. UV-induced damage; repaired mainly by nucleotide excision repair.
• D. Single-strand breaks; mismatch repair.

Answer: (C)

UV light creates pyrimidine dimers, such as thymine-thymine connections, which distort the DNA helix. The repair system specialized for bulky, helix-distorting damage is nucleotide excision repair. It recognizes the distortion, makes incisions on both sides of the lesion, and removes a short DNA segment containing the dimer. Then DNA polymerase fills the gap and DNA ligase seals the strand. This approach is well-suited to thymine dimers caused by UV exposure. Other repair pathways handle different types of damage—base excision repair fixes small, non-helix-distorting base damage; mismatch repair corrects replication errors; double-strand breaks are repaired by non-homologous end joining or homologous recombination.