Which type of DNA lesion is primarily repaired by nucleotide excision repair?

• A. Base excision repair fixes bulky lesions.
• B. NER repairs single-base mismatches.
• C. NER repairs breaks by ligation.
• D. NER repairs bulky DNA lesions such as thymine dimers.

Answer: (D)

Nucleotide excision repair is for bulky, helix-distorting lesions. When DNA is damaged by UV light, thymine dimers form and distort the DNA backbone. This distortion is recognized by the NER machinery, which removes a short patch of DNA surrounding the lesion and then DNA polymerase fills the gap followed by ligase sealing the strand. That’s why bulky lesions like thymine dimers are best repaired by this pathway.

Base excision repair, in contrast, handles small, non-helix-distorting base damages. Mismatch repair fixes replication errors such as single-base mismatches. Breaks are repaired by other mechanisms (for example, double-strand break repair pathways), with ligation being one step in that broader process.