Which DNA repair pathway is primarily responsible for removing bulky DNA lesions such as thymine dimers?

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

Answer: (D)

Bulky, helix-distorting DNA damage such as thymine dimers requires a repair system that can remove an entire section of DNA around the lesion and then restore the correct sequence. Nucleotide excision repair does exactly that: it recognizes the distortion caused by the dimer, unwinds the surrounding DNA, makes cuts on both sides of the damaged region, excises a short stretch that contains the lesion, and then fills in the gap with new DNA and seals it. This approach is well-suited for lesions that bother the double helix structure and cannot be fixed by simply changing a single base.

Base excision repair, by contrast, fixes small, non-helix-distorting changes to individual bases. Mismatch repair targets errors that occur during DNA replication, such as mispaired bases. Homologous recombination deals with more severe problems like double-strand breaks. Since thymine dimers are bulky, helix-distorting lesions, nucleotide excision repair is the primary mechanism to remove them.