Final answer:
b) Nucleotide excision. Nucleotide excision repair is a mechanism that repairs DNA damage, while photoreactivation is a process that specifically repairs thymine dimers. NER involves recognizing and removing damaged DNA segments, replacing them with correctly paired nucleotides, and sealing the gap. Photoreactivation uses visible light to break apart thymine dimers and restore base pairing.
Step-by-step explanation:
Nucleotide excision repair (NER) is a mechanism that repairs specific types of DNA damage, such as thymine dimers caused by UV exposure. In NER, enzymes recognize and cut the damaged region of DNA. The segment with the damaged bases is then removed and replaced with the correctly paired nucleotides by DNA polymerase. The remaining gap is sealed with a phosphodiester linkage catalyzed by DNA ligase.
Photoreactivation is another mechanism for repairing thymine dimers. In this process, the enzyme photolyase binds to the thymine dimer and, in the presence of visible light, breaks apart the dimer, restoring the base pairing of the thymines.