Final answer:
Proofreading by DNA polymerase is the initial mechanism to repair nucleotide errors during DNA replication, followed by mismatch repair for errors detected post-replication and nucleotide excision repair for more extensive UV-induced damage like thymine dimers.
Step-by-step explanation:
The initial mechanism for repairing nucleotide errors in DNA is proofreading by DNA polymerase. During DNA replication, DNA polymerase adds nucleotides to the growing DNA strand. It also proofreads every newly added base, and if it detects an incorrect base, it removes and replaces it with the correct one (Figure 9.13a). If errors are not corrected during replication, mismatch repair mechanisms come into play, where the incorrectly added base is detected after replication, excised, and replaced with the correctly paired base. However, for more extensive damage like thymine dimers caused by UV light, which is not a simple mismatch but a structural abnormality, nucleotide excision repair is employed. In nucleotide excision repair, enzymes make cuts on both ends of the damaged base, remove the segment, and replace it with correctly paired nucleotides, with the help of DNA polymerase and DNA ligase to seal the gap.