Final answer:
DNA polymerase performs a proofreading function during DNA replication to ensure fidelity by replacing incorrectly paired bases. Mismatch repair takes over post-replication when errors escape the proofreading process, removing incorrect bases and filling the gaps with correct ones. Nucleotide excision repair meanwhile corrects damage like thymine dimers.
Step-by-step explanation:
DNA Polymerase and Proofreading
In DNA replication, DNA polymerase detects and replaces bases that were incorrectly paired during DNA synthesis. This process is crucial for maintaining the DNA sequence integrity. DNA polymerase has a proofreading function that allows it to read every newly added base before adding the next one. If an erroneous base is detected, the exonuclease activity of DNA polymerase excises the incorrect nucleotide by cutting the phosphodiester bond, allowing for the proper base to be added.
Proofreading by DNA polymerase is highly efficient, but it is not infallible. Errors that slip through are addressed through mismatch repair mechanisms that operate post-replication. Mismatch repair enzymes identify the mispaired nucleotide in the newly synthesized strand and remove it by nuclease action, subsequently filling the gap with the correct nucleotide. These enzyme functions are essential for preventing mutations that can arise from misincorporated bases. An additional layer of DNA integrity is maintained by nucleotide excision repair, which specifically addresses larger-scale DNA damage such as thymine dimers. Thymine dimers, often the result of UV light exposure, are excised and replaced to prevent mutation and potential skin cancers in humans.