Final answer:
DNA polymerases use their 3' → 5' exonuclease activity to correct a mismatched base pair during DNA replication. This proofreading function removes the incorrect nucleotide and then DNA polymerase replaces it with the correct base.
Step-by-step explanation:
DNA polymerases use their 3' → 5' exonuclease activity to remove a mismatched base pair. This activity is part of the polymerase's proofreading function, which helps to ensure the accuracy of DNA replication. When an incorrect base has been added, DNA polymerase detects the mistake and employs its 3' → 5' exonuclease activity to make a cut at the phosphodiester bond and release the incorrect nucleotide. Once the incorrect base has been removed, it is replaced by the correct base as DNA replication continues. In addition to proofreading during replication, mismatch repair and nucleotide excision repair are important DNA repair mechanisms that also involve the removal and replacement of incorrect or damaged nucleotides. In the case of mismatch repair, the enzyme mismatch repair proteins detect and remove the incorrect base from the newly synthesized strand. For nucleotide excision repair, which operates to fix damages such as thymine dimers caused by UV light, the damaged bases are removed along with a few neighboring bases on both sides by repair enzymes, and the correct nucleotides are added back by DNA polymerase.