Final answer:
The DNA proofreading mechanism involves DNA polymerase checking and correcting newly added bases during replication. Mismatch repair fixes errors missed by proofreading, while nucleotide excision repair handles larger issues, like UV damage.
Step-by-step explanation:
DNA Proofreading and Repair Mechanisms
Cells have several repair mechanisms to minimize mutations. The proofreading ability of DNA polymerase is critical during DNA replication. As it synthesizes DNA, it checks whether each new base pairs correctly with the base in the template strand. If an incorrect base is added, the enzyme performs a 3' exonuclease action to make a cut at the phosphodiester bond, releasing the wrong nucleotide and allowing it to be replaced with the correct one.
When proofreading misses an error, the mismatch repair system can fix it. Mismatch repair enzymes recognize the wrongly incorporated base and excise it, after which the correct base is inserted. This system is crucial as it prevents the incorrect base from being passed on during future replications, which could lead to more permanent genetic changes.