Final answer:
In dam methylation mismatch repair, the protein A. MutH acts as the nuclease that nicks the unmethylated strand, allowing for the excision of the incorrect bases and the eventual repair of the DNA strand.
Step-by-step explanation:
In dam methylation mismatch repair, the protein that acts as the nuclease, which nicks the unmethylated strand, is A. MutH.
The process begins when MutS binds to the DNA mismatch, and then MutL binds and activates MutH.
MutH specifically nicks the newly synthesized DNA strand near the mismatch because it is not yet methylated.
The unmethylated strand is then extensively excised by an exonuclease, allowing DNA polymerase III to fill in the correct bases, followed by DNA ligase sealing the nick and completing the repair process.