Final answer:
In E. coli mismatch repair, MutH is signaled to nick the unmethylated strand by (C) the recognition of mismatched bases by MutL. The methylation status of the parent strand versus the newly synthesized strand allows repair proteins to determine the correct template for repairing mismatches.
Step-by-step explanation:
In dam methylation mismatch repair, the signal that causes MutH to nick the unmethylated strand is C. Recognition of the mismatched bases by MutL. When a mismatch is detected after DNA replication, the repair machinery uses methylation markers to distinguish the old DNA strand from the new, non-methylated DNA strand. Over time, adenine bases in the parent DNA strand are methylated, while the newly synthesized strand remains unmethylated for a period, providing a window of time for the repair system to correct any mismatches.
Mismatch repair proteins, specifically MutS and MutL, bind to and detect an incorrectly added base. MutL then interacts with MutH, signaling for it to introduce a nick in the non-methylated strand. Following this nicking, exonuclease removes a portion of the strand that contains the mismatch, followed by the action of DNA polymerase III to fill the gap with the correct nucleotide and DNA ligase to seal the strand.