Final answer:
Mismatch repair primarily occurs in the S phase of the Cell Cycle where DNA replication errors are corrected, but it might occur in G2 or M if earlier checkpoints are missed.
Step-by-step explanation:
Mismatch repair is an important DNA repair process that corrects errors that occur during DNA replication. The most relevant stage at which you would expect mismatch repair to be active is during the synthesis phase, or S phase, of the cell cycle. This is when newly synthesized DNA is checked for errors and corrected if necessary.
During the S phase, the cell undergoes DNA Synthesis Checkpoint to ensure that DNA has been replicated correctly. If the DNA is correctly replicated, the cell can proceed to the next stages, but if there are mistakes, the mismatch repair system fixes them. Additionally, the DNA repair and replication can still occur during later stages, such as G2 and M, if earlier checkpoints have been missed or are 'leaky'. However, the primary function of mismatch repair is to address errors immediately after DNA is replicated, which emphasizes the critical role of the S phase.