Final answer:
MAPK becomes active during the G1 phase of the cell cycle, where it aids in the decision process for a cell to progress to the S phase by ensuring all conditions are satisfied for DNA replication.
Step-by-step explanation:
The other point where MAPK can become active is the G1 phase of the cell cycle. The G1 checkpoint is crucial as it determines whether the cell is prepared for DNA replication which occurs in the subsequent S phase. If the necessary proteins and conditions are not met in G1, the progression to the S phase will be halted. MAPK, also known as mitogen-activated protein kinase, plays a significant role in signaling the cell to progress through these checkpoints.