Final answer:
The G2-M transition in the cell cycle is irreversible primarily due to the activation of Cyclin-Dependent Kinase (CDK). Once CDKs are activated and their cyclin partners degraded, the cell commits to mitosis, making this transition a one-way process.
Step-by-step explanation:
The transition from G2 to M phase of the cell cycle is irreversible primarily due to the activation of Cyclin-Dependent Kinase (CDK). CDKs are crucial for the cell cycle progression and once they are activated, they phosphorylate various substrates to promote the cell entry into mitosis. The activation of CDKs involves their binding to cyclins, forming a CDK/cyclin complex, which then gets phosphorylated by another kinase to become fully active. The complex orchestrates the entry into mitosis by phosphorylating a variety of proteins that initiate the process.
During the G2-M transition, specific cyclins that bind CDKs are degraded. Once these cyclins are destroyed, the CDKs they were bound to become inactive, which is a key step to ensure that the cell does not revert to an earlier phase of the cycle. This degradation is a one-way process, which makes the G2-M transition irreversible. The destruction of the cyclins ensures that the cell fully commits to entering mitosis.
Checkpoint Kinase (Chk) inactivation plays a role in cell cycle regulation, but it is not the primary reason the G2-M transition is irreversible. Replication Factor C (RFC) is essential for DNA replication and its inhibition would affect the S phase rather than the G2-M transition. Therefore, the correct answer to why the transition of G2-M is irreversible is the activation of CDK, not the inactivation of Chk, the destruction of cyclin, or RFC inhibition.