What initiates the breakdown of cyclin at the M checkpoint?

The breakdown of cyclin at the M checkpoint is primarily initiated by a decrease in MPF (Maturation Promoting Factor). MPF is a complex formed by cyclin and cyclin-dependent kinase (CDK), and it is crucial for the progression of the cell cycle, particularly through the G2/M transition. As the cell progresses through the mitotic phase, once it has completed cell division, the levels of cyclin begin to decrease, leading to a reduction in MPF activity.

This decrease in MPF is necessary because it signals that the cell can no longer progress through mitosis and begins the process of cell cycle exit, often leading to the transition into the next interphase. The reduction of MPF activity allows for the dissociation of cyclin from CDK, which initiates the degradation of cyclin through proteolysis. This mechanism ensures proper timing and regulation of the cell cycle, preventing premature exit from mitosis and maintaining genomic integrity.

Other factors like the presence of CDK and growth factors play different roles in cell cycle regulation but do not directly initiate the breakdown of cyclin at this specific checkpoint. The completion of DNA replication is crucial for entering mitosis but is not the trigger for cyclin degradation during the M checkpoint