• Cyclins accumulate during the G1, S, and G2 phases of the cell cycle.
• By the G2 checkpoint (the red bar in the figure), enough Cyclin is available to form MPF complexes (aggregations of Cdk and Cyclin) which initiate mitosis.
o MPF apparently functions by phosphorylating key proteins in the mitotic sequence.
• Later in mitosis, MPF switches itself off by initiating a process which leads to the destruction of Cyclin
o Cdk, the non-Cyclin part of MPF, persists in the cell as an inactive form until it associates with new Cyclin molecules synthesized during Interphase of the next round of the cell cycle.