V Figure 12.16 Molecular control of the cell cycle at the G, checkpoint. The steps of the cell gdle are timed by rhythmic fluctuations in the activity of cyclin-dependent kinases (Cdis). Here we focus on a cyclin-Cdk complex in animal cells called MPF, which acts at the G, checkpoint as a go-ahead signal, triggering the events of mitosis. M GIS G M G2 M MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cydlın concentration during the cell cycle O ynthesis of cyclin begins In late s phase and continues through G2. Because cyclin Is protected from degra- dation during this stage, It accumulates. O During G1. the degradation of cyclin continues, and the Cdk component of MPF Is recycled. cdk Degraded cydin G2 cdk checkpoint Cyclin i degraded Cyclin MPF ODuring anaphase, the cydlin com- ponent of MPF Is degraded, terminating the M phase. The cell enters the Gi phase. O MPF promotes mitosis by phos- phorylating varlous protelns. MPF's activity peaks during metaphase. Ogyclin combines with Cdk, producing MPF. When enough MPF molecules accumulate, the cell passes the G2 checkpolnt and begins mitosis. (b) Molecular mechantsms that help regulate the cell cycle cydin accumulation

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