Final answer:
Phosphorylation of cyclins does label them for degradation but it also activates the Cdk/cyclin complex, which is essential for regulating the cell cycle. Cyclin concentrations fluctuate, determining complex formation and progression through cell cycle checkpoints.
Step-by-step explanation:
The process of phosphorylation of cyclins is indeed a mechanism for labeling proteins for degradation; however, this is not the only function of phosphorylation in the context of cyclins. Cyclins, when bound to cyclin-dependent kinases (Cdks), regulate the cell cycle. For a Cdk/cyclin complex to be fully active and hence regulate the cell cycle, it must be phosphorylated at specific sites. Phosphorylation by Cdks changes the shape of the protein, thus activating it and allowing it to advance the cell to the next phase. The level of Cdk proteins is stable, but the concentration of cyclins varies, which determines when these complexes can form. Over the cell cycle, cyclins are produced and degraded; after a cyclin has done its job in moving the cell past a checkpoint, it is marked for degradation to ensure the timely progression through the cell cycle stages.