Final answer:
Cyclins gradually accumulate and bind to stable levels of Cdks to form complexes that undergo an abrupt activation through phosphorylation, allowing the cell to advance through the cell cycle checkpoints.
Step-by-step explanation:
Cyclins are made gradually throughout the cell cycle based on the timing rather than specific events. In contrast, Cdks (cyclin-dependent kinases) are activated abruptly when they bind to cyclins and are phosphorylated. This phosphorylation leads to the activation of proteins that are responsible for advancing the cell to the next phase of the cell cycle. The levels of Cdk proteins remain fairly stable, but it is the concentrations of cyclins that fluctuate and regulate the formation of Cdk/cyclin complexes. These complexes then enable the cell cycle to proceed through various checkpoints.
Abrupt activity changes in the cell cycle are possible through phosphorylation. Phosphorylation induces a change in the shape of the protein, thereby activating it. The abrupt changes are essential for the rapid transitions required to push a cell from one phase to the next in the cell cycle.