Final answer:
Cyclin is the molecule that must bind to Cdk to activate it, which in turn phosphorylates other proteins that are necessary for cell cycle progression. The levels of cyclins fluctuate and are crucial for the formation of active Cdk/cyclin complexes that regulate different checkpoints.
Step-by-step explanation:
As long as cyclin is bound to Cdk, the Cdk is active and happens in waves throughout the cell cycle. Cyclins and cyclin-dependent kinases (Cdks) play crucial roles in regulating the progression of cells through the cell cycle. The levels of Cdk proteins remain relatively stable, but it is the concentration of cyclins that fluctuates and dictates when Cdk/cyclin complexes form.
The fully active Cdk/cyclin complex, which must also undergo phosphorylation, is instrumental in phosphorylating other proteins that help advance the cell cycle past various checkpoints. These phosphorylation events change the shape and activity of target proteins, driving the cell into the next phase of the cycle at the appropriate times.