Final answer:
Cyclin-dependent kinases (Cdks) are key proteins in the cell cycle control system that phosphorylate selected proteins, with cyclins regulating their activity. The formation of active Cdk/cyclin complexes is crucial for cell cycle progression, and the levels of cyclins fluctuate throughout the cycle.
Step-by-step explanation:
One of the key proteins of the cell cycle control system is cyclin-dependent kinases (Cdks), which induce downstream processes by phosphorylating selected proteins on serines and threonines. Cyclins and Cdks are responsible for the progression of the cell through various checkpoints of the cell cycle. The concentration of cyclin proteins fluctuates and triggers the formation of Cdk/cyclin complexes, which are necessary for the cell to advance to the next phase.
Phosphorylation is a crucial regulatory mechanism by which Cdks activate proteins, altering their shape and function, thus advancing the cell to the next phase. The discovery of cyclins and Cdks and their role in regulating the cell cycle was recognized with the Nobel Prize in Physiology or Medicine in 2001. These discoveries underscored the importance of phosphorylation and the dynamic balance of kinase activities for cell cycle progression and regulation.