Final answer:
The cyclin-CDK complexes are the core components of the cell cycle clock, acting as the engine for progression and catalyzing serine/threonine phosphorylation to activate proteins necessary for advancing through cell cycle phases.
Step-by-step explanation:
The cyclin-CDK complexes constitute the core components of the cell cycle clock that together serve as the engine for the cell cycle progression by catalyzing serine/threonine phosphorylation. Cyclins regulate the cell cycle only when they are tightly bound to CDKs (cyclin-dependent kinases). To be fully active, a Cdk must be phosphorylated at specific locations after it has bound to a cyclin protein. This phosphorylation changes the shape of the protein, thus activating it. The activated Cdk/cyclin complex can then phosphorylate other proteins, facilitating progress through the cell cycle.
The levels of Cdk proteins remain relatively stable, whereas the concentrations of cyclins vary, peaking at different stages of the cell cycle to allow the formation of the necessary Cdk/cyclin complexes. These fluctuations are essential for the cell to advance from one phase to the next. Increases in cyclin concentrations are often initiated by both external and internal signals. Key stages in the cycle require freshly synthesized cyclins, which, once their task is completed, are degraded to ensure orderly progression through the cycle's checkpoints.
Notably, the work by Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse greatly contributed to our understanding of these processes, earning them the 2001 Nobel Prize in Physiology or Medicine for discovering the key roles of cyclins and Cdks in cell cycle regulation.