Final answer:
The Cyclin-Cdk holoenzyme forms when cyclin proteins bind to cyclin-dependent kinases (Cdks), with cyclin levels dictating complex formation. The Cdk/cyclin complex then becomes active through phosphorylation by another kinase. This activation enables the complex to phosphorylate other proteins and advance the cell cycle.
Step-by-step explanation:
The Cyclin-Cdk holoenzyme formation begins with the binding of a cyclin protein to a cyclin-dependent kinase (Cdk). The binding of cyclin to Cdk is crucial for the regulation of the cell cycle, as cyclins only regulate the cycle when tightly bound to Cdks. Cyclins undergo a fluctuating pattern in their levels throughout the cell cycle, which determines when Cdk/cyclin complexes are formed, with concentration peaks leading to the binding and activation at specific cell cycle checkpoints.
To achieve full activation, the Cdk/cyclin complex needs to be phosphorylated in specific locations. This phosphorylation is performed by another kinase and leads to a conformational change in the protein, activating it. The now active Cdk can phosphorylate other proteins, driving the cell to progress to the next phase of the cell cycle. The precise control and timing of these events are critical for the proper functioning and regulation of the cell cycle.
It's interesting to note that the levels of Cdk proteins remain relatively stable throughout the cycle, whereas cyclin levels vary significantly, indicating that the concentration of cyclins is the main determinant for the formation of active Cdk/cyclin complexes. The systematic rise and fall of cyclins trigger the advancement through various stages of the cell cycle, with the subsequent degradation of cyclins after they serve their function.