Final answer:
Cdks are enzymes that regulate the cell cycle via phosphorylation, needing to bind to cyclins and undergo phosphorylation themselves to become fully active. The levels of Cdks are stable, whereas cyclin levels fluctuate to control when Cdks can form active complexes. These mechanisms ensure cells progress through the cycle checkpoints properly.
Step-by-step explanation:
Cdks, or cyclin-dependent kinases, are enzymes that regulate proteins within the cell cycle through a process called phosphorylation. Phosphorylation involves the transfer of a phosphate group from ATP to a target protein, thereby activating the protein by changing its shape. This activation is critical for advancing the cell to the next stage in the cell cycle. Cyclins are another group of proteins that regulate the cell cycle, but only when they are tightly bound to Cdks. For a Cdk to be fully active and able to phosphorylate other proteins, it must first form a complex with a cyclin and then be phosphorylated at specific locations by another kinase. The levels of Cdk proteins are relatively stable throughout the cell cycle, but the concentrations of cyclin fluctuate, which determines when the Cdk/cyclin complexes form. The intricate balance between Cdks, cyclins, and phosphorylation ensures that the cell cycle progresses correctly through various checkpoints. The timely and tightly regulated formation of the Cdk/cyclin complex is essential for the positive regulation of the cell cycle, enabling cells to pass through these checkpoints and proceed to the next phase.