Final answer:
Cyclin-Cdks, like M-Cdk, are kept inactivated by Cdk inhibitors and are otherwise regulated by fluctuating cyclin concentrations and phosphorylation.
Step-by-step explanation:
Cyclin-dependent kinases (Cdks) are inactivated by a variety of mechanisms, ensuring that cell cycle progression is tightly regulated. One such mechanism involves Cdk inhibitors, which bind to the Cdk/cyclin complex and prevent their full activation.
For instance, the Mitotic Cdk (M-Cdk), which is essential for a cell to progress into mitosis, has to be carefully regulated. The concentrations of cyclins that bind to Cdks fluctuate throughout the cell cycle, determining when the complexes can form.
Additionally, for full activation, a Cdk must bind a cyclin and be phosphorylated at specific sites. However, if there is a cellular condition that needs to be resolved before proceeding, Cdk inhibitors will block the activation of Cdks, preventing them from phosphorylating other proteins, which is necessary to move the cell cycle forward.