CDKs (Cyclin-dependent kinases) are primarily regulated by which of the following?

Cyclin-dependent kinases (CDKs) are essential regulatory enzymes that control the progression of the cell cycle. The activity of CDKs is primarily regulated by the binding of specific proteins called cyclins, as well as through various post-translational modifications, such as phosphorylation and dephosphorylation.

Cyclins are proteins whose levels fluctuate throughout the cell cycle, and their interaction with CDKs is critical for activating the kinases. When cyclins bind to CDKs, they induce conformational changes that enable the kinases to phosphorylate target substrates necessary for advancing the cell cycle.

In addition to the cyclin binding, the regulation of CDKs is also significantly influenced by post-translational modifications. For instance, phosphorylation at specific sites can either activate or inhibit CDK activity, adding another layer of regulation. Conversely, some proteins also act as inhibitors by binding to CDKs, further emphasizing the multifaceted nature of CDK regulation.

This multidimensional regulation allows for precise control of the cell cycle, ensuring that cell division occurs in a timely and orderly manner. Thus, the correct answer acknowledges that both the presence of cyclins and various post-translational modifications (including phosphorylation) work together to regulate CDK activity effectively.