What regulates the activity of cyclin-dependent kinases (Cdks)? cAMP Specific cyclins Transcription factors The enzymatic activity of cyclins Histones

Cyclin-dependent kinases (Cdks) are pivotal in regulating the cell cycle and various cellular processes. Their activity is primarily regulated by specific cyclins. Here’s a detailed explanation breakdown of how this works:

1. Cyclin Binding: Cdks are inactive in their unbound form. When specific cyclins bind to Cdks, they undergo a conformational change that activates the kinase. This binding is fundamental as each Cdk is regulated by different cyclins at various phases of the cell cycle.

2. Phosphorylation: In addition to cyclin binding, Cdks are often regulated by phosphorylation. For instance, specific activating and inhibitory phosphorylation can occur, which modulates their activity.

3. Degradation of Cyclins: The levels of cyclins fluctuate throughout the cell cycle. Once their job is done, cyclins are tagged for degradation via the ubiquitin-proteasome pathway, which reduces Cdk activity.

4. Inhibitors: Cdk inhibitors (CKIs) also play a role in regulating Cdks' activity by binding to the cyclin-Cdk complex and preventing substrate phosphorylation.

In conclusion, while molecules like cAMP, transcription factors, and histones can influence cellular processes, it is the specific cyclins that directly regulate the activity of Cdks during the cell cycle. Therefore, the correct answer to this question is Specific cyclins.