Question

A cdk protein that is not bound to a cyclin is:

A. inactive and would be broken down into amino acids.
B. active and able to phosphorylate target proteins.
C. active because it is bound to gtp.
D. inactive but remains present in the cell.

Answer 1

A Cdk protein not bound to a cyclin is inactive but remains stable in the cell. It becomes active only when it binds to a cyclin and is phosphorylated. The levels of cyclin, not Cdk, fluctuate to regulate the formation and activation of these complexes.

Step-by-step explanation:

The activity of cyclin-dependent kinases (Cdks) is crucial for the progression of a cell through its cycle. Cdks require binding to a cyclin protein for their activation. Once bound, the Cdk/cyclin complex is then phosphorylated at specific locations which triggers its full activation. This phosphorylation process is essential as it enables the kinase activity of the complex, allowing it to phosphorylate other proteins that assist in moving the cell past certain checkpoints of the cell cycle.

In the absence of cyclin, Cdk proteins remain inactive but are stable within the cell. They do not get broken down into amino acids nor are they capable of phosphorylating target proteins without being part of the active complex. Their levels are relatively constant, while the levels of cyclin fluctuate, determining the timing of complex formation. Thus, lacking the cyclin binding, a Cdk is inactive but remains present and ready to be activated in the cellular environment when cyclin levels rise.