Question

For CDK4 to be active, which cyclin must be [HIDDEN]?

a) Absent cyclin B, present Cyclin D
b) Absent Cyclin D, present cyclin B
c) Presence of both cyclin B and Cyclin D
d) Absence of both cyclin B and Cyclin D

Answer 1

For CDK4 to be active, cyclin D must be present. Cyclins and Cdks form complexes necessary for cell cycle regulation, and the presence of cyclin D is required for CDK4 activity during the G1 phase.

Step-by-step explanation:

For CDK4 to be active, cyclin D must be present. Cyclins regulate the cell cycle only when they are tightly bound to Cdks (Cyclin-dependent kinases). The levels of Cdk proteins are relatively stable throughout the cell cycle; however, the concentrations of cyclin fluctuate and determine when Cdk/cyclin complexes form. Specifically, cyclin D is known to bind with CDK4 to regulate the progression of cells through the G1 phase of the cell cycle. The correct answer to the question is therefore option (b): "Absent Cyclin D, present cyclin B," as the presence of cyclin D is necessary for the activation of CDK4.