Question

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

a) Cyclin A
b) Cyclin B
c) Cyclin D
d) Cyclin E

Answer 1

For CDK4 to be active, Cyclin D must be elevated. In terms of regulating the cell cycle, Cyclin D1's expression is controlled by negative feedback, and it plays a crucial role in cell proliferation alongside CDK4.

Step-by-step explanation:

For CDK4 to be active, it must be bound to a specific type of cyclin. In this case, Cyclin D is the cyclin that must be elevated for CDK4 to become active. CDKs, or cyclin-dependent kinases, are a family of protein kinases which, upon being fully activated by association with a cyclin, can phosphorylate and thus activate other proteins critical in advancing the cell cycle past certain checkpoints. The levels of cyclin proteins fluctuate and these fluctuations determine when CDK/cyclin complexes are formed, which in turn are responsible for cell cycle progression.

The insulin growth factor (IGF-1) promotes cell proliferation, and it does so by influencing the expression of various proteins that contribute to cell cycle progression. Within the associated diagram indicating the cell cycle regulation, one of these proteins' expression is controlled by a negative feedback mechanism. That protein is Cyclin D1, as its levels are tightly regulated during the cell cycle to ensure proper cell division and proliferation.