Question

M-cyclin accumulates early in G2, but M-cdk is kept inactive until the early stages of mitosis. Select the correct mechanism for regulating the activity of M-cdk:

	A.	M-cyclin binds to M cdk and is phosphorylated by CAK and CDC25 and Wee1 is activated
	B.	M-cyclin binds to M cdk and is phosphorylated by CAK and Wee1 and Cdc25 is inhibited
	C.	M-cyclin binds to M cdk and is phosphorylated by CAK
	D.	M-cyclin is prevented from binding to M cdk through phosphorylation by CAK

Answer 1

cyclin are proteins, whose levels cycles ( increases and decreases at particular stages of the cell cycle) during the cell cycle. cyclin binds to the Cdk, Cdk is cyclin-dependent kinase, for Cdk to be in the activated, it has to bind to the cyclin.

M--cyclin accumulates early in G2, but M-Cdk is kept inactive until the early stages of mitosis, because once the M-CdK is formed it is phosphorylated by CAK, cyclin activating kinase, the phosphorylation by the CAK activates the M-Cdk, but the M-Cdk is kept inactive by the phosphorylation by the Wee1 kinase, but Cdc25 is a phosphatase which removes the phosphate groups added by the Wee1 kinase, so during early stages of the G2 the Cdc25 is inhibited so that the M-Cdk remains in the inactive state.

so the answer is M-cyclin binds to M cdk and is phosphorylated by CAK and Wee1 and Cdc25 is inhibited