The passage of a cell through the stages of the cell cycle is controlled by protein kinases that phosphorylate many different proteins at appropriate times. what are these protein kinases called?
a. cyclins
b. cyclin-dependent kinases
c. cdk activating kinases
d. tyrosine kinases

A. Cyclins

Explanation:

Cyclins are group of related proteins and are the most important core cell cycle regulators. Each cyclin is associated with a particular transition or phase in the cell cycle and helps drive the events of that phase or period by partnering with a family of enzymes called the cyclin-dependent kinases (Cdks).

Cdks are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins. The apg acts like a switch, making the target  less active or to have more protein. When cyclin attaches to Cdk, it can activate the Cdk as a kinase and can directs also the Cdk to a specific set of target proteins. For example, G1/S cyclins send Cdks to S phase targets (e.g., DNA replication), while M cyclins send Cdks to M phase targets (e.g.,breakdown of the nuclear membrane ).

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