Cell Cycle Control

The why of a cell moving through its cycle is pretty simple- the goal is to get big enough and strong enough, with enough correctly copied DNA to make another cell just like it. Replication is the point of the cell cycle- but HOW exactly does the cell know when to move from one phase to the next? The answer is a complex one, but in its simplest terms two very important factors are at work: cyclins and Cdk’s.

Some key terms for cell cycle control:

Cyclins- these are proteins that are associated with a particular phase, transition, or set of phases in the cell cycle. There are four of them, each corresponding to different phases of the cell cycle: G1 cyclins, G1/S cyclins, S cyclins, and M cyclins.

CDK’s- CDK stands for Cyclin dependent kinase. CDK’s are a family of enzymes that bind with cyclins- when they bind to cyclins they either activate or deactivate, and it is this step that helps move the cell cycle forward. Kinases are a type of enzyme that phosphorylate (attach phosphate groups to) specific target proteins. When the target proteins are phosphorylated it is like a switch has been flipped, either making the target protein more or less active.

Cyclins and CDK’s work together to help a cell progress through the cell cycle. When a cyclin attaches to a Cdk, it has two effects: it activates the Cdk, and it also directs the Cdk to its particular set of target proteins, ones appropriate to the cell cycle period controlled by the cyclin.

The levels of Cdk’s in a cell remain relatively stable throughout a cell’s lifetime, but the changing levels of cyclins are what keep things moving along- as levels of certain cyclins go up the cell transitions to the next phase, performing those specific tasks until the next group of cyclins start proliferating- see the cyclin expression cycle picture above for a cool pictorial representation.