The cell cycle contains checkpoints to prevent critical phases from beginning until the previous phases have been completed.
CDK & CYCLINS
CYCLIN-DEPENDENT KINASES (CDKs) are protein kinases, enzymes that add phosphate groups to target proteins. CDK enzymes are called cyclin dependent because they are switched on when they are combine with another protein called cyclin.
HOW DOES IT WORK?
Once cyclin has reached a certain concentration high enough to form a complex of CDK and activate it. The CDK then phosphorylates a number of cell cycle control target proteins which trigger the next phase. After transition is made, the cyclin is degraded and less is available for binding to CDK and therefore kinase activity decreases.
At each key checkpoint, regulatory events block the cyclin: CDK complex from triggering the associated cell cycle transition until the actions of a previous phase is completed.
Many of the external factors bind to the receptors at the cell surface, which respond to triggering reactions inside the cell. Signal molecules are referred to as growth or death factors. Cell-surface receptors also recognize contact with other cells or with molecules of the extracellular matrix. The contact triggers internal reactions pathways that inhibit division by arresting the cell cycle in the G1 phase. The response is called CONTACT INHIBITION, which stabilizes cell growth in fully developed tissues and organs. Thus they are shunted into the G0 phase and prevented from dividing.
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