Important aspect of cell cycle control

Important aspect of cell cycle control is ensuring that transitions such as entry into mitosis occur rapidly and completely.  The regulation of Cyclin-Cdk by Wee1 tyrosine kinase and Cdc25 (tyrosine phosphatase) gives a good appreciation of how this control can be achieved.

The balance of activities of Wee1 and Cdc25 determines the state of phosphorylation of tyrosine 15 of cdk.  When tyrosine 15 is phosphorylated, Cyclin-cdk is inactive; when tyrosine 15 is not phosphorylated, Cyclin-cdk is active.  Just as the activity of Cyclin-cdk itself is controlled by phosphorylation, so too are the activities of Wee1 kinase and Cdc25 phosphatase!