Which of the following is a protein maintained at constant levels throughout the cell cycle that requires cyclinto become catalytically active?
a) pdgfb) mpfc) protein kinased) cycline) cdk

The answer is; E

CDK is only active when they bind cyclin. In the CDK-cyclin complex, they are able to phosphorylate their substrates on serines and threonines. Particular CDKs are specific to their cell cycle phase. CDKs have maintained more or less constant levels in a cell and the cyclin levels are what fluctuate.