A cyclin has bound its cyclin dependent kinase. What else must happen before the complex is fully active?

1. Binding a cyclin is necessary for a Cdk to work, but it is not sufficient on its own. 2. The assembled complex still has to be phosphorylated at specific sites. 3. That phosphorylation is carried out by another kinase. 4. Adding a phosphate changes the shape of the protein and switches it on. 5. Only then can the Cdk phosphorylate the targets that push the cell past a checkpoint. 6. Breaking the complex apart would undo the cyclin binding that was just achieved. 7. Losing a phosphate is the reverse of the activating step and would switch the complex off. _Source: OpenStax Biology 1e (CC BY 4.0), Chs 10 and 11, section 10.3 Control of the Cell Cycle_