Two key classes of regulatory molecules, cyclins and cyclin dependent kinases (CDKs), determine a cell’s progress through the cell cycle. Cyclins form the regulatory subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle.
Different cyclin-CDK combinations determine the downstream targeted proteins. CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to various molecular signals.