cyclin-dependent kinase
/ SY-klin dee-PEN-dent KY-nayss /
Picture a powerful machine that is bolted to the floor and switched off, and stays harmlessly idle until someone clips in the right activation module — then it springs to life and starts stamping a particular mark onto everything that passes by. A cell uses just such a machine to drive each stage of division forward. A cyclin-dependent kinase (CDK) is an enzyme that, once switched on by a cyclin partner, kicks the cell from one phase of the cycle to the next.
A kinase is an enzyme that attaches a phosphate group onto other proteins — a small chemical tag that can switch those proteins on or off. A CDK is a kinase that is only active when bound to a cyclin (hence 'cyclin-dependent'). The cyclin both turns the CDK on and helps choose its targets. Once activated, a CDK phosphorylates a set of proteins that drive the cell into the next phase: for example, it can trigger DNA copying, or set off the dramatic reorganization of mitosis. Because cyclin levels rise and fall on schedule, the CDK is switched on and off at the right times, creating the cycle's orderly march.
CDKs matter because they are the central engines of cell-cycle control — different cyclin-CDK pairs act like gear shifts that move the cell from G1 to S to G2 to mitosis. Tampering with them is dangerous: overactive CDKs help drive cancer, which is why several cancer drugs are CDK inhibitors. The discovery of CDKs (first as a gene called 'cdc2' in yeast that was the same in humans) earned a Nobel Prize. A common misconception is that the CDK is what 'cycles' — actually it is the cyclin that comes and goes, while the CDK stays put and waits for its partner.
The CDK paired with mitotic cyclin B (historically called maturation-promoting factor) is the master 'enter mitosis' switch — inject active versions of it into a resting frog egg and the egg is forced straight into division.
An active CDK-cyclin B complex forces a cell straight into mitosis.
The CDK itself does not rise and fall — it is always present but inactive; what oscillates is its cyclin partner, so the CDK is a steady engine switched on and off by a changing key.