cyclin
/ SY-klin /
Think of a key that only works in a lock at a particular time of day, then is thrown away and replaced by the next time-stamped key. A cell driving its way through the cycle uses exactly this trick to make sure each stage happens in the right order and at the right moment. Cyclins are proteins whose amounts rise and fall in a repeating wave through the cell cycle, acting as the timing signals that switch the cycle's machinery on and off.
Cyclins are named for the way their levels cycle: a particular cyclin is built up at a certain phase, then abruptly destroyed once it has done its job. On their own, cyclins do nothing — their power comes from binding to a partner enzyme called a cyclin-dependent kinase (CDK). A CDK is inactive until a cyclin docks onto it; the cyclin both switches the CDK on and steers it toward the right targets for that phase. Different cyclins appear at different stages (for example, G1 cyclins, S-phase cyclins, and the mitotic cyclin B), so the rising and falling tide of cyclins is what tells the cell which phase to enter next.
Cyclins matter because they are the clock and the gears of cell division. Their abrupt destruction at the end of each phase is what makes the cycle move forward in one direction and not slide backward — for instance, destroying mitotic cyclin is required to exit mitosis. Their discovery, in dividing sea-urchin and clam eggs, earned a Nobel Prize and revealed that the same timing system runs in yeast, frogs, and humans. A useful caution: cyclins set the timing, but checkpoints can override them, halting the cycle even when cyclin levels say 'go.'
In a fertilized sea-urchin egg, the level of one cyclin climbs steadily, peaks just as the cell plunges into division, then crashes to zero — and this sawtooth rise-and-fall repeats with every single round of division, like a ticking clock.
A cyclin's level rises and crashes with each round of division.
Cyclins have no enzyme activity by themselves — they are the regulatory 'on switch' and target-selector for CDKs, which do the actual chemical work of phosphorylating other proteins.