Controlling the cycle

How does a cell know when to start the next part of its cycle? Timing is crucial - there is no point in a cell dividing into two new ones if it hasn't finished copying its DNA, for example. Like a washing machine cycle, the rinse must finish before the spin begins. Surprisingly, all cell cycles - in every living thing from yeast to a human being - are controlled in roughly the same way by special proteins made at certain stages of the cell cycle.