The Institute of Structural Molecular Biology, based at Birkbeck and University College, hosted a star performer as its first seminar speaker of 2008: Tim Hunt of the Cancer Research UK London Research Institute. Tim was awarded the Nobel Prize for Physiology or Medicine in 2001, with Leland Hartwell and Paul Nurse, for his discovery of cyclins - proteins that control the expression of the cyclin-dependent kinases (CDKs) which control a cell's passage through the cell cycle. So cyclins can be described as regulators of the regulators of the cell cycle.
Maybe only a Nobel Laureate could do it. Tim started his lecture with a quick tour through several hundred years of Physics, inspired by his small daughter's (unanswered) question "Why is the sky opaque?" He introduced (or re-introduced) his audience to Schrodinger's equation and Maxwell's idea of a "field" before confessing that he didn't understand quantum mechanics: no one should ever be ashamed of admitting as much.
The cell cycle, and its control, is, like quantum mechanics, "very interesting and very complicated". Tim's critical observation, which he made studying frog oocytes, was that cell division is controlled by the concentrations of the proteins that we now know as cyclins. They were given this name because their concentration in cells goes up and down according to where those cells are in the cell cycle - whether they are growing, replicating their DNA, undergoing mitosis...
Cyclins control the progress of cells through cell division by regulating the function of cyclin dependent kinases (CDKs). By binding to CDKs, cyclins control their activation state, and active CDKs drive the cells through the cell cycle. The press release for the 2001 Nobel Prize succinctly described CDKs as the cells' "motors" and cyclins as the gear boxes that control whether cells will be in idle or overdrive.
Cyclins are all-alpha proteins (link is to the PPS material) with 5-helical cores. PDB entry 1H1S shows human cyclin A bound to CDK2.
It is impossible to do justice to such a complex topic, and lecture, in a few paragraphs. To learn more, try the resources on the Nobel website would be a good first port of call.