Researchers have identified the "OK signal" required for a cell to progress from late interphase (G2) to mitosis. It is a complex of proteins called MPF (aka- M-phase promoting factor). The amount of MPF in the cell rises and falls in a cycle correlated with the cell cycle.
(1) Cyclin is synthesized throughout the cell cycle and (2) accumulates during early interphase. (3) The cyclin associates with another protein, cdc2, to form active MPF. (4) MPF functions as a protein kinase, triggering the activation of numerous proteins that facilitate mitosis. (5) One of the proteins activated by MPF is an enzyme that (6) brings MPF activity to a halt by degrading cyclin. (7) Since the cycling-degrading enzyme is only active in the presence of MPF, it ceases to function once it destroys the cyclin component. (8) The cdc2 component of MPF is recycled. (Modified from Campbell, Biology).