Checkpoints occur before S phase, before M phase, and before cytokinesis. The cell cycle control system works through enzymes which add a phosphate molecule to key proteins that initiate or regulate cell processes. These enzymes are called protein kinases. This kinase adds phosphate groups to key proteins which regulate the cell's entry into mitosis. MPF activity is dependent on the presence of cyclin.
Cyclin levels oscillate during the cell cycle, with a peak occurring during mitosis.
At the end of mitosis, cyclin levels fall rapidly. It is the necessity to condense chromosomes and divide the genetic information evenly between daughter cells that gives rise to the complexity of mitosis. Both the plane of cleavage and the timing of cytokinesis is determined by the placement of the microtubules. The plasma membrane puckers during anaphase to form the cleavage furrow, this pucker is perpendicular to the microtubules. The contractile ring is made of actin and myosin filaments, it is attached to the cytoplasmic face of the membrane.