3. Mitosis is assessed at the M checkpoint.
This checkpoint occurs during metaphase. It triggers and exit from mitosis and leads to cytokinesis and G1.
A set of proteins called cyclin dependent protein kinases (Cdks) and cyclins are sensitive to the conditions of the cell and interact at the checkpoints to trigger the next event in the cycle. The Cdks are enzymes that phosphorylate serine and threonine amino acids ofkey cellular enzymes and other proteins. Cyclins are proteins that bind to Cdks, allowing them to function as enzymes. They are destroyed and resynthesized during each turn of the cell cycle. Different cyclins regulate the G1 and G2 check points. At the G2 checkpoint, they phosphorylate histones, nuclear membrane filaments and microtubule-associated proteins that form the mitotic spindle. This initiates the activities that carry the cell cycle into the stages of mitosis.
The cell gradually accumulates G2 cyclin (also called mitotic cyclin). G2 cyclin binds to Cdk to form mitosis promoting factor (MPF). The MPF is not active at first in carrying the cycle past the G2 checkpoint. Eventually other cellular enzymes phosphorylate and activate a few molecules of MPF. Activated MPFs increase the activity of enzymes that phosphorylate MPF (positive feedback which leads to rapid increase in the cellular concentration of active MPF). G2 ends when the level of active MPF exceeds the threshold necessary to trigger mitosis. As mitosis proceeds to the end of metaphase, Cdk levels become constant for the most part, but G2 cyclin levels are degraded because MPF activates proteins that that destroy cyclin. So less MPF is available, mitosis ends, and gradual accumulation of new cyclin begins at the next turn of the cycle. .
The G1 checkpoint is regulated in an similar fashion. In unicellular eukaryotes such as yeast, cell size triggers DNA replication. They grow and divide as rapidly as possible and make the START decision by comparing the volume of cytoplasm to the size of the genome.
