Cell cycle is controlled by complexes of cyclins (protein) together with cyclin-dependant protein kinases (cdks: cyclin dependent protein kinases only active when combined with a cyclin, kinase adds phosphate group to target proteins when activated. Different cyclin: cdk combinations regulate cell cycle transitions at different checkpoints. Cancer is usually caused by a failure of this cell type, as it is unregulated cell division. G1 cell will not pass g1 checkpoint unless it is going to divide. Only cells that will divide are allowed to undergo replication. G2 - checks that dna replication is complete. Needs to have 2 complete sets of chromosomes before cell division will occur. M ensures mitotic apparatus is fully assembled and ready to physically separate chromosomes. Progression through the cell cycle is controlled by checkpoints. Cells that do not divide do not pass the g1 checkpoint and remain in g0. Once a cell passes the g1 checkpoint, it is usually committed to complete mitosis.