Checkpoints and the molecules that run them
The cycle is governed by cyclin-dependent kinases, which are present at constant levels and inactive until a cyclin binds. Cyclin concentration oscillates; CDK concentration does not. MPF — maturation promoting factor, cyclin B bound to CDK1 — accumulates through G2 and triggers entry into mitosis, then destroys its own cyclin by activating the anaphase-promoting complex, which is why the drive into M phase is self-limiting. Three checkpoints matter: G1, where the restriction point decides whether to divide at all and where most non-dividing cells exit into G0; G2, which verifies DNA replication is complete; and the spindle assembly checkpoint in M, which holds anaphase until every kinetochore is attached.