How does a cell know if it's in G(1) or G(2) and should proceed to S p
hase or mitosis! This is a restatement of the question of how a cell e
nsures mitosis is dependent upon S phase, and S phase is dependent upo
n mitosis. Several gene products have been identified which play impor
tant roles in maintaining these interdependencies. Central to these co
ntrols are oscillations between different complexes of cyclins and cyc
lin-dependent kinases.