Hysteresis in the activation and inactivation of MPF in frog egg extracts. (Data from Sha et al. [39].) (a) Frog egg extracts, treated with cycloheximide (a drug that blocks synthesis of cyclin B), arrest in interphase as evidenced by the morphology of sperm nuclei in the extract (dispersed chromatin, stained blue by DAPI, and intact nuclear membrane, visible in the phase contrast image). At t = 0 min, non-degradable cyclin B is added to separate aliquots in increasing amounts from 0 to 40 nM. By t = 90 min, nuclei in the 40 nM aliquot have entered mitosis (condensed chromatin and dispersed nuclear membrane), but nuclei in the other aliquots are still in interphase. The cyclin threshold for MPF activation in this experiment is approximately 35 nM. (b) Frog egg extracts are allowed to enter mitosis at t = 60 min, when separate aliquots are treated with cycloheximide (to block any further synthesis of cyclin B) and supplemented with increasing concentrations of non-degradable cyclin B. By t = 140 min, nuclei in the 16 nM aliquot have returned to interphase, but nuclei in the 24 nM aliquot are still in mitosis. The cyclin threshold for MPF inactivation in this experiment is approximately 20 nM. For [cyclin B] = 24 and 32 nM, the extract can persist stably in the low-MPF steady state (a) or in the high-MPF steady state (b), confirming the prediction of bistability and hysteresis in the MPF control system.