Fig. 3

The Novak-Tyson 1993 model of mitotic control in frog eggs and extracts. a Molecular mechanism. Adapted from figure 1 of [18]. Solid arrows represent chemical reactions; dashed arrows represent catalytic activities. IE is an ‘intermediary enzyme’ phosphorylated by MPF. b Simulation of Solomon’s experiment in Fig. 1c. Adapted from figure 4B of [18]. For a fixed concentration of cyclin B, we plot the steady state concentration of MPF, according to the model. The up arrows and down arrows indicate irreversible transitions at the turning points of the S-shaped curve. For any fixed value of [cyclin] between the two arrows, the control system is ‘bistable’, i.e., it may persist in either of two alternative stable steady states (on the upper and lower branches) separated by an unstable steady state (the middle branch). The black squares recapitulate Solomon’s experiment [18]: starting from the steady state of low MPF activity, cyclin concentration is increased until the control system jumps to the steady state of high MPF activity. The gray squares recapitulate the experiments of Sha et al. [21] and Pomerening et al. [22]: starting from the steady state of high MPF activity, cyclin concentration is decreased until the control system jumps back to the low steady state. c Simulation of Félix’s experiment in Fig. 1d. This is figure 7a of [18]; used by permission. We simulated Félix’s experiment [13] by computing the rate of cyclin degradation when k _s = 0 (i.e., an ‘interphase-arrested extract’) and the initial concentration of MPF is increased from 0 to 0.9 (where 1.0 is the maximum possible MPF concentration in the model)