Fig. 2

Three early models of the mitotic control system proposed by Norel and Agur [14] (a), Goldbeter [15] (b), and Tyson [16] (c). Left panels: Molecular mechanisms. Solid arrows represent chemical reactions; dashed arrows represent catalytic activities. E and H are enzymes that catalyze particular reactions; k _s is the rate constant for cyclin synthesis; −P indicates a phosphorylated protein. Middle panels: Oscillatory ranges. As a function of increasing rate of synthesis of cyclin, we plot MPF activity of each model for two types of solutions. The solid (dashed) lines correspond to stable (unstable) steady state solutions of the model’s differential equations. The blue circles correspond to the maximum (upper) and minimum (lower) activity of MPF during an oscillatory solution for a particular value of k _s. Notice that oscillatory solutions are observed only over a range of values of k _s. Right panels: Signal-response curves. For a fixed concentration of cyclin, we plot the steady state activity of MPF as predicted by each model. Black squares represent Solomon’s observations (adapted from Fig. 1c). In (a) and (c) the dashed up-arrow indicates the cyclin level where the control system would make an abrupt jump to a state of high MPF activity. In the Norel-Agur model, the MPF activity increases without bound (indicated by the question mark)