Figure 6. A mathematical model of Cdk1 activity captures the scaling of the speed with S-phase activity.

(A) Top: phase plane diagram depicting stable fixed points (filled circles) and unstable fixed points (open circles). Bottom: schematic view of the Cdk1 network. (B) Cdk1 (solid line) and Chk1 (dotted line) activity profiles for cycles 10–13, as obtained from the mathematical model described in the main text and Supplemental Information. (C) Phase plane analysis for different time points in cycle 11 (left) and 13 (right). The open circles denote unstable fixed points and filled circle denote stable fixed points. Notice that for both cycles bistability is observed transiently and for early time points, plots extend beyond cyclin-Cdk1 concentration. (D) Log-log plot of Cdk1 speed versus S-phase Cdk1 activation rate for experimental data (black) and simulated data (red). (E) Log-log plot of Cdk1 speed versus S-phase Cdk1 activation rate for simulations of the wild type (black) and simulations of the twe^3A wee1^9A mutants (purple). (F) Plot of the effective bistable potential appearing in the model (calculated at the time when Cdk1 transitions from the low to the high state). Arrows indicate the kinetic barrier. All error bars, 95% CI. See also Figure S5.