Deterministic simulations of the model. (*A*) Time courses for total amounts of cyclin B protein (*X*_{T}, black line), Cdc20 protein (*Z*_{T}, red line), and unphosphorylated Cdh1 protein (Ŷ blue line). (*B*) Time courses for free (uncomplexed) X (red line), Y_{P}–X (gray line), and cell volume (blue line). (*C*) The figure 8-shaped curve (red) is a cell cycle trajectory created by plotting, parametrically in *t*, the curves for *X*_{T}(*t*) and *V*(*t*) from *A* and *B*, respectively. The small arrows indicate the direction of motion around this trajectory. Cell division is indicated by the abrupt jump from *V* = 29.8 to *V* = 14.9, which is triggered when Ŷ increases above 1,200 molecules. The S-shaped curve (blue) is a one-parameter bifurcation diagram (computed by using XPP-AUTO) for steady-state solutions of the molecular control system, treating cell size (*V*) as a bifurcation parameter. Solid line, stable steady state; dashed line, unstable steady state. The two black lines above and below the unstable steady-state curve indicate the presence of stable limit cycle oscillations for *V* > 25; the *Upper* (*Lower*) curve indicating the maximum (minimum) value of *X*_{T} on the limit cycle for a particular value of *V*. (*Inset*) Period of limit cycle oscillations as a function of cell size. The control system generates oscillations by homoclinic (infinite period) bifurcations at *V* = 25 and *V* = 74.

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