Figure 9.

Bistability and hysteresis (schematic). Steady-state level of total Clb-dependent kinase activity depends on the expression of CLN2 and CDC20. When [Cln2] is large and [Cdc20] is small, the Clb1–6 regulatory system is in a state of high kinase activity (S/M), whereas in the other extreme, Clb-dependent kinase activity is low (G1). When [Cln2] and [Cdc20] are both small, the regulatory system is in neutral (background activity = A/B), and two stable states of Clb activity coexist (bistability). The system is driven around a hysteresis loop (dashed curve) by pulses of Cln2 (stages a and b) and Cdc20 (stages c and d).