 Cell-cycle hysteresis experiment

A 2003 study split a Xenopus laevis egg extract into portions with different histories. One received calcium to destroy endogenous cyclin B before receiving nondegradable cyclin B; the other received nondegradable cyclin B first, then calcium. The same sustained cyclin B dose was approached from low or high Cdc2 activity. A phosphorimager quantified histone H1 phosphorylation, sampling every $15$–$30$ minutes until an apparent stationary response. At $45$–$60\mathrm{nM}$, the dose maintained high activity but failed to induce it from the low state.^[ref]

Controls tracked labeled endogenous cyclin B disappearance after calcium treatment and persistence of nondegradable cyclin B, addressing unequal remaining input. The comparison supports hysteresis under extract conditions; finite observations cannot establish indefinite persistence. It does not isolate each reinforcing path or establish thresholds in intact biological cells.^[ref]