Figure 2

The threshold concentration of cyclin B to enter mitosis is higher than the threshold to exit mitosis. Cycling egg extracts in interphase of cycle 1 were supplemented with Δcyclin B (at t = 0). (a) To measure the activation threshold, CHX was added immediately (t = 0). (b) To measure the inactivation threshold, CHX was added 60 min later when the extract was in mitosis. Fluorescence micrographs of sperm nuclei are depicted. Triangles denote activation threshold () and inactivation threshold () concentrations. (Scale bars = 50 μm.) (c) Extracts prepared as in a and b without exogenous cyclin were immunoblotted for endogenous cyclin B1. (d) A CHX-treated CSF-released extract was supplemented with 150 nM Δcyclin B during interphase (t = 0). Samples were collected and blotted for Δcyclin B. Extracts are labeled M when >90% nuclei on a slide appear mitotic (condensed chromatin, no nuclear envelope). In unlabeled extracts, >90% nuclei were in interphase. Migration of molecular mass standards (in kDa) is indicated.