FIGURE 4.

Meiosis-specific cell cycle progression from meiosis I to meiosis II. (A) A graphical view of meiotic progression from metaphase of meiosis I (MI) to anaphase of meiosis II (MII). After anaphase I, only one of two nuclei is chosen for drawing to illustrate MII progression. In prophase II, the microtubules are nucleated from spindle pole bodies (SPBs) and form the bipolar spindle for meiosis II as in mitosis and MI. At the transition stage of metaphase to anaphase, each SPB is modified, and the globular forespore membrane (FSM) begins to grow to surround the nucleus. The leading edge of the FSM opening is decorated by leading edge proteins. During anaphase II, the barrier function of the nuclear envelope is invalidated, which is an incident called virtual nuclear envelope breakdown. After completion of MII, the rigid spore wall is assembled. (B) The kinetics of the CDK activity during meiosis. The horizontal axis (time) is shared with the time scale in (A). In wild-type cells (top), the CDK activity elevates until metaphase I and drops at anaphase I onset, which is triggered by APC/C. The APC/C inhibitor Mes1 modulates the activity of APC/C to a moderate level so that cells can enter anaphase I and to restart meiosis II, which requires re-accumulation of the CDK activity. In mes1Δ cells (middle), Fzr1, an APC/C coactivator, is prematurely activated to fully activate APC/C, and the cells cannot enter meiosis II, and terminates meiosis early instead. In fzr1Δ cells (bottom), meiosis I proceeds almost normally, but CDK repression after anaphase II onset is not sufficient as APC/C cannot be fully activated in the absence of Fzr1. The cells then start the aberrant third division albeit incomplete.