Figure 10.

Schematic representation of astral microtubule dynamics throughout the yeast cell cycle. The nucleus is indicated in blue, astral microtubules in green, and the intranuclear spindle microtubules in red. In unbudded cells and cells with small buds, the astral microtubules originate from the single SPB (G1, G1/S). The astral microtubules are organized most often into a conical array. Individual microtubules show asynchronous dynamic instability throughout the cell cycle with growth and shortening at ≅0.5 μm/min. As microtubules reach the cell cortex, continued growth results in nuclear movement in the opposite direction. Thus, astral microtubule growth pushes against the cell cortex to propel the nucleus around the cell interior (G1/S-S/G2). The nucleus and/or microtubules make no stable contact in unbudded cells. Once a bud has formed, the dynamically unstable microtubules penetrate the bud (S/G2). At this point, the nucleus moves toward the neck of the budded cell (G2/M). The second pole body is delayed in acquiring the dynein-GFP fusion protein (S/G2–G2/M). Only after the pole bodies have separated, is the second pole competent to nucleate astral microtubules (G2/M). Once both SPB's are labeled with dynein-GFP, and the nucleus is at the neck, biphasic spindle elongation ensues (M). Astral microtubules that contact the bud tip, shorten as the spindle elongates (Anaphase). Finally, when central spindle disassembly occurs (Telophase), the astral microtubule dynamic instability and the movements of the SPB and nucleus become typical of G1 cells.