Fig. 3. At the G2-M transition, cytoplasmic microtubules disappear as the spindle forms inside the nucleus. (A) Shown here are still images from a time-lapse movie of a cell entering mitosis. The cell shown here is strain 2694 expressing both GFP:-tubulin and GFP:Swi6, a heterochromatin binding protein (Pidoux et al., 2000). The early spindle is seen as a small bar of fluorescence near the SPB on the inside of the nucleus (arrow, time 144). As mitosis progresses, the cytoplasmic bundles at the SPB persist whereas other nuclear-associated microtubules disappear (time 265). When the spindle reaches a length of approximately 1.0 µm, the cytoplasmic bundles at the SPB have also disappeared (time 342). These images are representative of the time points marked by the rectangle on the time line of the kymograph in B. Bar, 2 µm. (B) The kymograph is made from the same time-lapse movie shown in A. Cytoplasmic microtubules are seen as triangular projections extending from the SPB (as in Fig. 2). The pattern becomes much less complex around time point 110 as additional cytoplasmic bundles disappear. At time 230, microtubule growth is restricted to one side of the SPB. Once the phase 1 spindle has formed, the cytoplasmic microtubules become progressively shorter as seen by the decreasing length of the growth lines shown by the arrow and arrowhead. Cytoplasmic microtubules are gone by the time the spindle reaches a length of 1.0 µm (time 265). Bar, 2 µm.