Treatment with 1 µM taxol reduces delta and activates the SAC. (A) Selected frames from time-lapse imaging of a preformed taxol monopole (top) and an MG132-treated bipolar spindle collapsing into a monopole after the addition of 1 µM taxol (bottom). Taxol monopoles delay in mitosis for 153 ± 52 min. (B) Taxol-induced spindle collapse is mediated by minus end–directed motor activity. The graph shows the percentage of each indicated structure with representative images after treatment with 1 µM taxol for 1 h. (C and D) Treatment of Ncd RNAi structures with 1 µM taxol (n = 240 kinetochore pairs) causes a reduction in both the interkinetochore distance and delta relative to Ncd RNAi alone (n = 95 kinetochore pairs). Ncd is undetectable by Western blotting (inset) after dsRNA treatment (96 h). (E) Ncd RNAi does not cause an increase in the number of mitotic cells in a cycling population. (F) Ncd RNAi cells divide with normal kinetics (51 ± 17 min; n = 100 cells), but addition of 1 µM taxol delays them in mitosis for 150 ± 47 min (n = 77 cells), indicating that the SAC is activated. The mean values ± standard deviations and the two-tailed p-values are shown in each graph. Error bars represent the standard deviations. Bars, 10 µm.