Fig. 5. The spindle checkpoint is compromised in mitch mutant mitotic cells treated with microtubule poisons. Brains from mitch¹ were treated with colchicine and hypotonic solution and examined for cyclin B (A,B; red), Bub3 (C; red), or Mad2 (D,E; red) and chromosomes (blue.) High levels of cyclin B are present in cells containing attached sister chromatids (A) whereas they are drastically lowered in PSCS cells (B). Two cells next to each other in the same field (C) show a difference in the distribution of Bub3 with respect to sister chromatid separation (PSCS). In cell 1, high levels of Bub3 are present at the kinetochores of the attached sister chromatids, but very low levels of Bub3 exist at kinetochores of PSCS cells (cell 2). Mad2 is present at much higher levels at the kinetochores of attached chromosomes (D) than at the kinetochores in PSCS cells (E). Bar, 5 µm.