Rapid completion of mitosis and cytokinesis in PtK cells following release from nocodazole arrest.

Mitotic PtK1 cells were arrested at nuclear envelope breakdown with the rapidly reversible microtubule inhibitor nocodazole. Addition of 1 microgram/ml nocodazole for 20 min resulted in complete blockage of spindle microtubule assembly but permitted other mitotic events to proceed such as chromosome condensation and cell rounding, typical of a c-mitosis. Upon release from nocodazole, mitosis and cytokinesis were completed approximately 35% faster than in a normal mitosis. The duration of each mitotic stage was compared in untreated and treated and released cells. Metaphase and cytokinesis were dramatically shortened while anaphase was unaffected. Cells treated for shorter periods of time with nocodazole and then released also reduced the length of time to complete mitosis and cytokinesis. The reduction in duration of these events was equivalent to the period of interruption of the mitotic cycle by nocodazole. With respect to the cell division cycle, PtK1 cells display a timing mechanism which is independent of the presence of microtubules.