Since the discovery of mitotic spindle microtu-bules, there have been two competing ideas about the origins of forces for chromosome segregation during cell division: the dynamics of microtu-bules, and the action of motor enzymes. When I entered this research field in the late 1960s, the sliding filament theory of muscle contraction was already well established, and contemporary structural and biochemical work on cilia made a motor-based mechanism for mitosis seem likely. Moreover, my colleagues and I discovered a remarkably simple model for mitosis based on the postulate of a single, microtubule-sliding motor and some — then plausible — assumptions about spindle microtubule polarity. So in spite of the beautiful work by Shinya Inoue and his students showing the importance of microtubule depolymerization for permitting chromosome motions, I was committed to the idea that motors were the drivers. Microtubule dynamics might be a mitotic regulator, but they seemed unlikely to provide the motive force.
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