Visualization of the microtubules in a mitotic apparatus and study of the mechanism of chromosome movement.

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Chromosome movement is performed by aid of microtubule filaments in a mitotic spindle, so that the studies on the dynamic hehabiour of microtubules are closely related to the mechanism of the chormosome movement. I visualized individual microtubules in vitro by high intensity dark-field light microscopy. Real-time video recording reveales the following evidences.1) Two populations of microtubules coexist in a dynamically unstable manner in vitro: those in one population elongate while those in the other shorten. 2) Both ends of a microtubule exist in either the growing or the shortening phase and alternate guite frequently between the two phases in a stochastic manner. 3) Growing and shortening ends can coexist even on a single microtubule, one end continuing to grow simultaneously with shortening at the other end. 4) There is no correlation in the phase conversion either among individual microtubules or between the two ends of a single microtubule. 5) The two ends of any given microtubule have remarkably different charactrstics; the active end grows faster, alternates in phase more frequently and fluctuates in length to a greater extent than the inactive end. 6) Microtubule-associated proteins suppress the phase conversion and stabilize microtubules in the growing phase.