Distribution Patterns of Microtubules in the Mitotic Spindles of Spirogyra.

Distribution patterns of microtubules (MTs) in metaphase and anaphase spindles in Spirogyra were examined by electron microscopy. At metaphase, 9 to 12 MTs were attached to each kinetochore. Polar MTs terminated near the chromosomes at one end and never extended into the other half-spindle. At anaphase, polar MTs overlapped in the mid zone between the separating two chromosome-groups. The polar separation is interpreted as being powered by a driving force generated by the sliding of MTs at overlapping regions. Calculation of the number of MTs at several sites in the mitotic spindles yielded results consistent with this interpretation. The distribution patterns of MTs in the half-spindles remained almost unchanged during the polar separation, consistent with the fact that the size and shape of the half-spindle during anaphase also remained almost unchanged.