Chromosome micromanipulation

Two types of unusual motion within the spindle have heen studied in a grasshopper (Melanoplus differentialis) spermatocyte. The first is the motion of granules placed by micromanipulation within the normally granule-free spindle. The most specific motions are poleward, approximate the speed of the chromosomes in anaphase, and occur in the area between the kinetochores and the nearer pole during both metaphase and anaphase. Exactly the same transport properties were earlier observed by Bajer inHaemanthus endosperm spindles. The absence of significant motion in the interzone between the separating chromosomes at anaphase has been unequivocally demonstrated inMelanoplus spermatocytes. Thus very specific motion of non-kinetochoric materials is probably a general spindle capability which would much restrict admissible models of mitotic force production,if the same forces move both granules and chromosomes. The second unusual motion is seen following chromosome detachment from the spindle by micromanipulation during anaphase. These tend to move toNearer pole rather than to the pole the chromosome's kinetochoresFace. The latter preference was earlier demonstrated after detachment during prometaphase or metaphase and has been confirmed without exception in the present studies. The apparent preference for motion to the nearer pole in anaphase provides the first evidence for poleward forces within each half-spindle which cannot be entirely specified by the chromosomal spindle fibers. Almost certainly these would be the usual forces responsible for chromosome motion since they act specifically at the kinetochores of detached chromosomes. This evidence requires interpretation, however because additional factors influence chromosome motion following detachment at anaphase. On thesimplest interpretation, certain current models of mitosis clearly are not satisfactory and others are favored.