Chromosomes move poleward in anaphase along stationary microtubules that coordinately disassemble from their kinetochore ends.

Abstract

Abstract. During the movement of chromosomes in anaphase, microtubules that extend between the kinetochores and the poles shorten. We sought to determine where subunits are lost from these microtubules during their shortening. Prophase or prometaphase cells on coverslips were injected with fluoresceinated tubulin and allowed to progress through mitosis. Immediately after the onset of anaphase, a bar-shaped beam of laser light was used to mark a domain on the kinetochore fibers by photobleaching a band, ",,1.0 Ixm wide, across the spindle. In different cells, spindles were photobleached at varying distances from the chromosomes. Cells were allowed to continue in anaphase until the chromosomes had further separated. They were then lysed, fixed, and prepared for double-label immunofluorescence with an antibody to fluorescein that does not bind appreciably to bleached fluorescein, and with an antibody to tubulin. Photobleached domains of microtubules appeared as bands of reduced fluorescence in the anti-fluorescein image. However, the anti-tubulin labeling revealed that microtubules were present and continuous through the photobleached domains. In all cases, the chromosomes approached and invaded the bleached domain while the bleached domain itself remained stationary with respect to the near pole. These results demonstrate that the chromosomes move along stationary kinetochore microtubules and that depolymerization of these microtubules during anaphase takes place at the kinetochore. In contrast to the generally accepted older view that chromosomes are passive objects pulled by "traction fibers; we suggest that the kinetochore is an active participant in generating the motive force that propels the chromosome to the pole.

Prime movers: the mechanochemistry of mitotic kinesins

“…Anaphase A -the shortening of the kinetochore to pole distance -can be driven by depolymerization of kinetochore-attached microtubules 145 (termed "Pacman") or by poleward flux 146 (FIG. 1d, insets 10 and 11).…”

Intracellular free calcium and mitosis in mammalian cells: anaphase onset is calcium modulated, but is not triggered by a brief transient.

“…These cells remain flat during mitosis and have been used in previous studies of microtubule dynamics and mitotic phosphoproteins at anaphase onset (Gorbsky et al, 1987;Vandre et al, 1984). Although we observed an average gradual Cai elevation from 75 to 165 nM beginning at metaphase or anaphase in all LLC-PK cells monitored, peaking after cell division, similar to that seen in Swiss 3T3 cells, we never saw Ca~ transients during the entire prometaphase to interphase period, regardless of the method of fura-2 introduction or content of the extracellular medium (Table I; Fig.…”

The perpetual movements of anaphase

“…Drosophila embryos, S2 cells, Xenopus oocyte extract spindles, newt lung cells, mouse, pig and human cells are all examples of the latter [49,50,[57][58][59][60][61][62][63][64][65]. Indeed, in these systems microtubule minus-end depolymerization is turned off or significantly attenuated during anaphase [54,65,66]. A possible explanation can be brought up by evoking anaphase B. Brust-Mascher and colleagues proposed a model for anaphase B in which the velocity of spindle elongation is governed by the extent that microtubule minus-end depolymerization is suppressed [58,67].…”

“…An explanation advanced was that a kinetochore "motor" could be turned on in anaphase but is inactive or less active in metaphase. Subsequently, through photobleaching studies on spindle microtubules, Gorbsky and collaborators showed that chromosomes move to and through a persistent bleach mark, with little change of position of the mark relative to the poles [66,80]. In a different set of experiments, Nicklas cut microtubules across the middle of the spindle between chromosomes and the pole, and upon anaphase onset in partially lysed preparations of grasshopper spermatocytes, chromosomes moved to the ends of microtubules at the edge of the cut [81,82].…”