Written Assignment on Cytokinesis

Take home essay critique of a cytokinesis paper by Kevin Burton and Lance Taylor (and a "News and Views" short review paper by Julie Theriot and Lisa Satterwhite) Nature vol 385, January 30, 1997, pages 388-9 and 450-454. (in which they may have discovered something more, or something different, than what they concluded)

Please write at least three pages single spaced, explaining the observations and conclusions, and finding at least one conclusion to argue against (i.e. explain why it is a misinterpretation, such as that some force was actually exerted perpendicular to the direction which Burton and Taylor concluded, and/or that some tension wrinkles were interpreted as compression wrinkles, and/or that forces used to pull actin into the cleavage furrow may have been misinterpreted as forces constricting the cleavage furrows, or force pulling or pushing the daughter cells apart. Or if you think they were correct on all these issue (which they may well have been; they are both excellent scientists), then please explain their reasoning, and why you agree with them completely.

BACKGROUND ISSUES: Cytokinesis (= Animal Cell Division = "Cleavage") occurs just after mitosis, at locations controlled by signals from the mitotic spindle. Contrary to the expectations of most cell biologists, the signals that induce formation of the cleavage furrow come from the poles of the mitotic spindle, and not from the equatorial part of this spindle, even though the cleavage furrow is formed next to the equator of the mitotic spindle. That paradox was proven by one of the most elegant (simple, but decisive) experiments ever done in cell biology. I refer to Ray Rappaport's classic doughnut experiment. Ray used a micromanipulator to push a cell-sized glass sphere down on the top of a dividing sand dollar (sand dollars are a kind of sea urchin) during or just before the first cell division after fertilization. This squeezed the embryo into the shape of a doughnut (torus). The key observation was that at the time of the second cleavage division, three contractile furrows form: not just the two next to the equators of the two mitotic spindle, but also a third furrow half-way between the poles of these two spindles.

This proved that furrows are stimulated to form half-way between spindle poles, whether or not a spindle connects them, and independent of the location of the chromosomes.


    #1) Do the signals from the poles of the spindle stimulate weakening of acto-myosin contraction in the cortex of dividing cells? The key idea that if the ends of the cells contract more weakly, and the equatorial region contracts with the same force as before, then that weakening near the poles will leave the equatorial area of cortex contracting more strongly. (Polar weakening theory)

    #2) (Polar strengthening theory) ; The signals from the spindle poles stimulate stronger contractility in the areas near them, and this stimulation over-laps at the equator of the cell.



THEORY A) Actin fibers could be stimulated to assemble (from freely-diffusing actin monomers), and more assembly might occur at the equator.

THEORY B) Actin and/or myosin are (or might be) pulled mechanically toward the equator, by the force of their own contractility

(This would also help to cause circumferential alignment of the actin (and myosin) fibers.)


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