A friend declares that chromosomes are held at the metaphase plate by microtubules that push on each chromosome from opposite sides
Which of the following observations does not support your belief that the microtubules are pulling on the chromosomes?
(a) the jiggling movement of chromosomes at the metaphase plate
(b) the way in which chromosomes behave when the attachment between sister chromatids is severed
(c) the way in which chromosomes behave when the attachment to one kinetochore is severed
(d) the shape of chromosomes as they move toward the spindle poles at anaphase
(a) The jiggling movement [choice (a)] is simply a sign that the chromosomes are subject to forces from both sides, which could be the microtubules pulling, pushing, or both. All the other observations suggest pulling. When the attachment between sister chromatids is severed [choice (b)], both daughter chromosomes move toward their respective poles, which suggests that they are being pulled. When the attachment to one kinetochore is severed [choice (c)], the whole chromosome moves to the opposite pole, showing that the kinetochore microtubules are pulling on their attached chromatid, not pushing it. Similarly, the shape of the chromosomes as they move toward the pole [choice (d)] suggests that the chromosomes are being pulled.
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