Cytokinesis-the actual process of cell division-has attracted theorists for well
ID: 7724 • Letter: C
Question
Cytokinesis-the actual process of cell division-has attracted theorists for well over 100
years. It has been said that all possible explanations of cytokinesis have been proposed; the problem is to decide which one is correct.
Consider the following three hypotheses:
1. Chromosome signaling: When chromosomes split at anaphase, they emit a signal to the
nearby cell surface to initiate furrowing.
2. Polar relaxation: Asters
relax the tension in the region of the cell surface nearest the spindle poles
(the polar region), allowing the region of the membrane farthest from the poles (the equatorial plane) to
contract and initiate furrowing.
3. Aster stimulation: The
asters stimulate contraction in the region of the cell surface where oppositely
oriented spindle fibers overlap (that is, the equatorial region), thereby initiating furrowing.
These
hypotheses have been tested in a number of ways, One particularly informative experiment involved pressing a glass ball onto the center of a dividing sand dollar egg so as to deform it into a torus (donut shape). As illustrated in Figure 17-35, the egg divided into a single sausage-shaped cell at the first division; at the second division, it divided into four cells.
A. What does the chromosome-signaling hypothesis predict for the
result of this experiment? Do the predictions match the experimental observations?
B. What does the polar-relaxation hypothesis predict for the experimental outcome?
Do the predictions match the experimental observations?
C. What does the aster-stimulation hypothesis predict? Do the predictions match the experimental observations?
Image could not be uploaded but it is portraying the
First and second divisions in a torous shaped sand dollar egg
Explanation / Answer
a). chromosome-signaling hypothesis predict that when chromosomes split at anaphase, they emit a signal to the nearby cell surface to initiate furrowing by astral MT contact with the cortex that is required for furrow initiation, and that this contact must occur after anaphase onset reinforcing the idea that the metaphase–anaphase transition results in a biochemical environment that is supportive of cytokinesis but that furrow ingression requires the partitioning of cytoplasm that is accomplished by MTs as they elongate during anaphase.
-- No the experiment do not match as the pressing of glass ball doe not emit any internal signals in the organism.
b). The polar relaxation hypothesis of furrow initiation suggests that due to a local concentration of astral influence at the poles, relative to the equator, cortical tension in the poles is decreased and therefore furrows form at the equator. Meanwhile, the equatorial stimulation hypothesis proposes that a maximum of stimulatory signals arise at the equator due to the overlapping influence from both asters in this region.
-- The prediction can match the expeimental obssrvations as the pressing a glass ball onto the center of a dividing sand dollar egg so as to deform it into a torus, involves the relaxing the tension in the region of the cell surface nearest the spindle poles.
c). Aster stimulation hypothesis predict that asters stimulate contraction in the region of the cell surface where oppositely oriented spindle fibers overlap (that is, the equatorial region), thereby initiating furrowing.
-- experimental observations and prediction cannot be aacurately matched. Because the pressing of glass at the center of organism can or cannot stimulate the asters at the equlateral region. If it stimulates the asters at the center of cell region then furrowing can be seen.
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