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 out-come? Do the predictions match the experimental observations?
C. What does the aster-stimulation hypothesis predict? Do the predictions match the experimental observations?