Biology Reference
In-Depth Information
orientation of cytokinesis is determined by the position of the mitotic spindle (
Figure 23.4
).
Cytokinesis does, after all, always run transversely across the line that was the axis of the
spindle, as it must if it is to separate daughter nuclei. The relationship between spindle orien-
tation and the cleavage plane holds even in cells from which the chromosomes have been
removed,
8
demonstrating clearly that it is the spindle apparatus in its widest sense, and not
the chromosomes that it separates, that gives the cue. There is still some debate about whether
spatial information for guiding cytokinesis comes from the interpolar parts of the spindle or
from the astral microtubules.
The astral model is supported mainly by an experiment in which the central portion of the
large egg of the echinoderm Echinarachnius parma ('sand dollar') is crushed with a glass rod
so that the whole cell becomes a biconcave disk similar in shape to a mammalian red blood
cell (
Figure 23.5
). The first mitosis of these crushed eggs takes place normally except that cyto-
kinesis fails to cross the crushed area and the daughter nuclei separate to opposite sides of the
remaining ring of cytoplasm. The next mitosis also begins normally, and the effect is to produce
asters that are spaced approximately equally around the cytoplasm. When cytokinesis begins,
four furrows form, including a furrow between adjacent asters that have no spindle between
them.
9
This observation is difficult to explain by a model in which interpolar tubules specify
cytokinesis but easy to explain by a model in which cleavage furrows form between asters.
Evidence against the aster-driven model comes mainly from observations of the meiotic
divisions of a mutant of Drosophila melanogaster, called asterless. As its name suggests, the
asterless mutant cannot form asters in its meiotic divisions, apparently because it lacks peri-
centriolar material, but cytokinesis takes place in the correct place anyway.
10
Positive support for the cleavage plane being specified by interpolar microtubules comes
from experiments in which communication between the spindle and the cell cortex is disrup-
ted. If a blunt glass needle is inserted into a cell to act as a barrier between the spindle and the
nearby cortex, it blocks cytokinesis when present from metaphase but fails to block it when
FIGURE 23.5
Evidence from manipulation of the sand dollar Echinarachnius parma that cleavage planes form
between asters. Please see text for a rival point of view.
