Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CYTOKINESIS
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astral stimulation model central spindle stimulation model astral relaxation model
Figure 17–45 Three current models of
how the microtubules of the anaphase
spindle generate signals that influence
the positioning of the contractile
ring. No single model explains all the
observations, and furrow positioning is
probably determined by a combination of
these mechanisms, with the importance
of the different mechanisms varying in
different organisms. See text for details.
astral microtubules carry furrow-inducing signals, which are somehow focused
in a ring on the cell cortex, halfway MBoC6 between m17.53/17.45 the spindle poles. Evidence for this
model comes from ingenious experiments in large embryonic cells, which demonstrate
that a cleavage furrow forms midway between two asters, even when
the two centrosomes nucleating the asters are not connected to each other by a
mitotic spindle (Figure 17–46).
A second possibility, called the central spindle stimulation model, is that the
spindle midzone, or central spindle, generates a furrow-inducing signal that specifies
the site of furrow formation at the cell cortex (see Figure 17–45). The overlapping
interpolar microtubules of the central spindle associate with numerous
signaling proteins, including proteins that may stimulate RhoA (Figure 17–47).
Defects in the functions of these proteins (in Drosophila mutants, for example)
result in failure of cytokinesis.
A third model proposes that, in some cell types, the astral microtubules promote
the local relaxation of actin–myosin bundles at the cell cortex. According to
this astral relaxation model, the cortical relaxation is minimal at the spindle equator,
thus promoting cortical contraction at that site (see Figure 17–45). In the early
embryos of Caenorhabditis elegans, for example, treatments that result in the loss
of astral microtubules lead to increased contractile activity throughout the cell
cortex, consistent with this model.
In some cell types, the site of ring assembly is chosen before mitosis. In budding
yeasts, for example, a ring of proteins called septins assembles in late G 1 at
the future division site. The septins are thought to form a scaffold onto which
other components of the contractile ring, including myosin II, assemble. In plant
cells, an organized band of microtubules and actin filaments, called the preprophase
band, assembles just before mitosis and marks the site where the cell wall
will assemble and divide the cell in two, as we now discuss.
chromosomes
dividing egg cell
both nuclei
enter mitosis
centrosome
a glass bead pushed
into the cell displaces
the spindle
glass bead
furrow forms only
on one side of cell,
producing a binucleate
egg
cleavage occurs both between the centrosomes
linked by mitotic spindles and between the two
centrosomes that are simply adjacent, and
four daughter cells are formed
Figure 17–46 An experiment
demonstrating the influence of the
position of microtubule asters on the
subsequent plane of cleavage in a
large egg cell. If the mitotic spindle is
mechanically pushed to one side of the
cell with a glass bead, the membrane
furrowing is incomplete, failing to occur on
the opposite side of the cell. Subsequent
cleavages occur not only at the midzone
of each of the two subsequent mitotic
spindles (yellow arrowheads), but also
between the two adjacent asters that are
not linked by a mitotic spindle—but in this
abnormal cell share the same cytoplasm
(red arrowhead). Apparently, the contractile
ring that produces the cleavage furrow
in these cells always forms in the region
midway between two asters, suggesting
that the asters somehow alter the adjacent
region of cell cortex to induce furrow
formation between them.