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

MITOSIS
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Figure 17–28 Spindle self-organization by motor proteins. Mitotic chromosomes stimulate the local activation of proteins
that nucleate and promote the formation of microtubules in the vicinity of the chromosomes. Kinesin-5 motor proteins (see
Figure 17–25) organize these microtubules into antiparallel bundles, while plus-end directed kinesins-4 and 10 link the
microtubules to chromosome arms and push minus ends away from the chromosomes. Dynein and kinesin-14 motors,
together with numerous other proteins, focus these minus ends into a pair of spindle poles.
The ability of chromosomes to stabilize
MBoC6
and
m17.34/17.28
organize microtubules enables
cells to form bipolar spindles in the absence of centrosomes. Acentrosomal spindle
assembly is thought to begin with the formation of microtubules around the
chromosomes. Various motor proteins then organize the microtubules into a
bipolar spindle, as illustrated in Figure 17–28.
Cells that normally lack centrosomes, such as those of higher plants and
many animal oocytes, use this chromosome-based self-organization process to
form spindles. It is also the process used to assemble spindles in certain animal
embryos that have been induced to develop from eggs without fertilization (that
is, parthenogenetically); as the sperm normally provides the centrosome when it
fertilizes an egg, the mitotic spindles in these parthenogenetic embryos develop
without centrosomes (Figure 17–29). Even in cells that normally contain centrosomes,
the chromosomes help organize the spindle microtubules and, with the
help of various motor proteins, can promote the assembly of a bipolar mitotic
spindle if the centrosomes are removed. Although the resulting acentrosomal
spindle can segregate chromosomes normally, it lacks astral microtubules, which
are responsible for positioning the spindle in animal cells; as a result, the spindle
is often mispositioned in the cell.
spindle poles
aster
Kinetochores Attach Sister Chromatids to the Spindle
Following the assembly of a bipolar microtubule array, the second major step
in spindle formation is the attachment of the array to the sister-chromatid pairs.
Spindle microtubules become attached to each chromatid at its kinetochore,
a giant, multilayered protein structure that is built at the centromeric region of
the chromatid (Figure 17–30; also see Chapter 4). In metaphase, the plus ends
of kinetochore microtubules are embedded head-on in specialized microtubuleattachment
sites within the outer region of the kinetochore, furthest from the
DNA. The kinetochore of an animal cell can bind 10–40 microtubules, whereas
a budding yeast kinetochore can bind only one. Attachment of each microtubule
depends on multiple copies of a rod-shaped protein complex called the Ndc80
complex, which is anchored in the kinetochore at one end and interacts with the
sides of the microtubule at the other, thereby linking the microtubule to the kinetochore
while still allowing the addition or removal of tubulin subunits at this end
(Figure 17–31). Regulation of plus-end polymerization and depolymerization at
the kinetochore is critical for the control of chromosome movement on the spindle,
as we discuss later.
Kinetochore attachment to the spindle occurs by a complex sequence of
events. At the end of prophase in animal cells, the centrosomes of the growing
spindle generally lie on opposite sides of the nuclear envelope. Thus, when the
envelope breaks down, the sister-chromatid pairs are bombarded by microtubule
10 µm
Figure 17–29 Bipolar spindle assembly
without centrosomes in parthenogenetic
embryos of the insect Sciara (or fungus
gnat). The microtubules are stained
green, the chromosomes red. The top
fluorescence micrograph shows a normal
MBoC6 m17.35/17.29
spindle formed with centrosomes in a
normally fertilized Sciara embryo. The
bottom micrograph shows a spindle
formed without centrosomes in an
embryo that initiated development without
fertilization. Note that the spindle with
centrosomes has an aster at each pole of
the spindle, whereas the spindle formed
without centrosomes does not. Both
types of spindles are able to segregate
the replicated chromosomes. (From B. de
Saint Phalle and W. Sullivan, J. Cell Biol.
141:1383–1391, 1998. With permission
from The Rockefeller University Press.)