Essential Cell Biology 5th edition

632 CHAPTER 18 The Cell-Division Cycle
spindle pole
duplicated
chromosome
(sister chromatids)
kinetochore
astral microtubules kinetochore microtubules interpolar microtubules
(A)
(B)
5 µm
Figure 18−24 Three classes of microtubules make up the mitotic spindle. (A) Schematic drawing of a spindle
with chromosomes attached, showing the three types of spindle microtubules: astral microtubules, kinetochore
microtubules, and interpolar microtubules. In reality, the chromosomes are much larger than shown, and usually
multiple microtubules are attached to each kinetochore. (B) Fluorescence micrograph of duplicated chromosomes
aligned at the center of the mitotic spindle. In this image, kinetochores are red dots, microtubules are green, and
chromosomes are blue. (B, from A. Desai, Curr. Biol. 10:R508, 2000. With permission from Elsevier.)
spindle poles
aster
the spindle; thus, each duplicated chromosome becomes linked to both
spindle ECB5 poles. e18.24/18.24 The attachment to opposite poles, called bi-orientation,
generates tension on the kinetochores, which are being pulled in opposite
directions. This tension signals to the sister kinetochores that they
are attached correctly and are ready to be separated (Movie 18.7). The
cell-cycle control system monitors this tension to ensure correct chromosome
attachment (see Figure 18−3), a safeguard we discuss in detail
shortly.
The number of microtubules attached to each kinetochore varies among
species: each human kinetochore binds 20–40 microtubules, for example,
whereas a yeast kinetochore binds just one. The three classes of microtubules
that form the mitotic spindle are highlighted in Figure 18−24.
10 µm
Figure 18−25 Motor proteins and
chromosomes can direct the assembly
of a functional bipolar spindle in the
absence of centrosomes. In these
fluorescence micrographs of embryos of
the insect Sciara, the microtubules are
stained green and the chromosomes red.
The top micrograph shows a normal spindle
formed by centrosomes in a fertilized
embryo. The bottom micrograph shows a
spindle ECB5 formed e18.25/18.25
without centrosomes in an
embryo that initiated development without
fertilization and thus lacks the centrosome
normally provided by the sperm when it
fertilizes the egg. Note that the spindle
with centrosomes has an aster at each
pole, whereas the spindle formed without
centrosomes does not. As shown, both
types of spindles are able to segregate
chromosomes. (From B. de Saint Phalle
and W. Sullivan, J. Cell Biol. 141:1383–1391,
1998. With permission from The Rockefeller
University Press.)
Chromosomes Assist in the Assembly of the
Mitotic Spindle
Chromosomes are more than passive passengers in the process of spindle
assembly: they themselves can stabilize and organize microtubules
into functional mitotic spindles. In cells without centrosomes—including
some animal cell types and all plant cells—the chromosomes nucleate
microtubule assembly, and motor proteins then move and arrange the
microtubules and chromosomes into a bipolar spindle. Even in animal
cells that normally have centrosomes, a bipolar spindle can still be formed
in this way if the centrosomes are removed (Figure 18−25). In cells with
centrosomes, the chromosomes, motor proteins, and centrosomes work
together to form the mitotic spindle.
Chromosomes Line Up at the Spindle Equator
at Metaphase
During prometaphase, the duplicated chromosomes, now attached to
the mitotic spindle, begin to move about, as if jerked first this way and
then that. Eventually, they align at the equator of the spindle, halfway
between the two spindle poles, thereby forming the metaphase plate.
This event defines the beginning of metaphase (see Figure 18–24B and