Essential Cell Biology 5th edition

Mitosis
631
Figure 18−22 A bipolar mitotic spindle is formed by the selective
stabilization of interacting microtubules. New microtubules grow
out in random directions from the two centrosomes. The two ends
of a microtubule (by convention, called the plus and the minus ends)
have different properties, and it is the minus end that is anchored
in the centrosome (discussed in Chapter 17). The free plus ends
are dynamically unstable and switch suddenly from uniform growth
(outward-pointing red arrows) to rapid shrinkage (inward-pointing blue
arrows). When two microtubules from opposite centrosomes interact
in an overlap zone, motor proteins and other microtubule-associated
proteins cross-link the microtubules together (black dots) in a way
that stabilizes the plus ends by decreasing the probability of their
depolymerization.
astral
microtubules
microtubules
centrosome
the network of fibrous proteins that underlies and stabilizes the nuclear
envelope (see Figure 17−7). The spindle microtubules, which have been
lying in wait outside the nucleus, now gain access to the duplicated chromosomes
and capture each and everyone (see Panel 18−1, pp. 628–629).
Spindle microtubules attach to the chromosomes at their kinetochores,
protein complexes that assemble on the centromere of each condensed
chromosome during late prophase (Figure 18−23). Kinetochores recognize
the special DNA sequence that forms a chromosome’s centromere: if
this sequence is altered, kinetochores fail to assemble and, consequently,
the chromosomes fail to segregate properly during mitosis.
stabilized
interpolar
microtubules
spindle pole
Once the nuclear envelope has broken down, a randomly probing microtubule
encountering a kinetochore will bind to it, thereby capturing that
chromosome and linking it to a spindle pole (see Panel 18−1, pp. 628–
629). Of course, each duplicated chromosome has two kinetochores—one
on each sister chromatid. Because these sister kinetochores face in opposite
directions, they tend to attach to microtubules from opposite poles of
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replicated
chromosome
kinetochore
centromere region
of chromosome
kinetochore
plus end of microtubule
kinetochore
microtubules
connecting protein complex
(A)
(B)
chromatid
(C)
Figure 18−23 Kinetochores attach chromosomes to the mitotic spindle. (A) A fluorescence micrograph of a duplicated mitotic
chromosome. The kinetochores are stained red with fluorescent antibodies that recognize kinetochore proteins. These antibodies
come from patients with scleroderma (a disease that causes progressive overproduction of connective tissue in skin and other organs),
who, for unknown reasons, produce antibodies against their own kinetochore proteins. (B) Schematic drawing of a mitotic chromosome
showing its two sister chromatids attached to kinetochore microtubules, which bind to the kinetochore at their plus ends. Each
kinetochore forms a plaque on the surface of the centromere. (C) Each microtubule is attached to the kinetochore via interactions with
multiple copies of an elongated connecting protein complex (blue). These complexes bind to the sides of the microtubule near its plus
end, allowing the microtubule to grow or shrink while remaining attached to the kinetochore. (A, from R.P. Zinkowski et al., J. Cell Biol.
113:1091–1110, 1991. With permission from The Rockefeller University Press.)
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