Essential Cell Biology 5th edition

630 CHAPTER 18 The Cell-Division Cycle
centrioles
centrosome
nucleus
duplicated
centrosome
G 1
S/G 2
Figure 18–21 The centrosome in an interphase cell duplicates to
form the two poles of a mitotic spindle. Most animal cells contain
a single centrosome, which consists of a pair of centrioles (gray)
embedded in a matrix of proteins (light green). The volume of the
centrosome matrix is exaggerated in this diagram for clarity. Although
the centrioles are made of a cylindrical array of short microtubules,
they do not participate in the nucleation of microtubules from the
centrosome (see Figure 17−13). Centrosome duplication begins at
the start of S phase and is complete by the end of G 2 . Initially, the two
centrosomes remain together, but, in early M phase, they separate,
and each nucleates its own aster of microtubules. The centrosomes
then move apart, and the microtubules that interact between the
two asters elongate preferentially to form a bipolar mitotic spindle,
with an aster at each pole. When the nuclear envelope breaks down,
the spindle microtubules are able to interact with the duplicated
chromosomes.
aster
forming
mitotic
spindle
duplicated
chromosome
metaphase
spindle
nuclear
envelope
spindle
pole
ECB5 e18.21/18.21
M phase
Centrosome duplication begins at the same time as DNA replication and
the process is triggered by the same Cdks—G 1 /S-Cdk and S-Cdk—that
initiate DNA replication. Initially, when the centrosome duplicates, both
copies remain together as a single complex on one side of the nucleus.
As mitosis begins, however, the two centrosomes separate, and each
nucleates a radial array of microtubules called an aster. The two asters
move to opposite sides of the nucleus to form the two poles of the mitotic
spindle (Figure 18–21). The process of centrosome duplication and separation
is known as the centrosome cycle.
The Mitotic Spindle Starts to Assemble in Prophase
The mitotic spindle begins to form in prophase. The assembly of this
highly dynamic structure depends on the remarkable properties of
microtubules. As discussed in Chapter 17, microtubules continuously
polymerize and depolymerize by the addition and loss of their tubulin
subunits, and individual filaments alternate between growing and shrinking—a
process called dynamic instability (see Figure 17−14). At the start of
mitosis, dynamic stability rises—in part because M-Cdk phosphorylates
microtubule-associated proteins that influence microtubule stability. As
a result, during prophase, rapidly growing and shrinking microtubules
extend in all directions from the two centrosomes, exploring the interior
of the cell.
Some of the microtubules growing from one centrosome interact with
the microtubules from the other centrosome (see Figure 18−21). This
interaction stabilizes the microtubules, preventing them from depolymerizing,
and it joins the two sets of microtubules together to form the
basic framework of the mitotic spindle, with its characteristic bipolar
shape (Movie 18.6). The two centrosomes that give rise to these microtubules
are now called spindle poles, and the interacting microtubules
are called interpolar microtubules (Figure 18−22). The assembly of the
spindle is driven, in part, by motor proteins associated with the interpolar
microtubules that help to cross-link the two sets of microtubules and
push the two centrosomes apart.
Chromosomes Attach to the Mitotic Spindle at
Prometaphase
Prometaphase starts abruptly with the disassembly of the nuclear envelope,
which breaks up into small membrane vesicles. This process is
triggered by the phosphorylation and consequent disassembly of nuclear
pore proteins and the intermediate filament proteins of the nuclear lamina,