Essential Cell Biology 5th edition

626 CHAPTER 18 The Cell-Division Cycle
single sister
chromatid
cohesin rings
condensin rings
(A)
(C)
(D)
sister
chromatids
1 µm
(B)
100 nm
Figure 18−18 Cohesins and condensins help to configure
duplicated chromosomes for segregation. (A) Cohesins tie together
the two adjacent sister chromatids in each duplicated chromosome.
They are thought to form large protein rings that surround the sister
chromatids, preventing them from coming apart, until the rings are
broken late in mitosis. (B) Condensins help coil each sister chromatid
(in other words, each DNA double helix) into a smaller, more compact
structure that can be more easily segregated during mitosis. These
cartoons illustrate one way that condensins might package chromatids;
the exact mechanism is not known. (C) A scanning electron micrograph
of a condensed, duplicated mitotic chromosome, consisting of two
sister chromatids joined along their length. The constricted region
(arrow) is the centromere, where each chromatid will attach to the
mitotic spindle, which pulls the sister chromatids apart toward the
end of mitosis. (D) An electron micrograph of a duplicated mitotic
chromosome in which condensin is labelled with antibodies attached
to tiny gold particles (dark dots), showing that condensins are found
mainly in the central core of the chromosome. This centralized location
of condensins is also represented in the cartoon model shown in (B).
(C, courtesy of Terry D. Allen; D, adapted from N. Kireeva et al., J. Cell
Biol. 166:775–785, 2004. With permission from Rockefeller University Press.)
helping each of these double helices to coil up into a more compact form
(Figure 18−18B–D). Together, these proteins help configure replicated
chromosomes for mitosis.
Figure 18−19 Two transient cytoskeletal
structures mediate M phase in animal
cells. The mitotic spindle assembles first
to separate the duplicated chromosomes.
Then, the contractile ring assembles to
divide the cell in two. Whereas the mitotic
spindle is based on microtubules, the
contractile ring is based on actin and
myosin. Plant cells use a very different
mechanism to divide the cytoplasm,
as we discuss later.
Different Cytoskeletal Assemblies Carry Out Mitosis and
Cytokinesis
After the duplicated chromosomes have condensed, a pair of complex
cytoskeletal machines assemble in sequence to carry out the two
ECB5 e18.18/18.18
mechanical processes that occur in M phase. The mitotic spindle carries
out nuclear division (mitosis), and, in animal cells and many unicellular
eukaryotes, the contractile ring carries out cytoplasmic division (cytokinesis)
(Figure 18−19). Both structures disassemble rapidly after they have
performed their tasks.
The mitotic spindle is composed of microtubules and the various proteins
that interact with them, including microtubule-associated motor proteins
(discussed in Chapter 17). In all eukaryotic cells, the mitotic spindle is
microtubules of the
mitotic spindle
chromosomes
PROGRESSION
THROUGH
M PHASE
actin and myosin filaments of the
contractile ring