Essential Cell Biology 5th edition

636 CHAPTER 18 The Cell-Division Cycle
Figure 18−30 The nuclear envelope
breaks down and re-forms during
mitosis. The phosphorylation of nuclear
pore proteins and lamins helps trigger the
disassembly of the nuclear envelope at
prometaphase. Dephosphorylation of these
proteins at telophase helps reverse the
process.
CONTINUED FUSION
OF NUCLEAR
ENVELOPE VESICLES
TELOPHASE
chromosome
nuclear pore
lamins
DNA
INTERPHASE NUCLEUS
inner nuclear
membrane
outer nuclear
membrane
duplicated
chromosome
P
nuclear envelope
vesicle
P
P P
phosphorylated
lamins
P
nuclear
envelope
PHOSPHORYLATION
OF NUCLEAR PORE
PROTEINS AND LAMINS
phosphorylated
pore protein
P
PROMETAPHASE
P P
P
P P
P
DEPHOSPHORYLATION
OF NUCLEAR PORE
PROTEINS AND LAMINS
QUESTION 18–7
Consider the events that lead to
the formation of the new nucleus
at telophase. How do nuclear and
cytosolic proteins become properly
re-sorted so that the new nucleus
contains nuclear proteins but not
cytosolic proteins?
CYTOKINESIS
Cytokinesis, the process by which the cytoplasm is cleaved in two, completes
M phase. It usually begins in anaphase but is not completed until
after the two daughter nuclei have re-formed in telophase. Whereas
mitosis depends on a transient microtubule-based structure, the mitotic
spindle, cytokinesis in animal ECB5 E18.30/18.30
cells depends on a transient structure based
on actin and myosin filaments, the contractile ring (see Figure 18−19).
Both the plane of cleavage and the timing of cytokinesis, however, are
determined by the mitotic spindle.
The Mitotic Spindle Determines the Plane of
Cytoplasmic Cleavage
The first visible sign of cytokinesis in animal cells is a puckering and furrowing
of the plasma membrane that occurs during anaphase (Figure
18−31). The furrowing invariably occurs along a plane that runs perpendicular
to the long axis of the mitotic spindle. This positioning ensures
that the cleavage furrow cuts between the two groups of segregated chromosomes,
so that each daughter cell receives an identical and complete
set of chromosomes. If the mitotic spindle is deliberately displaced (using
Figure 18−31 The cleavage furrow is
formed by the action of the contractile
ring underneath the plasma membrane.
In these scanning electron micrographs of
a dividing fertilized frog egg, the cleavage
furrow is unusually well defined. (A) Lowmagnification
view of the egg surface. (B) A
higher-magnification view of the cleavage
furrow. (From H.W. Beams and R.G. Kessel,
Am. Sci. 64:279–290, 1976. With permission
of Sigma Xi.)
(A)
200 µm (B) 25 µm