Essential Cell Biology 5th edition

Mitosis
635
(A)
ANAPHASE A CHROMOSOMES ARE PULLED POLEWARD (B) ANAPHASE B POLES ARE PUSHED AND PULLED APART
kinetochore microtubules
1 1
2
2
interpolar microtubules
kinetochore microtubules
shorten, dragging chromosomes
toward their spindle pole
a sliding force between interpolar
microtubules from opposite poles (1)
pushes the poles apart; a pulling force
at the cell cortex (2) drags the two
poles apart
plasma membrane
Figure 18−29 Two processes segregate chromosomes at anaphase.
(A) In anaphase A, the sister chromatids are pulled toward opposite
poles as the kinetochore microtubules depolymerize. The force driving
this movement is generated mainly at the kinetochore. (B) In anaphase
B, the two spindle poles move apart as the result of two separate
forces: (1) the elongation and sliding of the interpolar microtubules
past one another pushes the two poles apart, and (2) forces exerted on
the outward-pointing astral microtubules at each spindle pole pull the
poles away from each other, toward the cell cortex. Both forces
are thought to depend on the action of motor proteins associated
with the microtubules.
ECB4 e18.29/18.29
microtubule growth at
plus ends of interpolar
microtubules helps push
the poles apart
remain glued together. Thus, none of the duplicated chromosomes can
be pulled apart until every chromosome has been positioned correctly on
the mitotic spindle. The absence of APC/C also prevents the destruction
of cyclins (see Figure 18–9), so that Cdks remain active—thus prolonging
mitosis. This spindle assembly checkpoint thereby controls the onset
of anaphase, as well as the exit from mitosis, as mentioned earlier (see
Figure 18−12).
The Nuclear Envelope Re-forms at Telophase
By the end of anaphase, the chromosomes have separated into two equal
groups, one at each pole of the spindle. During telophase, the final stage
of mitosis, the mitotic spindle disassembles, and a nuclear envelope
reassembles around each group of chromosomes to form the two daughter
nuclei (Movie 18.9 and Movie 18.10). Vesicles of nuclear membrane
associate with the clustered chromosomes and then fuse to re-form the
nuclear envelope (see Panel 18−1, pp. 628–629). During this process,
the nuclear pore proteins and nuclear lamins that were phosphorylated
during prometaphase are now dephosphorylated, which allows them to
reassemble and rebuild the nuclear envelope and lamina (Figure 18−30).
Once the nuclear envelope has been re-established, the pores restore the
localization of cytosolic and nuclear proteins and the condensed chromosomes
decondense into their interphase state. A new nucleus has
been created, and mitosis is complete. All that remains is for the cell to
complete its division into two separate daughter cells.