Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

MITOSIS
995
ANAPHASE A
ANAPHASE B
Segregated Chromosomes Are Packaged in Daughter Nuclei at
Telophase
By the end of anaphase, the daughter chromosomes have segregated into two
equal groups at opposite ends of the cell. In telophase, the final stage of mitosis,
the two sets of chromosomes are packaged into a pair of daughter nuclei. The first
major event of telophase is the disassembly of the mitotic spindle, followed by
the re-formation of the nuclear envelope. Initially, nuclear membrane fragments
associate with the surface of individual chromosomes. MBoC6 These m17.46/17.40 membrane fragments
fuse to partly enclose clusters of chromosomes and then coalesce to reform
the complete nuclear envelope. Nuclear pore complexes are incorporated
into the envelope, the nuclear lamina re-forms, and the envelope once again
becomes continuous with the endoplasmic reticulum. Once the nuclear envelope
has re-formed, the pore complexes pump in nuclear proteins, the nucleus
expands, and the mitotic chromosomes are reorganized into their interphase
state, allowing gene transcription to resume. A new nucleus has been created, and
mitosis is complete. All that remains is for the cell to complete its division into two.
We saw earlier that phosphorylation of various proteins by M-Cdk promotes
spindle assembly, chromosome condensation, and nuclear-envelope breakdown
in early mitosis. It is thus not surprising that the dephosphorylation of these same
proteins is required for spindle disassembly and the re-formation of daughter
nuclei in telophase. In principle, these dephosphorylations and the completion
of mitosis could be triggered by the inactivation of Cdks, the activation of phosphatases,
or both. Although Cdk inactivation—resulting primarily from cyclin
destruction—is mainly responsible in most cells, some cells also rely on activation
of phosphatases. In budding yeast, for example, the completion of mitosis
depends on the activation of a phosphatase called Cdc14, which dephosphorylates
a subset of Cdk substrates involved in anaphase and telophase.
Figure 17–40 The two processes of
anaphase in mammalian cells. Separated
sister chromatids move toward the poles
in anaphase A. In anaphase B, the two
spindle poles move apart.
Summary
M-Cdk triggers the events of early mitosis, including chromosome condensation,
assembly of the mitotic spindle, and bipolar attachment of the sister-chromatid
pairs to microtubules of the spindle. Spindle formation in animal cells depends
largely on the ability of mitotic chromosomes to stimulate local microtubule nucleation
and stability, as well as on the ability of motor proteins to organize microtubules
into a bipolar array. Many cells also use centrosomes to facilitate spindle
assembly. Anaphase is triggered by the APC/C, which stimulates the destruction of