Cytokinesis—division of cytoplasm
Contractile
ring at
cleavage
furrow
Nuclear
envelope
forming
Nucleolus forming
Spindle
Metaphase
plate
Daughter
chromosomes
Metaphase
Metaphase—second phase of mitosis
Anaphase
Anaphase—third phase of mitosis
Telophase
Cytokinesis
Telophase—final phase of mitosis
Telophase
Telophase
begins as soon as chromosomal
movement stops. This final phase is like
prophase in reverse.
The identical sets of chromosomes at the
opposite poles of the cell uncoil and resume
their threadlike chromatin form.
A new nuclear envelope forms around each
chromatin mass, nucleoli reappear within the
nuclei, and the spindle breaks down and
disappears.
• Mit
osis is now ended. The cell, for just a brief
period, is binucleate (has two nuclei) and each
new nucleus is identical to the original mother
nucleus.
Cytokinesis begins during late anaphase and
continues through and beyond telophase. A
contractile ring of actin microfilaments forms
the
cleavage furrow
and pinches the cell apart.
The shortest phase of mitosis, anaphase
begins abruptly as the centromeres of the
chromosomes split simultaneously. Each
chromatid now becomes a chromosome in its
own right.
The kinetochore microtubules, moved along
by motor proteins in the kinetochores,
gradually pull each chromosome toward the
pole it faces.
At the same time, the polar microtubules slide
past each other, lengthen, and push the two
poles of the cell apart.
The moving chromosomes look V shaped.
The centromeres lead the way, and the
chromosomal “arms” dangle behind them.
• Mo
ving and separating the chromosomes is
helped by the fact that the chromosomes are
short, compact bodies. Diffuse threads of
chromatin would trail, tangle, and break,
resulting in imprecise “parceling out” to the
daughter cells.
The two centrosomes are at opposite poles of
the cell.
The chromosomes cluster at the midline of
the cell, with their centromeres precisely
aligned at the
equator
of the spindle. This
imaginary plane midway between the poles is
called the
metaphase plate
.
• Enzymes act
to separate the chromatids from
each other.
101
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