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Mitosis |
Stephen
M. Wolniak |
|
|
Professor |
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Department
of Cell Biology &
Molecular Genetics |
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| University
of Maryland |
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| College
Park, Maryland 20742 |
|
An
Introduction to Mitosis
Mitosis
is the process that facilitates the equal partitioning
of replicated chromosomes into two identical groups.
Before partitioning can occur, the chromosomes must
become aligned so that the separation process can
occur in an orderly fashion. The alignment of replicated
chromosomes and their separation into two groups
is a process that can be observed in virtually all
eukaryotic cells.
Both
the alignment and separation processes are the consequence
of the chromosomes interacting with filamentous proteinaceous
structures, known as microtubules. The microtubules
become organized into a biconical array known as
a spindle, which forms early in mitosis, and then
disassembles as mitosis nears completion. Mitotic
spindles are visible in living cells with the polarizing
light microscope. Some of the spindle microtubules
become attached to the chromosomes at sites known
as kinetochores. The kinetochores cannot be seen
with the light microscope, but they reside near the
place on the chromosome known as its centromere,
which can be observed with the light microscope.
There are two kinetochores on each replicated chromosome
(one on each chromatid), and when the replicated
chromosome splits apart at its centromere at the
onset of anaphase, each daughter chromosome possesses
one centromere and one kinetochore. The linkages
between kinetochores and microtubules are thought
to be central in controlling both the positioning
of the replicated chromosome at the central portion
of the spindle during the alignment phase, and in
moving the daughter chromosomes apart after they
split at their centromeres. The separation of daughter
cells from each other is a process known as cytokinesis,
and is separate from mitosis. In cytokinesis, animal
and plant cells differ considerably from each other.
These differences are the consequence of having or
not having a cell wall. Cytokinesis in fungi reveals
some similarities with plant cells, and exhibits
other features unique to the group.
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The
purpose of this page is to show interested individuals
how mitosis occurs in a stamen hair cell of the spiderwort
plant, Tradescantia virginiana. The stages of
mitosis are highlighted in a single cell, and the time/date
generator information on each image provides the viewer
with a sense of how long each step actually takes.
The
stages of mitosis: a tour with pictures.
| Prophase |
During
prophase, the replicated chromosomes undergo
extensive condensation (i.e., coiling).
The chromosomes are greatly thickened and
shortened but are still contained within
the nuclear envelope. Late in prophase, within
about 6 min of nuclear envelope breakdown,
the mitotic spindle begins to grow, and two
triangular 'clear zones' become visible,
with one on each side of the nucleus. In
three dimensions, the clear zones are actually
conical and the nucleus is spherical. With
continued spindle expansion, the nucleus
rapidly becomes compressed and appears eliptical
in the optical section provided by differential
interference contrast optics. Prophase ends
with the sudden dispersion of the nuclear
envelope (nuclear envelope breakdown), and
the chromosomal mass is no longer occupying
a discrete, spherically-shaped zone in the
cell. Prophase in stamen hair cells can last
for as long as several hours. It can also
be observed by downloading the movie. |
| Prometaphase |
Once
the nuclear envelope has broken down, the
spindle microtubules and the chromosomes
are no longer separated by a (double) membrane
boundary. The microtubules begin to interact
with the chromosomes, and the chromosomes
undergo what is known as congressional movement,
where they ultimately end up with their centromeres
all situated in middle of the spindle, at
a site known as the metaphase plate. Each
kinetochore of the replicated chromosome
is pointed toward one side of the spindle;
later, in anaphase, each kinetochore moves
to one of the two spindle pole regions as
the daughter chromosome The congression of
chromosomes and the alignment of centromeres
on the metaphase plate represent essential
prerequisites for the orderly separation
of the replicated genome into two equal parts.
The mechanisms underlying congression are
under intense scrutiny. |
| Metaphase |
The
replicated chromosomes converge toward the
center of the spindle, and once they get
there, significant movements cease. On either
side of each centromere are sites for microtubule
attachment to the chromosome; electron microscopists
called these plate-like structures kinetochores.
The kinetochores are not visible with the
light microscope. At several points during
metaphase, the chromatid arms may unwind
from each other. This unwinding is especially
apparent late in metaphase, just 1 or 2 minutes
before the chromatids will split apart at
their centromeres, with each replicated chromosome
giving rise to two daughter chromosomes.
The interval between nuclear envelope breakdown
and anaphase onset includes prometaphase
and metaphase, and requires approximately
33 minutes. |
| Anaphase |
Anaphase
commences with the initial splitting of sister
chromatids at their centromeres. These daughter
chromosomes then begin to separate from each
other, each moving away from the metaphase
plate and toward one of the two spindle pole
regions. The rate of chromosome separation
in stamen hair cells is approximately 1.4
micrometers/min. The mechanisms that control
chromosome separation clearly involve the
interactions between microtubules and components
in or near the kinetochore. Anaphase chromosomal
movement is a topic of intense interest in
the Cell Biology research community. |
|
Telophase/Cytokinesis
- Cell
Plate Vesicle Aggregation
- Plate
Coalescence
|
About
19 minutes after anaphase onset, the chromosomes
have moved close to the spindle pole regions,
and the spindle midzone begins to clear.
In this middle region of the spindle, a thin
line of vesicles begins to accumulate. The
vesicle aggregation event is a harbinger
to the assembly of a new cell wall that will
be positioned midway along the length of
the original cell. It will form the boundary
between the newly separating daughter cells.
Vesicles movement and aggregation in the
spindle midzone is facilitated by a microtubule
network known as a phragmoplast. This basket
shaped structure forms in late anaphase or
early telophase and disassembles about the
time that the vesicles begin to coalesce. |
Top
Technical
Details
The cell
was imaged with differential interference contrast
microscopy, with a 40X 0.85 NA objective and a 0.63
NA condenser. (This particular cell was actually
part of an ongoing set of microinjection experiments
that are aimed at discerning the regulatory steps
that lead to anaphase onset. The pointed object that
is present at the bottom of the field of view is
a micropipette. In the experiment, the cell was impaled
and loaded with a small quantity of a phosphorylated
peptide that acts as a protein phosphatase substrate.
The cell did not exhibit a change in its metaphase
transit time as a consequence of this injection.
For more information about these experiments, see
S.
Wolniak' s home page.) All of the optical components
were purchased from Carl Zeiss. The images were initially
generated with a Newvicon video camera, digitized
with a Scion LG-3 frame grabber, using the NIH Image
Program with a Power Macintosh 8100/110 computer.
Each stored image represents the average of four
video frames. The images appear essentially as they
do through the microscope; none of the images has
had any background substraction, and the levels of
contrast and brightness are comparable to those observed
through the oculars.
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