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.
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.
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.
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.
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.
