Mitosis: (Cell Division
via Mitosis)
This page describes the context of mitosis - explaining
its position in the series of processes that, together, form the
"cell cycle"
for somatic cells (cells relating to the non-reproductive
parts of the body) .
The four stages of mitosis - prophase, metaphase, anaphase and
telophase - are also listed and described.
For an illustration
of this process see the page - diagram
of mitosis.
Background:
This follows the page about an introduction
to cell division.
Reminder: Mitosis is defined as the
type of cell division by which a single cell divides in
such a way as to produce two
genertically identical "daughter cells".
This is the method by which the body produces new cells
for both growth and repair of aging or damaged tissues
throughout the body - as opposed to for sexual reproduction
(when meiosis applies).
Mitosis is the simplest of the two ways (mitosis and meiosis)
in which the nucleus of cells divide - as part of a process of
cell
division. The context in which mitosis occurs during the "cell
cycle" is explained as follows:
The Stages of the "Cell Cycle" for Somatic Cells
In all somatic cells (that is, all cells relating to the non-reproductive
parts of the body = all cells except for those of the
gametes) the "cell cycle" consists of two periods:
- Interphase (also
known as "interkinesis")
is the period in which the cell is not dividing.
This does not mean that little is happening as interphases
are very active periods during which cells perform
all the functions necessary for life, and also synthesise
DNA (deoxyribonucleic acid) so that both
of the new cells formed by the miotic phase will contain
a complete copy of the original, and so have everything
necessary.
and
- Miotic phase (M)
- when the cell is dividing.
The miotic phase of the "cell cycle" consists of two stages:
- Mitosis
Mitosis is the division of the cell nucleus, and is followed
by:
- Cytokinesis
Cytokinesis is the division of the cytoplasm of the cell
into two daughter
cells.
Mitosis (Nuclear Division)
0.
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Interphase
Interphase is not part of mitosis but is included here
as a reminder that interphase preceeds
mitosis.
(Hence, it has the number 0.)
Note: Chromatin is a material
located in the nucleus of a cells and resembling a thread-like
mass. It exists in the form called "chromatin" when
the cell is not dividing but forms chromosomes when the
cell divides. Chromatin consists of DNA and protein.
It can be stained with dyes in order to watch the process of mitosis using a
light microscope. |
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1. |
Prophase
- Early in the prophase stage the chromatin fibres shorten
into chromosomes that are visible under a light microscope. (Each
prophase chromosome consists of a pair of identical double-stranded
chromatids.)
- Later in prophase, the nucleolus disappears, the nuclear
envelope breaks down, and the two centrosomes begin to
form the miotic spindle (which is an assembly
of microtubules).
- As the microtubules extend in length between the centrosomes,
the centrosomes are pushed to opposite "poles" (extremes)
of the cell.
- Eventually, the spindle extends between two opposite
poles of the cell.
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2. |
Metaphase
Metaphase is characterized by the "metaphase
plate". This
is a mid-point region within the cell that is formed/defined
by the centromeres of the chromatid pairs aligning along
the
microtubules
at the centre of the miotic spindle. |
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3. |
Anaphase
- The centromeres split seperating the two members of each
chromatid pair - which then move to the opposite poles
of the cell: When they are seperated the chromatids
are called chromosomes.
- As the chromosomes are pulled by the the microtubules
during anaphase, they appear to be "V"-shaped because the
centromeres lead the way, dragging the trailing arms of
the chromosomes
towards the pole/s.
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4. |
Telophase
- Telophase begins after the chromosomal movement stops.
- The identical sets of chromosomes - which are by this
stage at opposite poles of the cell, uncoil and revert
to the
long, thin, thread-like chromatin form.
- A new nuclear
envelope forms around each chromatin mass.
- Nucleoli
appear.
- Eventually
the miotic spindle breaks-up.
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... then the cytoplasm begins to divide around the two new nuclei:
Cytokinesis (Cytoplasmic Division)
Cytokinesis is the process by which the cytoplasm
of the original cell forms the two new ("daughter") cells around
the two new ("daughter") nuclei formed by the process of mitosis
(or meiosis - cytokinesis being a part of both types of processes
of cell division).
In the case of animal (as opposed to plant) cells, a cleavage
furrow forms around the cell's equator then constricts as a ring
until it cuts completely through the cell.
When cytokinesis is complete, interphase begins (see further up
this page). This begins the next "cell cycle".
Now see the page illustrating a diagram
of mitosis, and also read the page about cell division
via meiosis.
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