Definition of Histology
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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.

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.

   

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 mitotic spindle (which is an assembly of microtubules, which are components of the cytoskeleton).
  • 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.
   

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.

   

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.
   

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.

... 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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Definition of Histology

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