Mitosis//equational cell division// mitotic cell division//Cytokinesis

 

CELL DIVISION OR CELL REPROUCTION

 

Part:1  Mitosis or equational cell division or mitotic cell division

 

Firstly cell division was studied by Prevost and Dumas, when they described cleavage in zygote of frog.

Cell division is the process by which cell divides or reproduce to form new daughter cells.

Cell division is the means of reproduction in unicellular organisms.

In almost all organisms which reproduce sexually, the cell division is of two types-

A. Mitosis

B. Meiosis

 

A.  Mitosis or equational cell division or mitotic cell division

It was first discovered by Strasburger in 1870 in plant cells and in animal cell it was first discovered by W. Fleming in 1879.

The term mitosis was given by W. Fleming in 1882.

It occurs in somatic cells example; Meristematic cells of root and stem tip, skin cells, bone marrow etc.

Mitosis is a kind of division, in which two daughter cells formed have the same number of chromosomes as that of the parent cell.

Mitosis is completed in two phases-

A.   Karyokinesis or Nuclear division  (division of nucleus)

B.   Cytokinesis or  Cytoplasm division  (division of cytoplasm)

 

1.     Karyokinesis or Nuclear division:

                                                              Karyokinesis is the division of nucleus.

It has four phases-

                   i.            Prophase

                 ii.            Metaphase

              iii.            Anaphase

              iv.            Telophase

 

1.Prophase:

 

Prophase is the longest phase.

During early prophase nuclear membrane, nucleolus and other cell organelle start to dissociate.

Chromatin gets highly condensed to form chromosome.

At the late prophase, each chromosome consists of two chromatids.

 In animal cell, Centrosome divides into two centrioles which move to opposite poles and form spindle fibres or astral rays.

At the end of prophase, cell do not show Golgi complexes, Endoplasmic  reticulum, and nuclear envelope.

 

2.Metaphase:

 

In metaphase, chromosomes arrange at equatorial plane or metaphase plate.

Each centromere is joined by two chromosomal spindle fibres one from each pole.

Chromosomes are shorter at metaphase and so this is the best phase to study chromosomes.

 

3.Anaphase:

 

It is the shortest phase.

Two chromatids of a chromosome get separated from each other by the splitting of the centromere.

These separated chromatids start moving towards  the poles.

In anaphase chromosomes appear ‘V’ shaped, ‘L’ shaped, ‘J’ shaped or ‘I’shaped.

 

4.Telophase:

 

Telophase is the last phase of karyokinesis.

It involves changes just opposite to prophase.

At each pole, around the chromosomes, nuclear membrane, nucleolus, Golgi complexes, Endoplasmic reticulum reformed.


      

                                  Various stages of mitotic division

 

B.  Cytokinesis or Cytoplasm division:

 

Cytokinesis is the division of cytoplasm.

The process of cytokineis is different in plants and animals.

In animals cell, cytokinesis is occur by the appearance of a furrow in the plasma membrane.

The furrow gradually deepens and ultimately joins in the entre dividing the cell cytoplasm into two.

In plant cell, cytokinesis takes place by cell plate formation that represents the middle lamella between the walls of two adjacent cells.

 

      Significance of Mitosis:

 

1)    A very significant contribution of mitosis is cell repair.

2)    In lower organism, it is the main method of asexual reproduction example, by budding, fission etc.

3)    It keeps the chromosome number constant and maintains genetic stability in daughter cells.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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