Mitosis

Mitosis is divided into 4 phases or stages:

Prophase
Metaphase
Anaphase
Telophase

These phases refer to the changes taking place in the nucleus.

The nucleus divides first and then the whole cell divides. Division of one nucleus to produce two daughter nuclei a is called karyokinesis. Division of cytoplasm to give two daughter cells is called cytokinesis.

Prophase

It shows three subphases:

(i) Early prophase

(a) Centriole divides and each of the two centrioles start moving towards opposite poles of the nucleus of the dividing cell.
(b) Chromosomes appear as long threads, and start coiling.
(c) Nucleus enlarges and becomes less distinct.

(ii) Middle prophase

(a) Chromosome condensation is complete and they become short and thick.
(b) Each chromosome is made up of two chromatids held together at their centromeres.
(c) Each chromatid contains newly replicated daughter DNA molecule.

(iii) Late Prophase

(a) Centrioles reach the opposite poles of the dividing cell.
(b) Some spindle fibres extend from pole to the equator of the dividing cell.
(c) Nuclear membrane disappears.
(d) Nucleolus is not visible.

Metaphase

(a) Chromosomes are brought towards the equator of the cell, with the help of spindle fibres.
(b) Each chromosome becomes attached to the two spindle fibres by centromere whereas each centromere is joined to the opposite poles.
(c) The sister chromatids are not yet separated because the centromere has not divided.

Anaphase

(a) Centromeres of all the chromosomes divide and then each chromatid becomes a chromosome.
(b) Spindle fibres contract and pull the centromeres to the opposite poles.
(c) As the chromosomes are pulled by spindle fibres to opposite poles by their centromeres, they acquire various shapes such as V, J or I depending upon the position of centromere.
(d) Half the number of chromosomes move towards one pole and the other half to the opposite pole.
(e) Cytokinesis begins as the cleavage furrow starts from the periphery towards the centre in animal cells, and in plants, cell plate appears in the centre that grows centrifugally towards periphery.

Telophase

(a) Chromosomes uncoil to form a chromatin network as in the parent nucleus.
(b) New nuclear membrane is formed around each daughter nucleus.
(c) Nucleolus reappears again in each newly formed daughter nucleus.

Cytokinesis

It is the process of the division of cytoplasm of a dividing cell into two. It is initiated in the beginning of telophase and is completed by the end of telophase. The mechanism of cytokinesis is different in plant and animal cells.

In an animal cell, invagination of plasma membrane proceeds from the periphery of the cell towards the interior. In plant cell phragmoplast (cell plate) begins to form in the centre of cell and then expands towards the periphery.

Siginificance of Mitosis

It is an equational division, and the two newly formed daughter cells are identical in all respects. They receive the same number and kind of chromosomes as were in the mother cells.

It is the only mode of reproduction in unicellular organisms.
It is the process by which growth takes place in multicellular animals and plants by constantly adding more and more cells.
It also plays a role in repair during growth, for example in wound healing, regeneration of damaged parts (as in the tail of lizard), and replacement of cells lost during normal wear and tear (as the surface cells of the skin or the red blood cells).

Growth by mitosis occurs in a limited or controlled manner to the extent it is required in the body. But at times due to some special cases the number of cells may increase abnormally which may cause Cancer.