Mitosis

Mitosis is a kind of cell division in which a somatic cell divides to form two daughter cells which have same chromosome number as in mother cell. So, the daughter cells are both qualitatively and quantitatively similar to the mother cell. It is also called somatic cell division and equational cell division. In plants, it occurs in meristematic cells of root and shoots apex. In animals, it commonly occurs in skin cells, bone marrow cells as well as during development of an embryo.

Phases of mitosis

It consists of two phases;

• Interphase
• Dividing phase

The dividing phase of mitosis consists of two more phase i.e.karyokinesis and cytokinesis.

Karyokinesis

It is the division of nucleus which occurs in four stages i.e prophase, metaphase, anaphase, telophase.

Prophase

It is first and the longest phase which can be divided into early and late prophase.

Early prophase

• Chromatin fibres condensed into chromosomes by dehydration and coiling.
• A chromosome is thin thread-like and they overlapped each other giving an appearance of a wollen ball and this stage is called spireme stage.
• Each chromosome consists of two chromatids which join at the common centromere.
• Nuclear membrane and nucleolus start to disappear.
• In an animal cell, centrioles start to move towards poles.

Late prophase

• Chromosomes become shorter and thicker by further condensation.
• Nuclear membrane and nucleolus disappear completely.
• In the animal, cell centrioles reach the opposite poles.
• Spindle apparatus is organised between two poles. Spindle apparatus is of two types; ancestral and amphiastral.

Aster is a star-shaped structure consisting of centrioles and spindle fibres.

Types of spindle fibres

They are two types;

• Tractile fibres
• Polar fibres

Tractile fibres

They are discontinuous fibres which extend from one pole to the equator. They attached with the centromere of the chromosomes.

Polar fibres

Polar fibres are continuous fibres which extend from one pole to another pole and they do not hold any chromosomes but instead, give stability to the spindle apparatus.

Metaphase

• All the chromosomes get arranged at the equator in a single metaphase plate which is formed by the centromere of chromosomes.
• The process of drawing all the chromosomes onto the equator is called congression.
• At metaphase, only the centromere lie at the equator, whereas their arms are directed towards poles or outside.
• The centromere of each of the chromosome is joined by two tractile fibres coming from coming from opposite poles.
• Chromosomes are shortest and thickest at metaphase.

Anaphase

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Anaphase

• It is the shortest phase of karyokinesis.
• The centromere of each of the chromosome divides and the chromatids are separated.
• The newly separated chromatid are pulled towards the pole by the contraction of tractile fibres.
• During the movement of chromosomes from the equator to the poles, the centromere moves ahead and the arms come behind.
• At anaphase chromosomes appear in the different shape like v, u, l, j depending upon the position of the centromere.
• The anaphase finally ends when all the chromosomes, chromatids reach the respective opposite poles.

Significance: It results in the equal distribution of the chromosomes into the daughter cell.

Telophase

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Telophase

• It is the last phase of karyokinesis which is characterised by the changes that occur in reverse sequence as that of prophase.
• Chromosomesdecondensed into chromatin fibre by the process of uncoiling and hydration.
• Nuclear membrane and nucleolus reappear.
• Spindle fibre or apparatus disappear.
• Two nuclei are formed at the end.
• Phragmoplast appears in between two nuclei and is followed by cytokinesis.

Cytokinesis

It occurs by two methods they are;

• Cell plate method
• Cell cleavage method

Cell plate method

This method occurs in the plant cell. In this method, small vesicle-like bodies called phragmoplast appear in the equatorial region between two daughter nuclei. Gradually phragmoplast fused together and form a cell plate in the centrifugal direction. The cell plate divides cytoplasm into two parts.

Cell cleavage method(Furrowing or Constriction method)

This method occurs in an animal cell. In this method, small construction or furrow appears at the side of the cell. Then this furrow or construction dipped inward in centripetal direction dividing cytoplasm into two halves.

Significance of Mitosis

• It helps in growth and development of organisms.
• It brings genetic stability by maintaining chromosome number constant.
• It helps in regeneration and repairing of tissues.
• It helps in healing of wounds.
• It maintains the nucleocytoplasmic ratio.
• It helps in asexual reproduction.

Uncontrolled mitosis results in the formation of tumour or cancer.