Mitosis | Definition, Stages, Diagram & Significance

Definition: Mitosis is a process in which a parent cell divides into two identical daughter cells. These cells are identical to each other as well as to the parent cell. It is a type of division in which already duplicated chromosomes are distributed into two daughter cell equally, so that the both cells have the same numbers of chromosome. Mitosis is called equational division because the two daughter cells have the same number of chromosomes as that present in the parent cell.

Occurrence : Mitosis is also called somatic cell division because it occurs in the somatic cells (body cells) of the animals. It also occurs in the gonads for the multiplications of undifferentiated germ cells. In plants, it occurs in the dividing meristematic tissue and also in leaves, flowers and fruits during growth.

Mitosis | Definition, Stages, Diagram & Significance

Mechanism of Mitosis

Mitosis is an elaborate process, which involves a series of important changes in the nucleus as well as in the cytoplasm. There are two major events that occurs during mitosis.

Karyokinesis
Cytokinesis

(I) Karyokinesis:

karyon means “nucleus” and kinesis means “movement”, therefore karyokinesis is the division of the nucleus. Karyokinesis is a continuous process in which a parent nucleus divides into two daughter nuclei. There is no pause during the process but for our convenience, mitosis has been divided into four stages. Clear cut lines cannot be drawn between two stages because karyokinesis is a progressive process.

The four stages of mitosis are:

1. Prophase:

‘Pro’ means first and ‘phase’ means stage, therefore prophase is the first stage of mitosis which follows the interphase phase of the cell cycle. The following events occur during prophase:

1. The condensation of chromatin material takes place and during condensation, the DNA strands get untangled to form compact mitotic chromosomes.
2. Each chromosome appears double and consists of two coiled sister chromatids joined by a centromere. There ends are not visible in early prophase. Therefore, the chromosomes appear Like a ball of wool. It is also called spreme stage.
3. The centrioles in the animal cells, begins to move towards the opposite poles of the cell.
4. Initiation of the assembly of mitotic spindle occurs during prophase.
5. Cells at the end of prophase, when viewed under the microscope, do not so Golgi complex, ER, nuclear membrane and nucleolus.

2. Metaphase:

‘Meta’ means second and ‘phase’ means stage, hence metaphase is the second stage of mitosis. The following events occur during metaphase

1. The disintegration of the nuclear envelope marks the beginning of the metaphase. The nuclear envelope disappear, and chromosomes spread through the cytoplasm of the cell.
2. The chromosome contain two chromatids attached to each other through the centromere. The chromosomes during this stage are the thickest and shortest and hence, it is easy to study their morphology. The chromosomes condensed to assume short rod-like forms during mitosis because it is easier for short, compact chromosomes to move through the cytoplasm in anaphase than it is for long, slender, twisted interphase chromosomes.
3. Mitotic spindle formation is complete. The phenomenon of bringing the chromosomes on the equator of spindle is called Congression.
4. All the chromosomes align themselves at the equator. The plane of alignment of centromeres of all the chromosomes of the cell, at metaphase is referred as metaphasic plate.
5. The centromere joins the two sister chromatids together. The surface of the centromere which holds the two sister chromatids of a chromosome is surrounded by a small disc-shaped structure called kinetochore.
6. The spindle fibres are made up of microtubules. The kinetochores present around the centromere forms the site of attachment of these microtubules. The microtubules of the spindle fibres attached to the kinetochore during metaphase.

3. Anaphase:

Ana means up, and phase means stage. Anaphase is the phase where chromatids move towards the pole. Following events occur during anaphase:

1. The centromere which holds the two chromatid together splits and separated daughter chromatids are now referred to as chromosomes of the future daughter nuclei.
2. The spindle fibre attached to the kinetochore now shorten and daughter chromosomes begin to migrate towards the opposite poles. Formation of interzonal fibres occur.
3. During migration, chromosomes always move away from the equatorial plates. The centromere of the daughter chromosome move towards the pole and the arms of chromosomes trail it. The anaphase ends when the chromosomes reach the poles. It is the best stage to study shapes of chromosomes (V, L, J, I).

4. Telophase:

‘Telo’ means end and ‘phase’ means stage, hence, telophase is the end stage of mitosis. The following events occur during telophase :

1. The chromosomes (sister chromatids) reach their respective poles. The mitotic spindle disappears.
2. After reaching the poles, the chromosomes gradually uncoil and become thin, slender, long and loss their identity. The decondensation of chromosomes occur and finally they become indistinguishable mass and collect at the poles.
3. Nucleolus, endoplasmic reticulum, golgi bodies, and other organelles reappear in the daughter cell.
4. The nuclear envelop of assembles around the chromatin

(II) Cytokinesis:

‘Kytos’ means ‘cell’ or ‘hollow’ and kinesis means ‘movement’. Cytokinesis is the division of cytoplasm. This phase marks the end of the cell division. Mitosis is not only concerned with the division of nucleus but is also concerned with the division of the cytoplasm. The division of cytoplasm occurs during cytokinesis. After nuclear division, i.e., karyokinesis, the cytoplasm of the parent cell divide into two daughter cells, so that two daughter cells have their own nucleus and cytoplasm. The cell organelles present in the cytoplasm also distribute into daughter cells.

In some organisms, karyokinesis is not followed by cytokinesis. In these organisms, division of nucleus occurs but division of cytoplasm does not occur. As a result, large number of nuclei are present or embedded in the cytoplasm. This leads to the formation of syncytium i.e., a single cell containing large number of nuclei.

The cytokinesis occurs differently in animal cell and plant cell.

In animal cells, cytokinesis is achieved by the formation of a furrow. The furrow in the plasma membrane of the cell deepens gradually. It moves centripetally and ultimately joins in the centre dividing the cell cytoplasm into two. The constriction of the plasma membrane deepens further and finally joins and divides the animal cell into two daughter cells.

In plant cells, the cytokinesis is achieved by the formation of a cell plate. Phragmoplast is formed by golgi complex and grows centrifugally to form cell plate. A plant cell is surrounded by an inextensible cell wall, hence it undergoes cytokinesis by a different mechanism. The cell plate starts from the centre of the cell and moves or grows centrifugally outwards to meet the lateral cell wall. It, therefore, divide the cell into two halves.

Cytokinesis by cell plate in a plant cell

Table: difference between cytokinesis in Plant cell and Animal cell.

S. No.	Cytokinesis in Plant Cell	Cytokinesis in Animal Cell
1.	The division of the cytoplasm (cytokinesis) occurs by formation of cell plate.	The division of the cytoplasm occurs by formation of furrow.
2.	Cell plate formation starts at the centre of the cell and grows outward, towards the lateral walls, dividing the cell into two halves.	Furrow start at the Periphery and then moves inward, dividing the cell into two halves.

Significance of Mitosis

1. Growth: Mitosis is essential for the growth and development of multicellular organism. A fertilised egg develops into an embryo which finally form an adult by repeated mitotic division. Somatic cell of an organism are formed by Mitosis.

2. Maintenance of cell size: Mitosis maintains the size of the cell. An overgrown somatic cell is induced to divide, so that it maintains a proper surface area to volume ratio, which is essential for proper functioning of the cell. If a cell becomes large, then it enters into mitosis.

3. Maintenance of chromosome number (genetic stability): Mitosis maintains the same type and number of chromosomes into two daughter cells.

4. Repair: Mitosis is a mechanism for replacing old dead and worn out cells by the new cells.

5. Reproduction: Mitosis brings about reproduction (multiplication) in unicellular organism.

6. Healing and regeneration: Mitosis produces new cells for healing the wounds and for regeneration.