It is a common type of cell division that occurs in all the cells of an organism. Hence, it is commonly called as somatic cell division. In mitosis, the resulting daughter cells will have the same number of chromosomes and contain the same amount of DNA, as that of the parent cell. Hence, mitosis is commonly described as equational division.

Mitosis occurs in two stages namely karyokinesis, the division of nucleus and cytokinesis, the division of cytoplasm. Just prior to karyokinesis, the cell will be in interphase.

fig. 17.2 - Stages of Mitosis

Interphase

It is the preparing phase. It is of varying duration depending on the cell type function. It is the period in which the cell carries out synthesis of organelles and increases in size. The nucleoli are prominent and actively synthesising ribosomes. Just prior to division, the DNA undergoes replication. Each chromosome exists as a pair of chromatids joined together by a centromere.

Karyokinesis

It is the division of nuclear material, represented by a sequence of events in the cell. It can be distinguished into four phases namely prophase, metaphase, anaphase and telophase.

Prophase

It is the longest stage of the division cycle. It is characterised by significant changes.

• Chromatids shorten (to about 4% of their original length) and thicken by spiralisation and condensation of DNA

• Centrioles move to the opposite poles of the cell

• Short microtubules develop, radiating from the centrioles. These are called asters

• Nucleolus gradually decreases in size and disappears

• Nuclear membrane disintegrates

• Spindle fibres appear in the cytoplasm

Metaphase

In this phase, chromosomes move to the equator of the cell.

• Pairs of chromatids become attached to the spindle fibres at their centromeres

fig. 17.3 - Stages of Mitosis

Anaphase

It is a rapid stage.

• Each centromere splits into two

• Spindle fibres pull the daughter centromeres to the opposite poles

• The separated chromatids, now called chromosomes, are pulled along with centromeres to the opposite poles

Telophase

It is the last phase of Karyokinesis.

• Chromosomes reach the poles of the cell, uncoil and lengthen to form chromatin

• Spindle fibres disintegrate and centrioles replicate

• A nuclear membrane is formed around chromosomes in each pole

• Two daughter nuclei are formed

As telophase is in progress, cytokinesis begins in the cell.

Cytokinesis

It is the division of cytoplasm. It occurs in animal cells by the appearance of a furrow in the middle of the cell. The furrow deepens and divides the cell into two. Two daughter cells are formed.

fig. 17.4 - Differences between Mitosis in Plant and Animal cells

Significance of Mitosis

Mitosis becomes significant for the following reasons.

• Mitosis forms two daughter cells which will have the same chromosome number and same genetic material as the parent cell.

• Daughter cells formed from mitosis are genetically identical to their parent cell and no variation would be introduced during mitosis. This results in genetic stability within the populations of cells derived from parental cells, as in a clone.

• The number of cells within an organism increases by mitosis and this process is called hyperplasia. It forms the basis for growth.

If mitotic division goes uncontrolled in any part of the body, it results in the formation of malignant cells. These cells continue to divide resulting in the formation of malignant tumours. This condition is called cancer.

• Mitosis is the basis of asexual reproduction in both plants and animals. This becomes the basis for vegetative propagation.

Mitosis is also responsible for repair and regeneration of the injured and lost parts of the body