Mitosis

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Mitosis is a type of cell division that occurs in the somatic (body) cells. A diploid (2n) parent cell gives rise to two diploid (2n) daughter cells.

For the purpose of description mitosis is divided into four stages:

• Prophase

• Metaphase

• Anaphase

• Telophase

It is important to understand that mitosis is a continuous process and there is no sharp break between one stage and the next. Before mitosis takes place the cell undergoes a so-called resting stage called Interphase. During this stage the cell has the same appearance as any non-dividing cell. The chromosomes are strung out in the form of long chromatin threads. Calling interphase as a resting stage would be a misnomer. The cell in this stage is far from inactive, but is growing and preparing for division.

Two things happen during interphase. First, the DNA doubles itself so that sufficient DNA is made available for each of the two daughter cells. A cell never divides until this new genetic material is formed. Secondly, the cell builds up a sufficiently large store of energy to carry out the process of cell division. New cytoplasmic organelles also are formed during interphase-mitochondria, ribosomes, chloroplasts etc. One of the most prominent organelles to double itself during mitosis is the centriole. Although plants do not have centrioles, the polar caps perform the same function. Interphase prepares the cell for prophase.

Prophase

The following changes take place during prophase:

• The nucleus enlarges

• Chromatin threads condense into visible, short and thick threads called chromosomes

• Chromosomes occur in sets of two of each kind. Each chromosome is made up of two identical threads called chromatids. The sister chromatids are connected at a particular point called centromere.

• The nucleolus and nuclear membrane disappear

• The centrioles, which have also divided into two pairs, move in opposite directions towards the poles of the nucleus

Metaphase

After the centrioles reach their positions corresponding to the two opposite poles, the chromosomes arrange themselves upon the equator of a spindle, which is formed in the cell by delicate cytoplasmic threads radiating from the centrioles. Each sister chromatid is attached at its centromere to a spindle fibre.

Anaphase

• Beginning at the centromere, the sister chromatids undergo mutual repulsion and are seen to split longitudinally into two halves. Each sister chromatid is now called a daughter chromosome

• The daughter chromosomes separate and begin to move towards opposite poles. The spindle fibres contract and pull the chromosomes towards the poles.

Telophase

• At each pole, the gathering chromosomes become less distinct as they merge into a network

• The nucleolus reappears at each pole. A newly formed nuclear membrane encloses the chromatin network. Thus two daughter nuclei are formed. The number of chromosomes in each daughter nucleus is the same as the number in the parent cell.

• The cytoplasm starts dividing (cytokinesis). The cell starts to constrict across the middle. The constriction deepens from outside towards the centre, and finally two daughters cells are formed.

• Cell organelles are equally distributed between the two daughter cells

In the plant cell, no constriction appears during cytokinesis. Instead, the division of the cytoplasm occurs by the formation of a new cell-wall in the middle region. During telophase, new cellulose particles are gradually deposited along the equatorial region. These particles fuse together to form a delicate plate membrane, dividing the cytoplasm into two new cells.