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Mitosis

MITOSIS 

This type of cell division is necessary for the maintenance and perpetuation of life. Thegrowingtissues and asexually reproducing cells divide by mitosis, where the parent cell gives rise to two daughtercells that are identical not only to each other but also to their parent cell. Thus, thegrowth of organisms occurs due to mitosis and increase in size of cells. In advanced multicellularorganisms, all the divisions from zygote (formed by the fusion of two gametes) to the adult are of mitosis type, giving rise to genetically identical cells of the individual. Generally, the organisms reproducing asexually give rise to genetically similar organisms (clones) within a species, except for variations arising due to mutations or other means, causing somaclonal variation.

Though, the cell division is a continuous process, but for convenience, it has been divided into various stages. The mitotic cell cycle has been divided into following stages:

1. Interphase: During this phase, since the level of coiling is very low, the individuality of the chromosomes is not prominent and they appear in the form of a mass, known as chromatin

During this phase, the cell prepares itself for division and it is divided into three phases

(i) G1 Phase: During this phase, the cell grows (G means growth or gap) in size and synthesizes the necessary RNAs and proteins fordivision.

(ii) S Phase: The DNA synthesis (S means synthesis of DNA) takes place during thisphase so that each chromosome, now, contains two DNA molecules still held together at centromere. Thus, though the chromosome number (the individuality of the chromosomes is not prominent and they appear as a mass, known as chromatin) remains the same, but due to DNA replication, the DNA content becomes double, e.g. if the diploid parent cell (having two sets of chromosomes) has 2n = 4 chromosomes (each chromosome consists of I DNA molecule) with DNA content = 4C, after S phase the chromosome number 2n = 4 remains the same (each chromosome, now, consists of 2 DNA molecules with 2 chromatids held at centromere) but, the DNA content becomes double (8C).

(iii) G2 Phase: Again, the cell synthesizes the proteins and RNAs required for mitosis. Thus, Gl and G2 are the resting (or gap) phases between DNA replication and the mitosis.

The Interphase varies from organism to organism, in different tissues, in different culture media, etc. from about 21-24 hours. It may be short in rapidly dividing cells.

2. Mitosis: In comparison to Interphase, the actual cell division process, Mitosis, is of short duration, about less than 1 hour. For convenience, Mitosis is again divided into four phases:

Prophase:Due to continually increasing level of coiling, the individual chromosomes become distinct as long, thin, thread-like structures and appear as mass of interwoven wool. As the prophase proceeds, chromosomes (consisting of two sister chromatids attached at centromere become more and more thicker and shorter in size), the nuclear membrane and the nucleolus start disappearing and ultimately disappear completely at the end of Prophase. In case of animals, the two centrioles, lying at right angle to each other on one side of the cell near nucleus, separate and get arranged at the two opposite poles

(ii) Metaphase:The spindle fibres start appearing and get attached to the centromere of the chromosomes.In animals, the spindle fibres (tubules) are produced from a pair ofcentrioles, lying at the two opposite poles of the cell, but in plants where centrioles areabsent, the mechanism of formation of spindle tubules is unknown. The chromosomes get arranged at the equatorial plate in the centre of the cell. At this stage, the degree of coiling is highest and the chromosomes become thickest and shortest, therefore, chromosome counting is generally performed at this stage (Fig. 5.4). [Colchicine obtained fromColchicum autumnale, inhibits the spindle fibre formation, causing polyploidy.]

iii) Anaphase:Each chromosome splits at the centromere and the two sister chromatids separate and becomes an individual chromosome.’ Due to the contraction of the spindle tubules, the two sister chromatids of each chromosome start moving towards the opposite poles and because of this pulling, they attain characteristic shapes, e.g., V -shape for metacentric chromosome (centromere in the middle), J-shaped for sub-metacentric chromosomes (centromere sub-terminal), l-shaped for acrocentric chromosomes (centromere terminal), etc. (Fig. 5.5). The thickness of the chromosomes starts decreasing and the length increasing due to uncoiling of chromosomes

(iv) Telophase:Ultimately, the two complete sets of chromosomes reach at the two opposite poles of the cell, around which the nuclear membrane is formed to form the nucleus, that contains nucleolus also. The chromosomes become thin and thread-like, as in
Prophase.

(v) Cytokinesis: After the formation of two nuclei (karyokinesis), the cytoplasm of the parent cell also divides into two daughter cells. In animals, since the cell wall is absent, cytokinesis occurs by the formation of a circular constriction (cell furrow method) in the flexible cell membrane in the middle that proceeds towards the centre of the cell, finally dividing the cell into two (Fig. 5.8).

In plants, having tough cell wall, the cytoplasm divides by the formation of a new cell wall plate in the central part of the cell that grows out gradually towards the periphery separating the parent cellinto two cells, each having a nucleus (Fig. 5.7).

Thus, each daughter cell now contains the chromosome number(e.g. 2n = 4 chromosomes)and DNA content (e.g. 4C) similar to that found in the parent cell (See Appendix-for Fig. 5.7) and (Fig.5.8).

The various stages of mitosis can be observed in rapidly dividing tissues, e.g., onion root tips. For the preparation of slides, about 5 mm long root tips are fixed in a solution of acetic acid andmethanol present in 1 : 3 ratios. These tips are then treated with IN Hel and heated at 60°C for 15minutes in a watch glass. After washing with tap water, the tips are put on a slide and stained with acetocarmine dye that colours the chromosomes. After covering the tips with coverslip, they are squashed with the thumb and warmed over the flame for a few seconds and finally observed under microscope.