Cell Division

Different types of cell division occur in all the types of organism from unicellular to multi-cellular. Because of a kind of cell division, bodily size of organisms increases and in other type of cell division, reproductive cells are developed and in some other cases organisms multiply through binary fission. In this chapter, you will know how the different types of cell division occur.

Cell Division

Every living body is a cell or cells. Life of every organism starts with a single cell. In fact, every cell is originated from a pre-existing one. The increase of cells in number through division is a very natural and important feature. The body of some organism is composed of a single cell. They are called unicellular organisms, such as bacteria, amoebas, plasmodium etc. These organisms multiply themselves from the division of a single cell. Other organisms are composed of more than one cell. They are called multi-cellular organisms. Humans, mango and Banyan trees etc. are composed of crore of cells. A giant Banyan tree also initiates its structure after the formation of a single cell (zygote by the union of two gametes). After the fertilization of an egg, the structure of a man is accomplish with the crore of cells dividing through the process of cell division. Again, male and female gametes are produced for the emergence of new generations. Cells divide through the process of cell division for the growth and reproduction of organisms.

Types of cell division

Three types of cell division take place in living body, such as, 1. Amitosis, 2. Mitosis and 3.Meiosis

Amitosis

In this division the nucleus of a cell divides directly into two parts. At the beginning of division, the nucleus elongates with two larger rounded ends and little narrower middle part. The middle part of the nucleus narrowing continuously once splits into two daughter nuclei. By this time cell wall of the cell pinching inward divides the cytoplasm of it into two parts and, ultimately, two daughter cells are produced. This type of cell division is found in bacteria, blue green algae, yeast etc.

Amitosis cell division

Mitosis

Through this process of division, a eukaryotic cell, stage by stage, divides into two identical daughter cells. In this process, nucleus and chromosomes are divided only once and two identical daughter cells are produced with the same type of chromosomes in number, physical and structural features as their mother cell contains. Mitosis occurs in somatic cells and through this division, as the number of cells increases, the plants and animals grow in length and breadth. The process usually take place in the body cells of animals and in the meristems of the plant parts growing, such as the tip of stems and roots, plumule and radicle, developing leaves, buds etc.

Stages of mitosis

The cell division mitosis is a continuous process. During mitosis, karyokinesis is usually followed by cytokinesis. Karyokinesis and cytokinesis represent the division of nucleus and the division of cytoplasm respectively. Before the starting of the cell division, a cell has to prepare itself. This stage is called interpahse. For the ease of description, the process mitosis can be divided into five stages. They are: 1. Pro-phase, 2. Pro-metaphase, 3. Meta-phase, 4. Ana-phase and 5. Telo-phase.

• Pro-phase: This is the first stage of mitosis. At the onset of the stage, nucleus becomes little larger and chromatin fibers start condensing into short, thick and tightly coiled structures called chromosomes. At this stage, chromosome can be seen under compound microscope. Though every chromosome then divides into two sister chromatids, at the centromere each one remains together. As the chromosomes are still in a mess form, it is little tough to count the number of chromosome easily in a cell.

Pro-phase

• Pro-metaphase: At the very early of the stage, spindle apparatus having two poles is developed in plant cells from fiber protein. The middle plane of the spindle apparatus is called equator. Spindle fibers are stretched from one pole to the other. They are called spindle fibers. At this stage kinetochores in centromeres of chromosomes get attached with some fibers of spindle apparatus. Kinetochores are protein structures assembled on centromeres and link the chromosomes with mitotic spindles. These fibers are generally called traction fibers. They are also called chromosomal fiber as chromosomes are attached with them. Chromosomes then start assembling on the equatorial plane. The nuclear membrane and nucleolus being disintegrated begin to disappear. In animal cells spindle apparatus is developed from centrioles, and the centrioles remain in two poles with astral rays spreading round from them.

Pro-metaphase

• Meta-phase: At the onset of the stage, all the chromosomes complete their assembling at the equator, middle plane from the two poles. The centromere of each chromosome remains on the equator but the two arms take position towards the poles. In this stage, chromosomes look most short and thick. Attraction between chromatids decline and repulsion increases. At the late of the stage, division of centromere starts. Nuclear membrane and nucleolus disappear completely.

Metaphase

• Ana-phase: The sister chromatids become the two sister chromosomes and they are pulled apart. The cleaved centromere moves first to the pole while the chromatids trail behind. Each chromosome splitting into two moves to two poles, and so the number of chromosome remains unchanged. On the position of centromere in the chromosome, chromosomes look V, L, J or I shaped, and then these chromosomes are called metacentric, sub-metacentric, acrocentric and telocentric respectively. At the end of the stage ana-phase, the daughter chromosomes are totally pulled to the two poles and start their elongation.

Anaphase

• Telo-phase: It is the last stage of mitosis and a reversal of pro-phase. Chromosomes being decondensed and surrounded by new nuclei elongate back into thin and long structures called chromatin fibers. Nucleolus also reappears. So, ultimately, two new nuclei are formed in two poles. The spindle apparatus is disintegrated and so, spindle fibers gradually disappear. At the end of the stage telo-phase, some small parts from endoplasmic reticulum aggregate in the equatorial plane and collectively form the structure cell plate. Equal distribution of cytoplasmic organelles is accomplished. As a result, two identical daughter cells are developed. In case of an animal cell, a contractile ring on the plasma membrane at the equatorial plane pinches off the two nuclei.

Telophase

Significance of mitosis

The significance of mitosis in the living body of organisms is immense. The balance in between the nucleus and cytoplasm of a cell in terms of volume and amount is maintained by the process of cell division mitosis. Through mitosis, the growth in the body multi-cellular organisms occurs. All the multi-cellular organisms start their life from a single cell zygote. The repeated division of this single cell produces innumerable cells and thus an organism grows to its complete level. As the number and feature of chromosome remain unchanged in the cells produced through mitosis, growth in organisms takes place systematically. Mitosis plays a role in maintenance of normal size, shape and volume of cells. Unicellular organisms reproduce through mitosis. Mitosis plays an important role in the vegetative reproduction of organisms and increasing the number of reproductive cells. Mitosis is essential in the body of living organisms to form new cells continuously for growth and healing of injuries. The life span of some cells is specific and they are, accordingly, replaced through the process of mitosis. As identical cells are produced through mitosis, qualitative features in the living world remain unchanged. Mitotic errors may result in abnormal mass of cells called tumors, with or without cancer cells.

The question is why meiosis occurs. In process mitosis, the number of chromosomes in the daughter cells remains the same as that of their mother cell. Mitosis is essential for the growth and asexual reproduction of organisms. In sexual reproduction, the union of male and female gametes is required. If the number of chromosomes in the reproductive cells would be the same as the vegetative cells, the zygote would contain twice number of chromosomes than that of the vegetative cell. Supposing the number of chromosomes in a vegetative and a reproductive cell of an organism is 4. In zygote the number of chromosome would be 8, and so the new organisms will have the body cells with chromosome number 8 and that is twice that of its mother organism. If every life cycle of an organism continues that way, the number of chromosome would be doubled again and again. In the second chapter, we have learnt that chromosomes bear the genes controlling the features of organisms. If the number of chromosome is increased life cycle after life cycle, the offspring will be basically different. In sexual reproduction, even through the union of male and female gametes, the number of chromosomes remains the same generation after generation as the number of chromosomes becomes half in the reproductive cells than that of the mother cell. When at the time of the development of reproductive cells and in any stage of the life cycle of plants in lower groups, the process meiosis occurs, then the feature of possessing half number of chromosome is called haploid. When the haploid cells are fused, the state of cell having two sets of chromosome is called diploid.

Meiosis

As the meiosis cell division occurs, the features in the species of living organisms keep on the same more or less, generation after generation.

Meiosis mainly occurs in the primordial germinal cells during the development of gametes. In the anthers and ovules of flowering plants, and in the testes and ovaries of animals meiosis occurs. During the development of pollen's from diploid pollen mother cells in mosses and ferns, meiosis occurs in their zygotes.

Concept of meiosis cell division

Two consecutive divisions occur with a cell in the process meiosis. The first and the second division are called meiosis-1 and meiosis-II respectively. In the first division meiosis I, the number of chromosome in a daughter cell becomes half than that of its mother cell, and the second division is simply mitosis.

Because of meiosis, the chromosome number in organisms remains constant. So generation after generation, the number of chromosomes remains the same in the cells of the body of offspring. Besides, genetic diversity is also found in species of organisms as the exchange of genes occurs during meiosis.

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