CELL DIVISION
Table of Contents
Definition of cell division –
Cell division occurs for growth, development, repair and reproduction of the body.
Cell division increases the number of cells.
The nucleus plays a major role in cell division.
The chromatin material present in the nucleus takes the shape of chromosomes during cell division.
Trait lines contain genes, which are genetic units that are passed on from one generation to the next.
Type of cell division
On the basis of nuclear division, there are three types of cell division.
(1) Amitosis
(2) Mitosis
(3) Meiosis
(1) Amitosis:
In this type of cell division, the nucleus is divided into two parts by contraction.
It is not necessary that the number of chromosomes be equal in both the parts.
Such as unicellular organisms, bacteria, yeast and blue green algae.
(2) Mitosis:
In this type of cell division the nucleus is divided into two equal parts.
The number of chromosomes in each part is equal to that of the parent cell.
This division is found in somatic cells.
(3) Meiosis:
In this type of cell division, the nucleus first divides into two (having half the chromosomes) and then four cells.
In this way, the number of chromosomes in each daughter cell is half that of the parent cell.
This division is found in germ cells.
Mitosis
Mitosis was first discovered by Walter Fleming.
In mitosis, after the interphase stage, first the nucleus is divided and then the cytoplasm is divided.
The complete method of mitosis can be divided into two parts.
(a) Nuclear division (Karyokinesis) –
The nucleus divides into two daughter nuclei.
(b) Cytokinesis –
Cytokinesis divides the cytoplasm into two parts.
Karyokinesis-
The process of cell division is in the following stages:
(1) Prophase –
Chromatin becomes in the form of coiled threads which are called chromosomes.
Chromosome is made up of two identical filaments which are called chromatids.
Nuclear membrane and nucleus gradually start disappearing.
The centriole divides and the two daughter centrioles move towards opposite poles.
(2) Metaphase-
In this stage the amorphism of the chromosome is studied.
Chromosomes get arranged in the equatorial region.
Spindle fibers begin to form.
In this stage, two chromosomes are formed from each chromosome.
(3) Anaphase –
In this stage the chromosomes start moving from the equatorial region towards the poles due to repulsion.
In this situation the shapes of chromosomes become clearly visible.
Spindle fibers disappear as the chromosomes approach the poles.
(4) Telophase-
In this stage the chromosomes reach the poles.
Chromosomes begin to increase in length and become coiled.
The central membrane and nucleus become clearly visible.
Spindle fibers begin to be destroyed.
And from one nucleus two offspring become nuclei.
Cell division (Cytokinesis) –
occurs through different methods in plants and animals.
(i) By cell furrow –
In animal cells, a furrow is formed in the plasma membrane on the midline of the cell.
This furrow becomes so deep that it divides into two cells.
Every two daughter cells are formed.
(ii) By cell plate –
In plant cells, microscopic granules gather in the middle of the spindle and form a strip.
A wall of cellulose is formed on both sides of this strip.
In this way two daughter cells are formed.
Significance of Mitosis
(4) The number of chromosomes in mitosis is equal to the number of parental chromosomes.
(2) This division is necessary for regeneration and repair.
(3) Mitosis is important for the growth and development of organisms.
(4) This division is the basis of growth in all organisms.
(5) Chromosomes are the carriers of heredity, that is, genetic traits are transmitted from parents to children.
Cell cycle
There is a cycle from the coming into existence of a cell till division which is called cell cycle.
It is divided into four demands.
(1) In the first growth phase G1-
synthesis activities take place inside the cell.
Preparations are made for the synthesis of genetic material.
(2) Synthesis stage S –
Genetic material is synthesized
(3) In the second growth phase G2-
The genetic material and all the substances inside the cell are synthesized and the cell is completely ready for division.
(4) Division phase M-
The nucleus divides into four phases.
Daughter cells are formed by cytoplasmic division.
Meiosis
In sexual reproduction in plants and animals, meiosis occurs at the time of gamete formation
in which the number of chromosomes is reduced to half.
Cell division occurs twice in the process of meiosis.
In the first division, the number of chromosomes in the daughter cells is halved,
whereas in the second division, mitosis, the number of chromosomes remains the same.
In this way, at the end of mitosis, four daughter cells are formed.
Significance of meiosis cell division
(1)Due to gene exchange,
there is exchange of genes in the chromosome,
due to which there is variation in the structure of the chromosome,
which leads to variation in the offspring.
(2) Four cells are formed from one parent cell.
(3) By this division, haploid gametes are formed.
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