WHAT IS CELL DIVISION ?
Cell division produces new cells from pre-existing ones in order to help in growth, replacement, repair and reproduction in all living organisms.
These characters are transmitted from parent cell to the daughter cells in the form of genes which are located on chromosomes. These chromosomes (the hereditary materials) are present in the nucleus of the cell.
Every cell capable of divisions passes through recurring events called the cell cycle. The sequence of events including duplication of DNA, synthesis of other cell constituents, growth and division, that a cell undergoes from the time of its formation upto its division into daughter cells is called cell cycle.
Duration of cell cycle i.e. ‘duration between two successive cell divisions is called generation time. A cell cycle consists of the following two phases i.e.
1. Interphase (Non-dividing phase)
2. Mitotic phase (Dividing phase)
1. Interphase :
(a) G1-Phase (First Growth Phase or Post-mitotic phase) :
G1 is the longest-phase of interphase. In this phase the cell is metabolically active. The RNA and proteins are synthesized and cytoplasm increases hence cell grows in size.
(b) S-Phase (Synthesis phase) :
Synthesis of DNA takes place and the chromosomes get duplicated (make its copy).
(c) G2-Phase (Second Growth Phase or Pre-mitotic phase) :
This is a shorter growth phase in which synthesis of DNA stops. However, formation of RNA and proteins, necessary for cell division, continues. It prepares the cell to undergo mitotic phase ( the next cell devision ) and thus the cell cycle goes on.
2. Mitotic Phase :
It is the dividing phase of the cell. During this phase the nucleus as well as the cytoplasm divides into two daughter cells, undergoing four important stages. These are Prophase, Metaphase, Anaphase and Telophase. After Telophase the new cells formed can again carry on with the cell cycle and this process goes on and on.
The cell cycle cannot go on endlessly. At some places it stops permanently like in brain and nerve cells, once formed in the embryo do not divide further. Once dead, they are not replaced.
Production of cells and their death in our body : In children, the new cells produced are more than the cells that die, hence, they grow. In adults the new cells produced are almost equal to the cells that die, hence, the cell population stays constant.
What are the two types of cell division ?
The cell division is of two types:
1. Mitosis ( First type of cell division )
Mitosis is a type of cell division during which a cell divides into two daughter cells, each containing similar and same the number of chromosomes as present in the parent cell.
This type of cell division takes place in somatic or body cells during growth, development and repair of an organism, hence, also known as somatic cell division.
The following three important events take place during interphase :-
(i) The cell grows to the same size as their mother cell.
(ii) RNA and proteins synthesized and volume of cytoplasm increases.
(iii) DNA synthesis takes place and chromosomes get duplicated.
During interphase, chromosomes exhibit a minimum degree of condensation or coiling but cannot be distinguished individually. There is an increase in volume of interphase nucleus as well as nucleolus. The cell is quite active metabolically.
Since DNA synthesis occurs, therefore, no alternative is left for the cell except to divide by entering the mitotic phase. At the end of interphase, series of changes start in the physical and chemical organization of the chromosomes.
The process of mitosis is studied in two parts
(A) Karyokinesis (Division of nucleus)
All the changes in nucleus that occur during cell division are collectively termed as Karyokinesis. During karyokinesis many events take place in a particular sequence.
All these events are studied in the following four stages :
1. Prophase : (Pro = First)
(i) It begins with the shortening and thickening of chromosomes. These chromosomes are clearly visible inside the nucleus.
(iv) The two asters start moving towards the opposite poles. By the end of prophase they reach at the opposite poles.
(v) Between the two asters develop very fine proteinaceous thread like structures called spindle fibres. Asters along with spindle fibres is called spindle or mitotic apparatus. Such a spindle is called amphiastral spindle. In plants, the spindle is without asters and is called anastral spindle.
2. Metaphase : (Meta = after)
(i) The chromosomes begin to show peculiar movements. They arrange themselves between the poles, on the equator of the spindle.
3. Anaphase : (Ana = back)
(i) Each centromere splits into two daughter centromeres, each retaining one of the two chromatids. The structure so formed are now known as sister chromatids or daughter chromosomes.
4. Telophase : (Telo = end)
(i) Each sister chromatid (daughter chromosome) which reaches at the opposite poles starts uncoiling, thinning and elongating. They form a network like structure called chromatin reticulum.
The process of karyokinesis is followed by cytokinesis.
(B) Cytokinesis ( Division of cytoplasm )
Division of the cytoplasm is known as cytokinesis. In plant cells, cytokinesis takes place by the formation of cell plate.
Identification points of Mitosis:
(1) Prophase :
(i) Each chromosome with two chromatids.
(ii) Disappearance of nuclear membrane.
(iii) Disappearance of nucleolus occur during Mitosis.
(iv) Formation of the spindle.
(2) Metaphase :
(i) Chromosome arranged at equator as equatorial plate while Metaphase.
(3) Anaphase :
(i) Splitting of each chromosome into two daughter chromosomes.
(ii) Movement of daughter chromosomes towards opposite pole.
(4) Telophase :
(i) Uncoiling of chromosomes,
(ii) Disappearance of spindle.
(iii) Reappearance of nucleolus and nuclear membrane.
Significance of Mitosis:
Mitosis has a manifold significance, such as:
- Maintenance of chromosome number (Genetic stability) : Mitosis keeps the number of chromosomes equal and constant in all the body cells of an organism.
- Maintenance of cell size : Mitosis helps to maintain the size of the cell constant. When a cell is fully grown up, it divides by mitosis producing two similar daughter cells.
- Growth : Every multicellular organism be it an alga, a worm, a mango tree or human being, starts its life as a single cell-the zygote. To produce billions or trillions of cells as in a whale or an elephant, this cell undergoes repeated mitotic divisions.
- Replacement : Every minute, about 3 billion cells die in our body. The same number of new cells are produced by cell division in the same period to replace the dead cells for survival.
- Repair : Apart from the normal wear and tear of the tissues in our body, there may be accidental injuries. These injuries, such as, fractures, cuts in the skin, surgical operations etc. are healed up due to addition of new cells by the divisions of adjoining cells of the injury.
- Asexual reproduction : In unicellular organisms like Amoeba, young ones are produced by the division of the cell that constitutes their body.
- Regeneration : The lost parts may be organs or tissues in certain organisms, are re-formed by the division of cells. For instance, the tail broken by a wall lizard in self defence is re-formed within few days by the division of the cells from where the tail is broken.
- Embryonic development : In living organisms the process of development of embryo takes place by mitosis.
2. Meiosis ( Second type of cell division )
The term meiosis was coined by Farmer and Moore in 1904. It is a special type of cell division that produces sex cells or gametes.
Since the number of chromosomes in this type of division is reduced to half in the daughter cells, hence, it is also called reductional cell division.
Meiosis as a reduction division :
These are as follows :
I) Meiosis-I (Reductional division)
II) Meiosis-II (Equational division).
I) Meiosis-l (Reductional division) :- During meiosis-I or First meiotic division, a diploid cell (2n) divides into two haploid (n) daughter cells.
Homologous chromosomes :- The chromosomes which are exactly similar in shape and size and bear same genes at same loci are called homologous chromosomes.
II) Meiosis-Il (Equational division) :- Meiosis-II division is much similar to mitosis. It is known as equational division, because it maintains the number of chromosomes received by the daughter cells at the end of meiosis-I.
Significance of Meiosis :
(i) Reproduction : Meiosis takes place during the formation of gametes or sex cells. It takes place inside the reproductive organs (testis and ovary) in animals to produce sperms and ova.
- What is cell division ?
- Cell Cycle
- What are the two types of cell division ?
- significance of Mitosis
- Significance of Meiosis