In this article we will discuss about:- 1. Meaning of Mutation 2. Types of Mutation 3. Classification.
Meaning of Mutation:
Mutation is the random, minute and irreversible, measureable change in the gene of an organism or a cell, with the necessary implications that any phenotypic manifestation of the change may potentially or actually be shown by all descendant organisms or cells.
Mutation in somatic cells is of the same general quality as in germ cells. Somatic mutation is associated with the functional activities of a cell type while germ cell mutation is responsible for introducing variations in the offspring’s.
Possible causes of mutation are cosmic radiation, atomic radiation, and different stresses, viz., chemicals, heat, ultraviolet radiation, etc. are mutagenic (mutation inducing).
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Mutations are always random and non-directional. They are mostly lethal or deleterious but some mutations are useful when organisms are placed in a changed environment Environmental changes disrupt the adapted-ness of an organism and the harmony between the environment and the organism can be restored only through a genotypic reconstruction by mutation.
Mutation and selection occur simultaneously. Most frequently selection works against mutation and the rate of change, if the mutations are not lethal, will be very slow. But if a particular mutation is advantageous to the organism, and mutation rate is appreciable, the combined action of mutation and selection might well cause a rapid change. It is believed that mutations are the raw materials for evolution.
Types of Mutation:
Mutations are of two types:
a. Spontaneous mutation. These occur in nature in animals or plants not subjected to any artificial stress.
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b. Induced mutation. These are produced when the organism is subjected to different mutagens, viz., X-rays, gamma- rays, chemicals, etc.
Stages of occurrence of mutation:
Mutations may take place in different stages in the life of an organism and also at different locations.
a. Gametic:
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It occurs prior to the maturation of gametes.
b. Zygotic:
It appears immediately after the fertilization of egg.
c. Somatic:
It occurs in somatic cells as a result of which the cells produced by the multiplication of the mutant cells become genotypically different from the non-mutant cells.
Classification of Mutation:
Mutational changes can be grouped into two major classes. The mutation proper is caused by chemical alteration of individual genes, and chromosomal aberrations leading to a grosser structural change involving destruction, multiplication or spatial arrangement of the genes in the chromosomes.
The chromosomal aberrations may be of the following types:
I. Numerical changes:
Affecting die number of chromosomes.
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a. Haploidy:
Higher organisms for the major part of their life are mostly diploid. Under experimental conditions, some diploid organisms have produced haploid aberrant with a single set of chromosomes in the cells instead of the normal double sets.
b. Polyploidy:
Normally diploid organisms in some case may produce forms having more than two sets of homologous chromosomes.
II. Structural changes:
Affecting the arrangement of the genes in the chromosomes.
A. Changes due to reduplication or loss of some genes.
a. Duplication:
In addition to its normal location a section of the chromosomes may be present elsewhere. In the salivary gland cells of Drosophila certain sections of the normal chromosomes are represented two or more times in the haploid set.
b. Deletion (deficiency):
A section of the chromosome containing one gene or a block of genes is lost resulting in a deficient chromosome.
B. Changes due to altered arrangement of genes.
a. Translocation:
Two chromosomes may exchange parts giving rise to a new chromosome.
b. Inversion:
Due to a rotation of 180° the arrangement of the genes in the chromosome may be altered.