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Chapter: 11 th 12th std standard Bio Botany plant tree Biology Higher secondary school College Notes

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Gene mutation : Significance, Classification, Mutagenic agents

In a species, variations are caused by changes in the environment or any changes in the innate genetic setup of an organism or by the combination of both. Sudden change in the genetical set up of an organism is defined as mutation. In 1901, Hugo de Vries first used the term mutation based on his observation on Oenothera lamarckiana.

Mutation

 

In a species, variations are caused by changes in the environment or any changes in the innate genetic setup of an organism or by the combination of both. Sudden change in the genetical set up of an organism is defined as mutation. In 1901, Hugo de Vries first used the term mutation based on his observation on Oenothera lamarckiana. Charles Darwin termed these sudden change as 'sports'. According to Bateson, mutation is a discontinuous change. Based on molecular basis of heredity, mutation is defined as sudden change in the sequence of nucleotides of gene. The mutation brings about a change in the organism. The organism which undergoes mutation, is called a mutant. eg. Oenothera lamarckiana.

 

Mutations that affect the biochemical reactions are called biochemical mutations. For example, biochemical mutants of Neurospora failed to synthesize certain amino acids. Some mutations drastically influence the genes and cause death to the individual. Such mutation is described as lethal mutation. For example, in the plant Sorghum, recessive mutant fails to produce chlorophyll and therefore they die in the seedling stage. Thus, most of the mutations are harmful, because they disturb the genic balance of the organism. Although most of the mutations are useless and even harmful, and some of the mutations play a significant role in the evolution of new species. Many new strains of cultivated crops and new breeds of domesticated animals are the products of gene mutations. Small seeded Cicer arietinum (bengal gram) suddenly get mutated to large seeded Cicer gigas is the case of gene mutation.

 

Classification of mutation

 

Mutations have been classified in various ways based on different criteria. Depending on the kind of cell in which mutations occur, they are classified into somatic and germinal mutation. They may be autosomal or sex chromosomal according to their type of chromosome in which they occur. They may be spontaneous or induced according to their mode of origin. They may be forward or backward according to their direction. They may be dominant or recessive according to their phenotypic expression of mutated genes.


Point or gene mutation

 

Point mutation is sudden change in small segment of DNA either a single nucleotide or a nucleotide pair. Gross mutation is a change involving more than one or a few nucleotides of a DNA.

 

The gene mutation may be caused by loss or deletion of a nucleotide pair. This is called deletion mutation and reported in some bacteriophages. Addition of one or more nucleotides into a gene results in addition mutation. Replacement of certain nitrogen bases by another base in the structure of DNA results in substitution mutation. The deletion and addition mutation alter the nucleotide sequence of genes and ultimately result in the production of defective protein and this leads to the death of the organism. The substitution mutations can alter the phenotype of the organism and have great genetic significance.

 

There are two types of substitution mutations - transition and transversion. When a purine or a pyrimidine is replaced by another purine or pyrimidine respectively this kind of substitution is called transition. When a mutation involves the replacement of a purine for pyrimidine or viceversa this is called transversion.

 

Mutagenic agents

 

The chemical substances and environmental conditions which cause mutations in the organisms are called mutagens or mutagenic agents. There are two kinds of mutagenic agents - physical and chemical mutagenic agents.

 

Physical mutagenic agents

 

Electromagnetic radiation, radiations like a, b and g, ultraviolet rays, temperature, etc. are some of the examples for physical mutagens. X-rays and gamma rays are ionizing radiations which induce mutation in seeds. UV rays are nonionizing radiations. Pollen can be treated with UV since pollen has germinal nucleus in which mutation can be caused.

 

Chemical mutagenic agents

 

Chemicals can also be used for inducing mutations in the organisms. Such chemicals are called chemical mutagenic agents. eg. Nitrous acid, Methyl methane sulphonate (MMS) and ethyl methane sulphonate (EMS). Ethyl methane sulphonate has been extensively used for inducing mutations in microorganisms, higher plants and animals.

 

Significance of mutation

 

1.        Mutations play an important role in the origin of new species and serves as a tool for evolution.

 

2.        Induced mutations are useful in agriculture, animal husbandry and biotechnology to produce new strains. For example, mutant strains of Penicillium produces more penicillin.

 

3.        It is one of the best approaches for improvement of crops.

 

4.        Induced mutants are reported in paddy, wheat, soyabeans, tomatoes, oats, and barley. Mutant varieties of wheat are early maturing, disease resistance and they are enriched with protein. Mutant varieties of paddy produce many tillers with long grains.

 

5.        The study of mutant strains of viruses helps us to know the fine structure of gene. The genes are made up of small functional units such as cistron, recon and muton. Cistron is an unit of function, recon is the unit of recombination and muton is the unit of mutation.

 

6.        Many types of mutations cause heritable diseases and cancer in human beings.

 

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