MUTATION AND MUTAGENESIS

Refers to any heritable change in nucleotide sequence of a gene of the organisms irrespective of altered phenotypic expression of characters of the organisms. A gene codes for a protein, therefore, the physical and chemical properties of proteins are changed due to alteration in genes. Genes are made up of nucleotide sequences. Hence, changes in a gene refer to changes in nucleotide or nucleotide sequence. For example, if the normal sequence was ATT, the change to AAT may lead to entirely different amino acids into a polypeptide chain. Therefore, mutation is the result of stable and heritable changes in nucleotide sequence of DNA. These changes may include alteration of a single base pair of nucleotides or addition/deletion of one or more nucleotides in the coding region of a gene.

When an amino acid substitution has no detectable effect on phenotype, it is known as silent mutation. On the other hand if a bacterium carries such a mutation in the enzyme that synthesizes an essential amino acid, it can grow very slowly unless the medium is supplied with that substance. This type of mutation is called leaky mutation.

For an organism to exist with stability, it is necessary that the nucleotide sequence of its gene must not be altered to such an extent that can promote instability. Changes to some extent brings about alteration in phenotypic expression. Though changes in genetic make up is harmful to an organism, but it is necessary to generate variability in organism and contribute to the process of evolution in nature.

The process of formation of a mutant organism is known as mutagenesis. Mutagenesis occurs in an organism by two mechanisms, (i) spontaneously (ii) through physical or chemical agents. The mutation that arises all of sudden without any effort is called spontaneous mutation. The mutation that arises through induction by addition of chemicals (i.e. mutagens) or radiation is called induced mutation.