Result Frequency of mutation

Result of mutation

The mutation causes the change in the structure of an organism. In most of the cases, such changes are undesirable and non-functional. The mutation results in the change in the sequence of nitrogenous bases in DNA which results in the change in the codon. Change in codon leads to the formation of different polypeptide chain which may or may not has desired function. Mutation in haemoglobin -A producing gene results information of haemoglobin-S. Due to haemoglobin-S, the shape of RBC changes to a sickle shape. The mutation also causes the formation of termination codon from normal codon. In this case, there is a formation of the incomplete polypeptide chain.

Frequency of mutation

The frequency of mutation is different in a different gene. In some organisms, the mutation occurs at a high rate. Some genes are so stable that they hardly mutate. Such genes are called stable genes. Genes which mutate easily are called, mutable genes. Bacterial gene mutates 1 in 10⁸ and a particular locus in Drosophila mutates in order of 1 in 1,00,000 to 2,00,000 flies. In man, genes associated with muscular dystrophy are estimated to mutate once in 10⁵ people. Emerson found that the R gene (controls anthocyanin) mutates more frequently than others, at the rate of 50 per 1,00,000 gametes.

Mutation in haploid organism

The mutation occurs in both diploid as well as haploid organisms. Diploid organisms have two sets of chromosomes. Each gene has two alleles. The homozygous organism has similar allele (either AA or aa) and the heterozygous organism has different alleles (Aa) so if the mutation occurs in the homozygous organism having dominant alleles, it might not be detected because AA mutates into Aa and dominant allele An interact with mutant allele a. Dominant alleles dominate action of mutant allele a.

But, haploid organisms have only one set of the chromosome so each gene has only one allele. In this case, if allele, which is dominant in the diploid zygote, undergoes mutation (suppose from A to a), mutant allele behaves as if it were a dominant alleles. As a result, mutant allele expresses and give changed character. So, in haploid organism mutation can be detected easily. An example can be taken from the mutation in Neurospora. Neurospora is the haploid fungus. It belongs to ascomycetes can synthesize all 20 kinds of amino acid and vitamins from minimal medium but mutant form cannot grow in the absence of one of them.

Neurospora is cultured in the simple medium having inorganic salts + glucose (as carbon source) + biotin. Such medium is also called minimal medium. If Neurospora is subjected to mutagen such as X-rays, the mutation occurs in few spores. Such obtained spores are cultured again in minimal medium. Some of the spores fail to grow in minimal medium because of mutation. such spores are called biochemical mutants. If such mutant spores are cultured in minimal medium amino acid + vitamins, they grow. From this, it is concluded that mutant spores are defective in synthesis of either amino acids or vitamins. If mutant spores grow in the minimal medium having amino acid, it means they are defective in amino acid synthesizing genes. If not, they have defected in haploid organisms.

Role of mutation in evolution

The mutation may be harmful or beneficial. A beneficial mutation is a raw material for evolution. As mutation is sudden and large change, offspring become distinct to the parents. The Bigger mutation could be more important, but in a majority of the cases, the smaller changes are more important for evolution. It is because if a genotype which is well adjusted into an environment is too drastically altered, it will not be able to adjust. Small changes in the genotype of an organism over a period of time results speciation (origin of new species).

Role of mutation in gene pool

Mutation is one of the factors which influence gene frequency in a population. A change of an allele at a locus is known as mutation. This alteration in the nature of the gene is a heritable change, the thus frequency of allele changes in subsequent generations.

Although mutations are rare events, they are continually occurring and bringing about changes in gene frequency. Mutations add new alleles to the gene pool and increase genetic diversity. The mutation, in fact, may change the allelic frequencies in a gene pool and also provide new genetic information, thus increasing the genetic variability in the gene pool and provide the raw material for evolution.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.