Genes Mutations

In most organisms genes are segments of DNA molecules. In the broad sense the term ‘mutation’ refers to all the heritable changes in the genome, excluding those resulting from incorporation of genetic material from other organisms. A mutation is an abrupt qualitative change in the genetic material of an organism. Mutations may be intragenic or intergenic. Intragenic mutations or point mutations include alterations in the structure of the DNA molecule within a gene.  Intergenic mutations, of which chromosomal changes in structure are examples, involve long regions of DNA, i.e. many genes. These include deletion or additions of segment of chromosomes, resulting in deficiency and duplication respectively.

Random Vs. Directed Mutations. It is usually stated that mutations occur in a random manner. By this it is meant that they are not directed according to the requirements of the organism. According to this theory mutations would have to be directed towards some objective.

Rate of mutation. It should be noted, however, that most methods for estimating the rate of mutations tend to underestimate their frequency due to many reasons. Firstly, lethal mutations which leave no progeny may be missed. Secondly, mutations which leave only a slight change in the phenotype may remain undetected.
Mutations occur much more frequently in certain regions of the gene than in others. The favored regions are called ‘hot spot’.

Effects of mutations on the phenotype. According to their effects on the phenotype mutations may be classified as lethal, sub vitals and super vitals. Lethal mutations result in the death of the cells or organisms in which they occur. Subvital mutations reduce the chances of survival of the organism in which they occur. Subvital mutations reduce the chances of survival of the organism in which they are found.

How does a mutation act? Genes act by controlling the rate of production of specific proteins (enzymes). The scheme of protein synthesis in most organisms is as follows. (1) The DNA (gene) produces a complementary mRNA strand which has condons consisting of nucleotide triplets. (2) tRNA molecules, each forming a complex with a specific  amino acid, have three free nucleotides which from the anticodon. (3) The alignment of tRNA molecules on mRNA depends upon complementary codon-anticodon pairing. (4) Thus the sequence of amino acid molecules in an enzyme depends upon the nucleotide sequences in DNA.