The following points highlight the three types of point mutation. The types are: 1. Non-Sense Mutations 2. Missense Mutation 3. Silent Mutation.
Type # 1. Non-Sense Mutations:
Non-sense mutation is one type of point mutation. There are 64 codons that code for amino acid out of which three codons (UAA, UAG, UGA) are known as termination codons that do not encode for any amino acid.
If any change occurs in any codon, it brings about changes in amino acids which specify an amino acid to termination codon. This process is called non-sense mutation. For example, UAC codes for tyrosine. If it undergoes base substitution (C-G), it becomes UAG i.e. a termination codon.
This results in synthesis of incomplete polynucleotide chain which remains inactive. Only a fragment of wild type protein is produced which has a little or no biological function unless the mutation is very near to the carboxyl terminus of the wild type protein.
The non-sense mutations bring about drastic change in expression of phenotypic characters because in this mutation the structure and function of enzymes are changed.
Type # 2. Missense Mutation:
Missense mutation is the second type of point mutation. When one amino acid in a polypeptide chain is replaced by the other amino acid, this type of mutation is known as missense mutation. For example, if a protein valine (non-polar) has been mutated to aspartic acid (polar) due to loss of activity; it can be restored by the wild type phenotype by a missense suppressor that substitutes alanine (non-polar) for asparatic acid.
A missense mutation occurs by insertion, deletion or substitution of a single base into a code, for example the codon GAG specifying glutamic acid could be changed to GUG which codes for valine. Missense mutation that arises from substitution synthesizes proteins which differ from the normal protein by a single amino acid.
Substitution occurs in three different ways:
(i) A mutant tRNA may recognise two codons perhaps by a change in anticodon loop,
(ii) A mutant tRNA can be recognised by a wrong aminoacyl synthetase and be mis-acylated, and
(iii) A mutant synthetase can change a wrong tRNA molecule.
However, if a suppressor that substitutes alanine for aspartic acid works with 20% efficiency, every protein to which a cell synthesizes at least one aspartic acid is replaced. In this situation a cell probably cannot survive.
In-spite of substitution of a single amino acid many proteins are still functional. This depends on type and location of amino acid. For example, if a non-polar amino acid in the polypeptide chain is replaced by a polar amino acid, it will drastically change the three dimensional structure of the protein and also change the function.
But if the polar amino acid is replaced by the another, there will be little or no effect on protein. Missense mutation plays an important role in providing new variability in organisms and driving the evolution because they are not lethal and remain in the genome.
Type # 3. Silent Mutation:
Silent mutation is another type of point mutation which could not be detected until the nucleic acid sequencing is done. Any change in gene does not affect the phenotypic expression because the code is degenerate i.e. more than one code specify an amino acid. For example, if the codon CGU is changed to CGC, still it would code for arginine.
Similarly, both AAG and AAA specify alanine. If the codon AAG is changed to AAA, the latter codon will still code for lysine even after change in base sequence of DNA. This mutation is of silent type because even after change in base sequence of DNA, there is no change in the amino acid sequence and expression of phenotype characters.