If the mutational changes occur in a second gene, it eliminates or suppresses a mutant phenotype which is called suppression or intergeneric reversion. This type of suppression has been studied carefully with conditional mutation which develops wild type phenotypes on certain conditions and produces mutant phenotype in other conditions. The major class of this mutation is called suppressor sensitive mutation. It acts like wild type when a suppressor molecule is present. For example, a phage mutant can grow in one strain. Supressor sensitive mutations are of two types, non-sense (chain termination) mutation and missense (amino acid substitution) mutations.
Most of the mutations affect only one base pair in a given location, therefore, these mutation are called point mutation or gene mutation. There are several types of point 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 code 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 mutations bring about drastic change in expression of phenotypic characters because in this mutation the structure and function of enzymes are changed.
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 to 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 that differs from the normal protein by a single amino acid. Substitution occurs in three different ways:1. A mutant tRNA may rceognise two codons perhaps by a change in anticodon loop, 2. A mutant tRNA can be recognized by a wrong aminoacyl synthetase and be misacylated, and 3. A mutant synthetase can change a wrong tRNA molecule. However, if a suppressor that substitute alanine for aspartic acid worked with 20% efficiency, every protein to which a cell synthesizes atleast one aspartic acid is replaced. In this situation a cell probably cannot survive.
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.
Frameshift mutation arises from insertion or deletion of one or two pairs within the region of the gene. It is also a gene mutation. It is very deleterious and yields mutant phenotypes resulting from the synthesis of non-functional proteins. If frameshift occurs near the end of the gene or there occurs a second frameshift down stream from the first and restores reading frame, the phenotypic effect would not be drastic.
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