Hugo de Vries used the term mutation for the first time to describe the phenotypic changes in evening primrose, Oenothera lamarckiana. Mutation refers to any heritable change in nucleotide sequence of a gene of the organisms. A gene codes for a protein and so 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. Mutation is the result of stable and heritable changes in nucleotide sequence of DNA.
If the mutational changes occur in a second gene, it eliminates or suppresses a mutant phenotype which is called suppression or intergenic reversion. Suppressor- 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. These mutations are called as point mutations or gene mutations. 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 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 as non- sense mutation. For example, UAC codes for tyrosine. If it undergoes base substitution (C-G), it becomes UAG (termination codon). This results in synthesis of incomplete polynucleotide chain which remains inactive. 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.
Missense mutations 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. A missense mutation occurs by insertion, deletion or substitution of a single base in to a code, for ex. 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: i. a mutant tRNA may recognize two codons by a change in anticodon loop, ii. A mutant tRNA can be recognized by a wrong aminoacyl synthetase and be misacylated, and iii. a mutant synthetase can change a wrong tRNA molecule. Missense mutation play an important role in providing new variability in organisms and driving the evolution because they are not lethal and remain in the genome.
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. 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 phenotypic characters.
Frameshift mutation arises from insertion or deletion of one or two base pairs within the coding region of the gene. It is also a gene mutation. It is very deleterious and yields mutant phenotypes resulting from the synthesis of non-fonctional 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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