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Index >> Gene Mutation >> Extragenic Suppression OR Intergenic Suppression

Extragenic Suppression OR Intergenic Suppression

	Extragenic Suppression OR Intergenic Suppression - If the deleterious effects of a mutation in a gene are overcome by a mutation in another gene, the process is called extragenic or intergenic suppression. In the strict sense the term suppressor mutation refers to intergenic events only, and not intragenic events. The essential feature of intergenic suppression is that the interacting mutational events take place in two separate genes. These two genes may even be located on different chromosomes.
The termination of polypeptide chain synthesis is brought about by the termination codons (VAA, VAG or VGA). A mutation which converts a codon specifying an amino acid into a termination codon (nonsense mutation) results in the formation of an incomplete polypeptide chain. Such chains are usually inactive. The effect of a nonsense mutation can be suppressed by mutations in other genes (intergenic suppression). Such suppressor mutations result in viable proteins
Altering the anticodon of a tRNA molecule is one method of suppressing the effects of nonsense codons. One of the codons for glutamine (Gin) is CAG. This codon is recognized by the anticodon GUC of Gln-tRNA. The glutamine codon CAG may undergo mutation to become VAG (CAG----->UAG) which is a termination codon.
This codon does not specify any amino acid and results in polypeptide chain termination. The incomplete chain formed is usually inactive. The effect of the nonsense codon VAG can be suppressed by mutations in other genes. The normal anticodon for tyrosine tRNA is 3' AUG 5'. A suppressor mutation can convert this anticodon to 3' AUC 5' through a G -->C substitution
	This mutated tyrosine tRNA anticodon can recognize the nonsense codon VAG as a codon for tyrosine. Thus tyrosine is added to the chain instead of glutamine. The mutant protein is active. In this case the suppressor gene functions by producing a tRNA that reads a termination codon. (Normally no tRNA has an anticodon that can be read by a termination codon). The type of suppression mutation mentioned above can only be selected if the tRNA which has undergone mutation is not essential for synthesis

E. coli has two different tyrosine tRNAs which recognize both tyrosine codons UAU and UAC. One tRNA species continues to recognize the two codons, while the mutated species reads the AUG termination codon as a codon for tyrosine.

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Suppressor Genes Frameshift Suppressions Molecular Basis of Mutations Spontaneous Mutations Tautomerism Induced Mutations Chemical Mutagens Base Analogues Agents Modifying Purines and Pyrimidines OR Agents Labilize the Bases Agents Producing Distortions in DNA Radiations Types of Mutations

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