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| Gene Metabolism Relationship |
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| The concept that genes have the information to produce enzymes which are proteinaceous in nature was proved by George Beade and Edward Talium in the early 1940s with their work on the fungus Neurospora crassa. |
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| This mould can normally grow in a simple minimal medium containing salts and sugar, making all other chemicals such as amino acids, purines, pyrimidines etc., through enzymecatalysed reaction. |
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| Beadle and Talium used X-rays to induce mutations in the mould and these mutants could not grow on a minimal medium. |
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| Each type of mutant required some extra nutrient and as such the nutritional mutants are called auxotrophs. |
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| Beadle and Talium obtained different nutritional mutants requiring the amino acids ornithine, citrulline or arginine for growth. |
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| In the cell, ornithene is converted into citrulline which is in turn converted into anginine, with each step being catalysed by an enzyme. |
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| Beadle and Talium reasoned that the mutant requiring onnithine for growth is defective in the enzyme producing onnithine as a result of a gene mutation. Similarly, a defect in another gene may lead to a defect in the enzyme catalysing the conversion of ornithine to citrulline and therefore this mutant would require citrulline for growth. |
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| Therefore, each gene probably has the information to produce one enzyme and thus, was born the one-gene one enzyme hypothesis. |
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| Two ideas soon modified the one gene-one enzyme hypothesis. |
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| Firstly, while nearly all enzymes are proteins, all proteins are not enzymes. Secondly, many protein are made up of subunits called polypeptides, with each distinct polypeptide under the control of a gene. |
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| Example: The adult haemoglobin consists of 4 polypeptides: 2 and 2 . A separate gene codes each type. |
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| This led to the re-evaluation of Beadle and Taliums hypothesis into “one gene one polypeptide”. |
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| fig. 22.1 Beadle and Tatums Experiment on Neurospora crassa |
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| In the above figure, Type 1 mutants have defective gene A and lack enzyme A, hence need ornithine, citrulline or arginine for growth. Type 2 mutants have defective gene B and lack enzyme B, hence need cirtulline or arginine for growth. Type 3 mutants have defective gene C and lack enzyme C, hence need arginine for growth. Wild type grows in minimal medium and does not need any additional material for growth. |
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