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In this article we will discuss about the gene action in drosophila.
Beadle and Ephrussi (1937) studied metabolic pathway for eye colour in Drosophila. There are three eye colours in the fruitfly, viz. wild, vermillion and cinnabar.
Procedure:
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The imagine disc or primordia was transplanted from one larva on to the abdomen of other larva. Reciprocal eye transplantation was made among wild, vermillion and cinnabar larvae. The eye colour was examined when the larvae, became adult.
Results:
In all cases transplanted eye developed into normal wild type except when cinnabar eye was transplanted to the abdomen of vermillion larvae which was cinnabar.
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Explanation:
They explained their results on the basis of biochemical pathway which is from tryptophan to ommochrome. Inactivation of enzyme at any biochemical step leads to mutant eye colour. Two types of pigments are found in the eye of Drosophila, viz. pterins and ommochromes.
The eye colour develops as given below:
(i) Pterin leads to development of bright red eye.
(ii) Ommochrome produces brown eye colour.
(iii) When both are present, the eye colour is wild.
(iv) When the conversion of tryptophan to kynurenine [step 1] is bocked, there is accumulation of tryptophan which produces vermillion eye.
(v) When the conversion of kynurenine to 3-hydroxy kynurenine [step 2] is bocked, there is accumulation of kynurenine which produces cinnabar eye-colour.
(vi) When there is no blockage, normal eye-colour (wild type) will develop. Inactivation of enzyme at any biochemical step will lead to mutant eye-colour i.e. vermilloin or cinnabar.
In 1945 American Scientists George Beadle [a geneticist] and Edward Tatum [a biochemist] proposed the one gene-one enzyme hypothesis working with pink bread mould [Neurospora crassa]. The hypothesis states that each gene determines the structure of a specific enzyme which in turn controls a single biochemical reaction.
Later on this theory was modified to the concept that one cistron controls the production of one polypeptide. This “one gene-one enzyme” concept won Beadle and Tatum (with Lederberg) the Nobel Prize in 1958. Mutation of a single gene alters the chemical reaction governed by such gene.
Experiments of Beadle and Tatum opened up new field of research with several useful applications. The work of Sir Garrod received importance only after publication of the work of Beadle and Tatum on Neurospora in 1941. Since then biochemical genetics has become popular field of genetic investigations.
Biochemical Pathway in Neurospora:
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Beadle and Tatum studied biosynthetic pathway of arginine in Neurospora. They reported that biosynthesis of arginine involves several sequential steps and each step is controlled by specific enzyme. The end product of one biochemical reaction serves as a precursor for the next.
Mutation of gene at three different steps will lead to the following situations:
(i) Mutation of gene responsible for the production of enzyme E3 will block the conversion of precursor into ornithine. Such mutations can grow in culture medium containing either ornithine, citrulline or arginine amino acids.
(ii) Mutation of gene which controls the production of enzyme E4 will block the conversion of ornithine into citrulline. Such mutations can grow in medium supplied either with citrulline or arginine.
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(iii) Mutation of gene responsible for the production of enzyme [E5] will inhibit the conversion of citrulline into arginine. Such mutations require arginine for their normal growth. Thus, it is obvious that the substance which is produced after the blockage is required for normal growth of the mutant.
