Question

Explain the “one gene – one enzyme hypothesis” and describe the experiments that led to the development of this theory.

Answer 1

Explanation:

1. It was first discovered by Beadle and Tatum 1941

2. They concluded that each gene control single specific biochemical reaction.

3. Genes control chemical reaction by controlling production of specific enzymes that catalyses these reactions.

4. Thus one-gene-one-enzyme hypothesis states that each gene specifies a single specific enzyme.

Drawbacks of this hypothesis:

1. All genes do not produce enzymes

2. All enzymes are proteins but not all proteins are enzymes. Thus ther came to existence, One-gene-one-polypeptide hypothesis.

Experiment A

Arginine biosynthesis pathway of E. coli

1. One of the first clear cut proof of this hypothesis came from arginine biosynthesis pathway of E. coli.

2. This Pathway consist of 8 biochemical reactions, each of which is catalyzed by specific enzymes.

3. In return, production of each enzyme is controlled by a specific single gene

4. All reactions are explained here.

5. here, as we can see, each reaction is controlled by single enzyme, which is controlled by the single gene.

6. Here, above, pathway for arginine biosynthesis in Ecoli. Mutants lacking each of the eight enzymes are known

7. Absence of each enzyme is due to single and distinct mutant gene.

Experiment B

Beadle and Tatum experiment

In 1948, Beadle and Tatum proposed that a gene controls the synthesis of one enzyme.

(a) Beadle found that the red eye colour of Drosophila melanogaster is controlled by two genes due to brown and vermillion pigments.

(b) In 1944, Beadle and Tatum irradiated Neurospora crossa with X-rays and obtained a number of nutritional mutants called auxotroph’s.

(c) An auxotroph or nutritional mutant is that mutant which is not able to prepare its own metabolites from the raw materials obtained from outside.

(d) Auxotroph requiring citrulline possesses ornithine but no arginine.

(e) When citrulline is supplied, the auxotroph comes to have arginine. The nutritional mutant requiring arginine contains both ornithine and citrulline.

(f) It seems that arginine is synthesized from ammonia and sugar of the minimal medium through at least three steps each requiring its own enzyme.

(g) Beadle and Tatum explained that defective enzymes are due to defective or mutant genes. Hence, genes express their effect by controlling the synthesis of enzymes.