What is a Gene? One – Gene One Polypeptide Hypothesis

What is a Gene?

Archibald Garrod and William Bateson concluded in 1902 that certain diseases among their patients were more prevalent in particular families. By examining several generations of these families, Garrod discovered that some of the diseases acted as if they were the product of basic recessive alleles. He concluded that these conditions were Mendelian traits and that they had aroused from modifications in the hereditary information in the forefather of the affected families.

Garrod examined several of these disorders in detail. In alkaptonuria, the patients produced urine that contained homogentisic acid. This substance oxidized rapidly when exposed to air, turning the urine black. In normal people, homogentisic acid is broken down into simpler substances. With significant insight, Garrod concluded that patients suffering from alkaptonuria did not have the enzyme needed to catalyze this breakdown. He hypothesized that lots of other inherited diseases might also reflect enzyme deficiencies.

From Garrod’s finding, it could be inferred that the details encoded within the DNA of chromosomes acts to define particular enzymes. This point was not in fact established, however, until 1941, when a series of experiments by Stanford University geneticists George Beadle and Edward Tatum provided definitive evidence on this point. Beadle and Tatum deliberately set out to produce Mendelian mutations in chromosomes and after that studied the effect of these anomalies on the organisms.

Beadle and Tatum exposed Neurospora spores to X-rays, expecting that DNA in some of these spores would experience damage in the areas encoding the capability to make substances required for normal growth. DNA modifications of this kind are called mutations and the organisms that have actually undergone such modifications are called mutants.

Initially, they permitted the progeny of the irradiated spores to grow on a defined medium containing all of the nutrients required for growth, so that any development deficient mutants arising from the irradiation would be kept alive.

To identify whether any of the progeny of the irradiated spores had anomalies causing metabolic deficiencies, Beadle and Tatum positioned subcultures of individual fungal cells on a “minimal” medium which contained only sugar, ammonia, salts, a couple of vitamins, and water. Cells that had actually lost the ability to make other compounds necessary for growth would not survive on such a medium. Utilizing this method, Beadle and Tatum were successful in recognizing and separating lots of growth deficient mutants.

Next, the scientists included different chemicals to the very little medium in an effort to find one that would make it possible for an offered mutant strain to grow. This procedure permitted them to determine the nature of the biochemical deficiency that strain had. The addition of arginine, for instance, allowed several mutant strains, called arg mutants, to grow. When their chromosomal positions were located, the arg anomalies were found to cluster in three areas.

One – gene/ one – polypeptide Hypothesis

For each enzyme in the arginine biosynthetic pathway, Beadle and Tatum were able to separate a mutant strain with a defective type of that enzyme, and the mutation was always located at one of a couple of specific chromosomal sites. Most importantly, they found there was a different site for each enzyme.

Hence, each of the mutants they examined had a flaw in a single enzyme, brought on by a mutation at a single site on one chromosome. Beadle and Tatum concluded that genes produce their effects by defining the structure of enzymes which each gene encodes the structure of one enzyme. They called this relationship one – gene/ one – enzyme hypothesis.

Because many enzymes consist of several protein or polypeptide subunits, each encoded by a different gene, the hypothesis is today more frequently referred to as “one gene/ one- polypeptide”.

Enzymes are accountable for catalyzing the synthesis of all the parts of an organism. They are also responsible for the assembly of nucleic acids, proteins, carbohydrates, and lipids. Therefore, by encoding the structure of enzymes and other proteins, DNA specifies the structure of the organism itself.

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