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History of Genetics - Fine Structure Of The Gene

Science EncyclopediaScience & Philosophy: Heterodyne to Hydrazoic acidHistory of Genetics - Gregor Johann Mendel, Rediscovery, The Chromosome Theory Of Heredity, Culture Of Heredity, Fine Structure Of The Gene

Fine Structure of the Gene

Morgan's introduction of the fruit fly to genetics revolutionized it because the fly's rapid life cycle and minute size enabled the scale of experimentation to be markedly increased. Contrast the whole year required between generations of peas and corn with the two weeks needed for the fruit fly. This meant that in a short time over a hundred characters had been studied and many mutants found. Drosophila became as a result the most prominent "model organism" of genetics. Bridges oversaw and maintained the growing stock of the mutant types and made them freely available internationally. The lab at Columbia, known as the "Fly Room," was an example of team effort, led by a genial, exuberant boss. Morgan had to undergo quite a conversion by his team, but the outcome was a giant step forward in genetics, crowned with the award of the Nobel Prize in 1933.

H. J. Muller was less close to Morgan than the others and did not long remain in the group. Their views on genetics differed. Whereas Morgan was happy to leave to one side the question of the material basis of the gene, Muller wanted to know the answer. His pioneer work on the production of mutations by X rays not only won him the Nobel Prize but offered him the hope of establishing the size of the gene. This approach was used by the brilliant Russian geneticist N. V. Timoféeff-Ressovsky, in Germany, to yield an estimate of the "sensitive volume" of the gene as that space needed by one thousand atoms, or about the size of an average protein. Unfortunately, as later work revealed, the methodology and interpretation of this experiment proved faulty.

Drosophila was by no means the only model organism for genetics. In addition to commercial cereal crops, poultry, mice, and yeast, the bread mold Neurospora figures prominently in the development of the field. Using this organism, George Beadle and Edward Tatum concluded that there is a 1:1 relation between a gene and a given enzyme, thus suggesting that the primary product of a gene is an enzyme. But for the fine structure analysis of the gene the model system that was to bring the analysis down to the molecular level was the viral infected colon bacillus (Escherichia coli). Here the bacterial virus (bacteriophage, or phage) has just one chromosome, and in mixed infections this chromosome can recombine with one from another, thus permitting recombination and making fine structure mapping possible. By 1957 Seymour Benzer had used this system to make an estimate of the likelihood of crossing-over between two mutants one DNA base apart in the bacteriophage T4 to be 1 in 10,000. His own analysis had then reached 1 in 20,000.

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