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Dates: during 1950-1959
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When Muller made this discovery, he may have heard a roll of distant thunder, but he could not have known what it meant. In the year 1926, long before Hiroshima, no man-made radioactivity was at large on earth outside the range of X-ray machines and radium capsules, and none was expected. No one suspected that in less than 20 years the mutation-producing effects of radiation would be a worldwide worry...
...wealth supplied by Mullers X rays gave genetics a big boost, and Beadle felt the benefit along with his colleagues. After getting his doctorate (in genetics) at Cornell in 1931, he went to the California Institute of Technology on a National Research Council fellowship. Dr. Morgan, grand maestro of the fruit flies, had moved there in 1928 to head the biology section, and several of his keenest disciples had come with him. Young Dr. Beadle found himself in the best genetic society...
Mutated Mold. The Beadle and Tatum plan for Neurospora was to try to create strains that differ from the normal mold in simple, chemical ways. Their method was simple, too. They irradiated mold with X rays to induce mutations. Then they gathered spores formed by sexual reproduction and laid them out on a sheet of agar jelly containing the minimum nutrients that natural wild mold requires. Some of the spores sprouted and grew normally, showing that they had not been mutated in any obvious way. Some were dead, perhaps mutated too much...
This was what Beadle had been hoping for. His explanation is that the gene damaged by X-ray violence was originally responsible for producing an enzyme (organic catalyst) needed in the mold's process of making vitamin B-6 out of simpler nutrients. With the gene out of action, the process stopped, and the mold could not grow without help. It was like a human diabetic who needs an external source of the insulin that his body cannot make...
...that year, James D. Watson and Francis H.C. Crick in England went a step farther. DNA, they said, is a double helix with two spirally rising chains of linked atomic groups and a series of horizontal members, like steps, connecting the two spirals. This molecular model, deduced mostly from X-ray diffraction photos, seemed complex and unlikely, but geneticists rejoiced when they heard about it. It was just what they" needed to explain many perplexing things that they had been observing for years (see diagram...