Ultraviolet-induced mutation and the chemical nature of the gene.

T EXTBOOKS tell us that it was generally believed that genes were made of protein until the demonstration by AWRY, MACLEOD and MCCARTY (1944) that the material that caused bacterial transformation was DNA. Most textbooks fail to note that studies done several years earlier on mutations induced by ultraviolet (W) radiation also indicated that DNA had genetic activity. And it is often forgotten that neither the W experiments nor the identification of the transforming principle really convinced scientists that genes were made of DNA. The first convincing demonstration of the mutagenic action of W was made in Drosophila by EDGAR ALTENBURG (1934). His close colleague, H. J. MULLER, had shown that X-rays induced mutations in Drosophila, but that natural sources of ionizing radiation could account for only a small fraction of the spontaneous mutations (MULLER and MOTT-SMITH 1930). Therefore, ALTENBURG set about testing other kinds of natural radiation for a possible role. UV seemed to him a good candidate. ALTENBURG first attempted to monitor UV-induced mutation by treating mature males and scoring sexlinked recessive lethals by MULLER’S ClB procedure. He immediately ran into trouble administering the W to the testes, because so much of the radiation was absorbed by the superficial tissues. (Perhaps this should have forewarned him that W could not be responsible for much of the spontaneous germinal mutation in animals.) ALTENBURG (1934) adopted a procedure developed by GEIGY (1931) for irradiation of Drosophila eggs at the polar cap end; these pole cells are destined to give rise to the germ cells. With this material, ALTENBURG was able to show highly significant frequencies of recessive lethal mutations resulting from W treatment. However, most of the further work on UV-induced mutations was done, not on Drosophila, but on other forms whose naked germ cells were readily accessible: pollen cells, fungus spores, and bacteria. During the late 1930s, three separate investigators set up experiments to determine the action spectrum of UV-