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What experiments transformed nonlethal bacteria into lethal bacteria—all thanks to DNA?
History of Nucleic Acids
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In 1928, an army medical officer, Frederick Griffith (1878–1939), was trying to find a vaccine against Streptococcus pneumoniae. In the course of his work, he found that two strains of the bacteria existed: one had a smooth coat S but was lethal; the other form had a rough coat R, which was nonlethal when injected into mice. He decided to investigate what would happen if he injected both heat-killed S bacteria and live R bacteria into mice. To his surprise, the mice injected with this combination died. Upon closer examination of the blood, living S bacteria were found. Something had occurred that transformed the nonlethal R bacteria into S bacteria. Subsequent experiments throughout the 1940s attempted to find the identity of the transforming factor—and it was eventually found to be DNA. (For more about bacteria, see the chapter “Bacteria, Viruses, and Protists.”)