main idea. A. From the 1920s through 1944, researchers used pneumococcus bacteria to discover the properties of DNA because the bacteria was relatively simple, having only two strains. B. Frederick Griffith discovered that a nonvirulent strain of bacteria could be transformed into a virulent strain by being exposed to dead cells from the virulent strain. C. By selectively destroying various substances in the cells of pneumococci bacteria, Oswald Avery and his colleagues identified DNA as the substance that caused bacterial transformation. D. Oswald Avery injected the combination of heat-killed, virulent cells and nonvirulent cells into mice because he hoped this would lead to a vaccine for pneumonia. E. Avery and his colleagues were able to isolate Griffith's transforming principle by injecting mice with the extract that contained the transforming principle. F. Scientists did not initially recognize the importance of the discovery that DNA could cause genetic transformation because the hereditary material was assumed to be a protein.