DNA Is the Transforming Factor

DNA was not again taken seriously as the hereditary material until 1944, when Oswald Avery (1877-1955) published a landmark paper outlining his experiments with two strains of Pneumococcus bacteria. The S ("smooth") strain was able to cause disease in mice, while the R ("rough") strain was not. Under the microscope, the S strain had a smooth, glistening surface, due to a sugar capsule it secreted. The R strain lacked the capsule.

Fifteen years earlier, Frederick Griffith had shown that injecting R bacteria plus heat-killed S bacteria into mice would cause disease just as surely as injecting live S bacteria, a result that Griffith attributed to a "transforming principle." When Avery grew R bacteria in a dish with the heat-killed S bacteria, he saw that the R bacteria were transformed into S bacteria, capable of making capsules and causing disease, just as Griffith had observed. Avery's group purified the components of the S bacteria, and showed that DNA alone could cause the transformation, while protein could not.

Though not immediately accepted by all scientists, Avery's discovery triggered intense interest among biochemists and geneticists, who turned their attention to discovering how DNA could be the genetic molecule. As in every other branch of biochemistry, the structure was presumed to hold the key to the function, and so the hunt was on for the structure of DNA.

One key was the discovery by Erwin Chargaff (1905-2002) that, contrary to Levene's conclusions, the nucleotide proportions were not all the

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