Discovery of bacterial transformation. Griffith’s experiments showed that a molecule in a lethal type of bacteria can transform nonkilling (nonvirulent) bacteria into killers

Figure 9.1 Discovery of bacterial transformation. Griffith’s experiments showed that a molecule in a lethal type of bacteria can transform nonkilling (nonvirulent) bacteria into killers (virulent).

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Page 165U.S. physicians Oswald Avery, Colin MacLeod, and Maclyn McCarty hypothesized that a nucleic acid might be Griffith’s “transforming principle.” They observed that treating broken-open type S bacteria with a protease—an enzyme that dismantles protein—did not prevent the transformation of a nonvirulent to a virulent strain, but treating it with deoxyribonuclease (or DNase), an enzyme that dismantles DNA only, did disrupt transformation. In 1944, they confirmed that DNA transformed the bacteria. They isolated DNA from heat-killed type S bacteria and injected it with type R bacteria into mice (figure 9.2). The mice died, and their bodies contained active type S bacteria. The conclusion: DNA passed from type S bacteria into type R, enabling the type R to manufacture the smooth coat necessary for infection. Once type R bacteria encase themselves in smooth coats, they are no longer type R.

Protein Is Not the Hereditary Molecule

Science seeks answers by eliminating explanations. It provides evidence in support of a hypothesis, not proof, because conclusions can change when new data become available. To identify the genetic material, researchers also had to show that protein does not transmit genetic information. To do this, in 1953, U.S. microbiologists Alfred Hershey and Martha Chase used Escherichia coli bacteria infected with a virus that consisted of a protein “head” surrounding DNA. Viruses infect bacterial cells by injecting their DNA (or RNA) into them. Infected bacteria may then produce many more viruses. The viral protein coats remain outside the bacterial cells.

Researchers can analyze viruses by growing them on culture medium that contains a radioactive chemical that the viruses take up. The “labeled” viral nucleic acid then emits radiation, which can be detected in several ways. Hershey and Chase knew that protein contains sulfur but not phosphorus, and that nucleic acids contain phosphorus but not sulfur. Both elements also come in radioactive forms. When Hershey and Chase grew viruses in the presence of radioactive sulfur, the viral protein coats took up and emitted radioactivity, but when they ran the experiment using radioactive phosphorus, the viral DNA emitted radioactivity. If protein is the genetic material, then the infected bacteria would have radioactive sulfur. But if DNA is the genetic material, then the bacteria would have radioactive phosphorus.