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|Chapter Six: DNA, RNA, and Protein Synthesis|
In 1928, Fred Griffith performed the first experiment which suggested that protein was not the genetic material. His experiment was actually fairly simple. He first injected mice with a live strain of virulent (deadly) bacteria, and not to anyone's surprise, all of those mice died. Then, he killed the virulent bacteria cells by heating them. Mice injected with these heat-killed virulent bacteria did not die. In another set of mice, Griffith injected a live non-virulent strain of bacteria, and these mice did not die, the result which Griffith expected.
The surprise came when Griffith injected a group of mice with both live non-virulent bacteria and heat-killed virulent bacteria. In that group, some of the mice died. When Griffith examined those mice, he found live virulent bacteria in their blood. Griffith drew the conclusion that the genetic information in the heat-killed virulent bacteria survived the heating process and was somehow incorporated into the genetic material of the non-virulent strain to cause them to become virulent. But Griffith knew that heat denatures protein, so he suggested that the genetic material must be something else. However, his results did not specifically point to DNA as a possibility.
Oswald Avery followed up on Griffith's experiment in the following decade. Like Griffith, Avery first used heat to kill virulent bacteria. He then extracted RNA (ribonucleic acid), DNA, carbohydrates, lipids, and proteins from these dead cells, all of which were considered to be possible candidates for the carriers of genetic information. Next, he added each type of molecule to a culture of live non-virulent bacteria to determine which was responsible for changing them into virulent bacteria as Griffith had observed. Only the non-virulent cells which were given DNA from the dead virulent strain became virulent, so Avery concluded that DNA must be the genetic material.