Concept 17 A gene is made of DNA.
Oswald Avery explains Fred Griffith's and his own work with Pneumococcus bacteria.
How do you do? I'm Oswald Avery. My colleagues and I did a series of experiments using strains of Pneumococcus bacteria, which cause pneumonia. Pneumococcus grows in the body of the host, but, like other types of bacteria, also can be grown on solid or liquid cultures. In 1928, Fred Griffith published a study on the different strains of Pneumococcus. Two in particular, S and R, look different. The S colonies have a smooth surface, and the R colonies look rough. The S colonies look smooth because each bacterium has a capsule-like coat made of sugars. This coat protects the S bacteria from the host's immune system, and so the S strain is infectious. The coat-less R strain is not. Griffith found that mice injected with the S strain develop pneumonia and die within days. Mice injected with the R strain do not get pneumonia. Griffith noticed that different strains of Pneumococcus could be cultured from one patient. He began to wonder if one strain could change into another. To test this idea, he did a series of experiments using the R and S strains. First, Griffith heated the S strain culture to kill the bacteria. As predicted, when injected into mice, the heat-killed bacteria did not produce an infection. Griffith co-injected the heat-killed S with live R into mice, and, much to his surprise, the mice developed pneumonia and died. Even more astonishing, Griffith was able to isolate live S strain from the blood of infected mice. These cultures could infect other mice. S strain cultured from infected mice remained active — showing that the change was stable and inherited. Griffith concluded that some "principle" was transferred from the heat-killed S to the R strain. The principle transformed the R into the infective S strain with a smooth coat. When I read about Griffith's results, I became very interested in the identity of this transforming "principle." Colin MacLeod, Maclyn McCarty, and I began experimenting using a test tube assay instead of mice. We used detergent to lyse the heat-killed S cells. Then we used this lysate for transformation assays. The test tube assays worked well, and showed us that the heat-killed S lysate could change R to S. The transforming principle was something in the lysate. We tested each of the lysate components for the transforming activity. First, we incubated the heat-killed S lysate with an enzyme, SIII, that completely chewed up the sugar coat. We tested the transforming ability of the sugar coat-less S lysates. The sugar coat-less S lysate was still able to transform. This told us that the R strain was not just assembling a new S sugar coat from the pieces. Next, we incubated the coat-less S extract with protein digesting enzymes — trypsin and chymotrypsin. Next, we incubated the coat-less S extract with protein digesting enzymes — trypsin and chymotrypsin. We tested this lysate's ability to transform. This protein-less lysate was still able to transform. So, the transforming principle is not protein. While we were testing and purifying the lysate, we precipitated nucleic acids — DNA and RNA — with alcohol. We were the first to isolate nucleic acids from Pneumococcus. While we were testing and purifying the lysate, we precipitated nucleic acids — DNA and RNA — with alcohol. We were the first to isolate nucleic acids from Pneumococcus. Since the transforming principle was not the sugar coat, and not protein, we suspected that it may be one of the nucleic acids. We dissolved the precipitate in water, and tested the transforming ability of the solution. Since the transforming prinicple was not the sugar coat, and not protein, we suspected that it may be one of the nucleic acids. We dissolved the precipitate in water, and tested the transforming ability of the solution. First, we destroyed the RNA using the RNase enzyme. We tested this solution for its ability to transform. The solution still had the ability to transform. Therefore, RNA could not be the transforming principle. What we had left was virtually pure DNA. As a final test, we incubated the solution with the DNA-digesting enzyme, DNase. We used this solution to test for transforming ability. This solution was unable to transform. My colleagues and I concluded that DNA is the transforming principle, and we published these results in 1944.
It's hard to imagine now the impact that Avery's experiments must have had. Until Avery's experiments, scientists weren't even sure that bacteria had genes.
Avery's experiments showed that DNA is the tranforming principle, but he didn't try to figure out how transformation works. How do you think transformation works?