Unit Three. The Continuity of Life
The Avery Experiments
The agent responsible for transforming Streptococcus went undiscovered until 1944. In a classic series of experiments, Oswald Avery and his coworkers Colin MacLeod and Maclyn McCarty characterized what they referred to as the “transforming principle.” Avery and his colleagues prepared the same mixture of dead S Streptococcus and live R Streptococcus that Griffith had used, but first they removed as much of the protein as they could from their preparation of dead S Streptococcus, eventually achieving 99.98% purity. Despite the removal of nearly all protein from the dead S Streptococcus, the transforming activity was not reduced. Moreover, the properties of the transforming principle resembled those of DNA in several ways:
Same chemistry as DNA. When the purified principle was analyzed chemically, the array of elements agreed closely with DNA.
Same behavior as DNA. In an ultracentrifuge, the transforming principle migrated like DNA; in electrophoresis and other chemical and physical procedures, it also acted like DNA.
Not affected by lipid and protein extraction. Extracting the lipid and protein from the purified transforming principle did not reduce its activity.
Not destroyed by protein- or RNA-digesting enzymes. Protein-digesting enzymes did not affect the principle’s activity, nor did RNA-digesting enzymes.
Destroyed by DNA-digesting enzymes. The DNA-digesting enzyme destroyed all transforming activity.
The evidence was overwhelming. They concluded that “a nucleic acid of the deoxyribose type is the fundamental unit of the transforming principle of Pneumococcus Type III”—in essence, that DNA is the hereditary material.
The Hershey-Chase Experiment
Avery’s result was not widely appreciated at first because most biologists still preferred to think that genes were made of proteins. In 1952, however, a simple experiment carried out by Alfred Hershey and Martha Chase was impossible to ignore. The team studied the genes of viruses that infect bacteria. These viruses attach themselves to the surface of bacterial cells and inject their genes into the interior; once inside, the genes take over the genetic machinery of the cell and order the manufacturing of hundreds of new viruses. When mature, the progeny viruses burst out to infect other cells. These bacteria-infecting viruses have a very simple structure: a core of DNA surrounded by a coat of protein.
In this experiment, shown in figure 11.2, Hershey and Chase used radioactive isotopes to “label” the DNA and protein of the viruses. Radioactively tagged molecules are indicated in red in the figure. In the preparation on the right, the viruses were grown so that their DNA contained radioactive phosphorus (32P); in another preparation, on the left side of the figure, the viruses were grown so that their protein coats contained radioactive sulfur (35S). After the labeled viruses were allowed to infect bacteria, Hershey and Chase shook the suspensions forcefully to dislodge attacking viruses from the surface of bacteria, used a rapidly spinning centrifuge to isolate the bacteria, and then asked a very simple question: What did the viruses inject into the bacterial cells, protein or DNA? They found that the bacterial cells infected by viruses containing the 32P label had labeled tracer in their interiors; cells infected by viruses containing the 35S labeled tracer did not. The conclusion was clear: The genes that viruses use to specify new viruses are made of DNA and not protein.
Figure 11.2. The Hershey-Chase experiment.
The experiment that convinced most biologists that DNA is the genetic material was carried out soon after World War II, when radioactive isotopes were first becoming commonly available to researchers. Hershey and Chase used different radioactive labels to "tag" and track protein and DNA. They found that when bacterial viruses inserted their genes into bacteria to guide the production of new viruses, 35S radioactivity did not enter infected bacterial cells and 32P radioactivity did. Clearly the virus DNA, not the virus protein, was responsible for directing the production of new viruses.
Key Learning Outcome 11.2. Several key experiments demonstrated conclusively that DNA, not protein, is the hereditary material.