“When asked what his idea of happiness would be, [Hershey] replied, ‘to have an experiment that works, and do it over and over again.'”
– Jonathan Hodgkin, 2001
In 1944 the Avery-MacLeod-McCarty experiments demonstrated that DNA, rather than proteins, is the carrier of genetic information. Though the work appeared to be well-supported, and was endorsed by other researchers, the trio met with resistance from much of the scientific community. For nearly a decade, the Avery group was forced to repel attacks on the validity of their experiments, defending both their findings and their reputations.
Finally, in 1952, Alfred Hershey, a Carnegie Institution researcher working at Cold Spring Harbor Laboratory, set out to conclusively settle the issue. Like many of his contemporaries, Hershey believed that proteins, with their complicated structures, were more likely to be the carriers of genetic information than was the simple DNA molecule. Hershey, however, was about to make a discovery that would turn his own notions on end.
In order to show that proteins carry genetic information, Hershey and his lab technician, Martha Chase, decided to track the transfer of proteins and DNA between a virus and its host. For their experiment, they chose to use the T2 bacteriophage as the vehicle for delivering genetic material. Like all bacterial viruses, the T2 is comprised of only a protein-based outer wall and a DNA core, its simple structure making it the perfect research candidate. The phage reproduces by injecting its genetic material into a bacterium, leaving its protein shell attached to the host. Then, through a microscopic takeover, the virus seizes control of the bacterium’s reproductive mechanisms and uses them to duplicate itself, destroying the host in the process.
Though it was known that the protein shell remained outside the bacterium, researchers thought it possible that certain proteins were transferred from the virus to the bacterium upon attachment. If genetic material was in fact carried by proteins, this would explain how a phage is able to reproduce within a bacterium without the entirety of the protein shell penetrating the bacterium’s membrane. In order to prove that proteins are the carriers of genetic information, Hershey and Chase needed to demonstrate that at least a portion of the phage’s protein mass was transferred to the interior of the bacterium.
In their first experiment, Hershey and Chase tagged the T2 phage DNA with Phosphorous-32, a radioactive form of the element. Because phosphorous can be found in large quantities in DNA, but in only trace amounts in protein, the researchers could track the location of DNA and protein according to the radiation concentrations. They then allowed the tagged phages to begin infecting samples of E. coli. After introducing to the phage culture to the bacterial sample, they used a Waring blender to violently disturb the infected bacteria, causing the protein shells to detach from their hosts. Then, using a centrifuge, they separated the bacterium from the phages and protein.
Once the separation was complete, they measured the radiation concentrations in the E. coli cells and the protein shells. The phosphorous tracer appeared in large quantities only in the bacterial sample, demonstrating that DNA was transferred from the bacteriophage to the host organism. Further, despite the protein shells being detached while reproduction of the phage should have been taking place, the virus was still copied in each of the host cells. This, in turn, suggested that the proteins shell itself was not necessary to the replication process following the initial insertion of genetic material.
Shocked by their findings, Hershey and Chase decided to perform the test once again, this time using a different tracer molecule. They chose sulfur for the second test, because it appears in the amino acids that make up proteins, but is not present in DNA. This allowed them to track the same process as in the first experiment, but in reverse. After tagging the proteins, infecting the E. coli cells, and separating the shells from the host, the researchers tested for the presence of sulfur. In accordance with their previous results, the sulfur could only be found in the protein shells and not in the bacteria. And again, the phage’s genetic material was replicated despite the protein shell being disconnected from the bacteria via the blending process.
Sufficiently impressed by the significance of his findings, Hershey returned to the phosphorous-tagged batch to engage in some follow-up research. Upon examining the offspring of the phages, the researchers found that the young bacteriophages also possessed phosphorous-tagged DNA, but their protein lacked any trace of radioactivity. The implications of their first experiments were reinforced.
At first, the pair was inclined to believe that the experimental or data-collection procedures were flawed. They rechecked the experiment design, the equipment, and the bacterial cultures. It was all in vain, though. Hershey was a notoriously cautious researcher and his experiments were always well-planned and precisely executed. The results were no mistake and the import of their work was clear: Hershey and Chase had elucidated direct, irrefutable evidence that DNA, not protein, is the source of genetic material.
Later that year, the pair reported their findings in a short paper in The Journal of General Physiology titled “Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage.” This publication catalyzed a storm of activity in the scientific community, with researchers all over the world clamoring for details on the experiments. Alfred Hershey’s lectures on the subject were attended by the greatest scientific minds in Europe and North America; Pauling was one of hundreds to hear him speak. In the years following the discovery, DNA became a major focus for researchers all over the world, resulting in Pauling’s own attempts to deduce its structure and the eventual success of Watson and Crick. Even today, our genetic research traces its roots from the work of Alfred Hershey and Martha Chase.