Hershey–Chase experiment

The experiment known as the Hershey-Chase experiment proved that genetic information is encoded in DNA and not in proteins. The experiment was conducted in 1952 by Alfred Hershey and Martha Chase. It provided independent confirmation of the result already obtained in 1944 by Oswald Avery, Colin MacLeod and Maclyn McCarty in their experiments on the genetic transformation of bacteria.

In the Hershey-Chase experiment, viruses were used that are specialised in attacking bacterial cells. Such viruses are also called bacteriophages ('bacteria eaters') or phages. They essentially consist of a protein envelope and DNA (rarely RNA) inside. When they encounter a bacterium, they attach their tails to the bacterial surface and inject their entire DNA into the bacterium. The protein coat, on the other hand, remains outside the bacterium. The injected DNA stimulates the bacterium to build new phages (self-assembly), which are finally released, destroying the bacterium.

Hershey and Chase now grew phages, which were designated T2 phages, once with the addition of radiolabelled sulphur (^35S) and in a second Petri dish with the addition of radiolabelled phosphorus (^32P). The radiolabeled sulfur was thereby incorporated into the proteins. The radiolabeled phosphorus in the other Petri dish, on the other hand, was incorporated into the DNA of the cultured phages.

Bacteria (Escherichia coli), which did not have any radiolabelled substances, were then added to each of the phages. Shortly after the phages had attached themselves to the bacteria and injected their DNA, Hershey and Chase placed the samples in a blender. The shear force in the blender was sufficient to dislodge the empty phage protein coat from the bacterial surface, but neither the coats nor the bacteria were destroyed. In a subsequent centrifugation, the heavy bacteria settled into the sediment while the lighter phage envelopes remained in the supernatant.

In the samples which were labelled with the ^35S, no radioactivity could be measured in the sediment after centrifugation. However, the liquid was clearly radioactive. In the samples, in which the DNA was labelled with the ^32P, a clear radioactivity could be measured in the sediment after centrifugation. The liquid, however, was not radioactive. This was evidence that DNA was entering the bacteria. DNA could stimulate carried genetic material into the bacterium to produce phages.