Hershey Chase experiment
With the experiment known as the Hershey Chase experiment, it was possible to demonstrate that genetic information is encoded in DNA and not in proteins . The experiment was carried out in 1952 by Alfred Hershey and Martha Chase . It provided an independent confirmation of the result that Oswald Avery , Colin MacLeod and Maclyn McCarty had already obtained in 1944 in their experiments on the genetic transformation of bacteria .
The Hershey-Chase experiment used viruses that specialize in attacking bacterial cells. Such viruses are also called bacteriophages ('bacteria eaters') or phages. They essentially consist of a protein shell and DNA (rarely RNA) in it. If they encounter a bacterium, they attach their tail appendages to the bacterial surface and inject all of their DNA into the bacterium. The protein shell, on the other hand, remains outside the bacterium. The injected DNA stimulates the bacterium to build new phages ( self-assembly ), which are finally released while destroying the bacterium.
Hershey and Chase then cultured phages, which were designated as T2 phages , once with the addition of radioactively labeled sulfur ( 35 S) and in a second Petri dish with the addition of radioactively labeled phosphorus ( 32 P). The radioactive labeled sulfur was incorporated into the proteins. The radioactively labeled phosphorus in the other Petri dish, on the other hand, was incorporated into the DNA of the cultivated phages.
Bacteria ( Escherichia coli ), which did not contain any radioactively labeled substances, were then added to the phages . Shortly after the phage attached to the bacteria and injected their DNA, Hershey and Chase put the samples in a blender. The shear force in the mixer was sufficient to detach the empty phage protein envelope from the bacterial surface, but neither the envelope nor the bacteria were destroyed. In a subsequent centrifugation, the heavy bacteria settled in the sediment, while the lighter phage coats remained in the supernatant.
In the samples for which the proteins were labeled with the 35 S, no radioactivity could be measured in the sediment after centrifugation. The liquid, however, was clearly radioactive. In the samples in which the DNA was marked with the 32 P, a clear radioactivity could be measured in the sediment after centrifugation. However, the liquid was not radioactive. That was evidence that DNA was entering the bacteria. Through the DNA, the genetic material carried into the bacterium could stimulate the production of phages.
Individual evidence
- ↑ AD Hershey, M. Chase: Independent functions of viral protein and nucleic acid in growth of bacteriophage . In: The Journal of General Physiology . 36, No. 1, 1952, pp. 39-56. doi : 10.1085 / jgp.36.1.39 . PMID 12981234 . PMC 2147348 (free full text).