The Hershey–Chase experiments were a series of experiments conducted in 1952 by Alfred Hershey and Martha Chase, confirming that DNA was the genetic material, which had first been demonstrated in the 1944 Avery–MacLeod–McCarty experiment. While DNA had been known to biologists since 1869, most assumed at the time that proteins carried the information for inheritance. Hershey and Chase conducted their experiments on the T2 phage, a
virus whose
structure had recently been shown by electron
microscopy.The phage consists of a protein shell containing its genetic
material. The phage infects a bacterium by attaching to its
outer membrane and injecting its genetic material and leaving its empty shell
attached to the bacterium.
First Experiment:
![Picture](/uploads/2/6/8/3/26839870/2307184.jpg?224)
- 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.
Second Experiment
![Picture](/uploads/2/6/8/3/26839870/1397786723.jpg)
- Shocked by the results derived from the first experiment and to further confirm their findings, they decided to perform the test once again, this time using a different tractor 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.
Final Experiment
- 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.
RESULTS
Radioactive sulfur is found predominantly in the supernatant.
- Radioactive phosphorus is found predominantly in the cell fraction, from which a new generation of infective phage can be isolated.
Conclusion:
The results were no mistake and the import of their work was clear:
Hershey and Chase had elucidated direct, irrefutable evidence
Hershey and Chase had elucidated direct, irrefutable evidence
- That DNA, not protein, is the source of genetic material.
- The active component of the bacteriophage that transmits the infective characteristic is the DNA.
- There is a clear correlation between DNA and genetic information.
Animation Related to the Above:
- http://nortonbooks.com/college/biology/animations/ch12a02.htm
- http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio21.swf::Hershey+and+Chase+Experiment
How well do you understand the Hershey-Chase experiment?
Follow the link to take the quiz and find out!