Hershey and Chase Experiment
Biologists have known that DNA exists since 1869, but until the 1950s, they didn’t know it carried the genetic information of the cell—they thought proteins held that honour, since they seemed to be so much more complex. An early, incorrect experiment proposed that the nucleotides in DNA are arranged in a repeating sequence instead of unique codes, which supported the idea that they were simpler molecules.
But in 1953, Alfred Hershey and Martha Chase showed for sure that the genetic material was not, in fact, proteins. Like many fundamental experiments in the field of biology, theirs was beautifully simple.
The key component was a virus called bacteriophage. As its name suggests, it infects bacteria, taking over their replication processes in order to make copies of itself—usually, so many replicates are made that eventually the host bacteria bursts, spreading the virus to other cells. A bacteriophage achieves this by attaching to the bacteria’s surface and injecting genetic material down into the core to start the replication process. Hershey and Chase worked with T2 bacteriophage and E. coli bacteria.
Since bacteriophages are simply composed of a protein “shell” that encloses their DNA or RNA genome, they were perfect for the experiment—all Hershey and Chase had to do was see whether protein or DNA was injected into the bacteria.
However, they had to uniquely label the two components so they could tell which was where. For this, they used their knowledge of the structure of protein and DNA: sulphur is contained in proteins but not DNA, and phosphorous is contained in DNA but not protein. So Hershey and Chase began to conduct two experiments side by side: one where they grew protein in radioactive sulphur-35 and used normal DNA, and one where they used normal protein and grew DNA in radioactive phosphorous-32.
Here’s how the rest went down:
Introduce bacteria to the two different bacteriophages.
The bacteriophages get to work. The shells remain on the outside of the bacteria, while its genetic material is injected into the bacteria.
Hershey and Chase centrifuged the mixtures to separate the bits out by density—a centrifuge is essentially a ultra-fast, ultra-cool blender. The lighter shells are shaken away from the denser bacteria cores, which still contain the genetic material.
Use a Geiger counter to check which part is radioactive, the shell or the material in the bacteria.
And as you might have guessed, here’s what they found:
In the experiment where protein was radioactive, the shells were found to be radioactive while the infected bacteria were not.
In the experiment where DNA was radioactive, the infected bacteria were found to be radioactive while the shells were not.
Therefore, Hershey and Chase concluded that DNA was injected into the bacteria and used to make copies of the phage, so DNA must be the genetic material.
Note for those interested: Hershey received a Nobel Prize for his efforts, but Chase did not, possibly because she was a lab technician (or, of course, a woman).
Further resources: McGraw Hill video and Biography of Martha Chase
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