Hershey and Chase Experiment

Hershey and Chase Experiment is a very popular experiment which provides evidence of DNA as “Genetic material”. It was introduced by the two scientists A.D. Hershey and Martha Chase in the year 1952. After seven years of an experiment given by the Avery, Hershey and Chase gave the further proof of DNA as genetic material by the use of radioactive bacteriophage.

Avery, Macleod and McCarthy have concluded that the DNA was the genetic material which transformed the avirulent rough strain to the virulent strain. Before this, Griffith by his experiment has concluded that the protein factor causes virulence of the rough strain, but it was not proved to be genetic material.

So to resolve the query of genetic material, many researchers were engaged to know whether the cause of inheritance is Protein or DNA. Many discoveries then led to the discovery of “DNA” as genetic material or the cause of inheritance. One of the best experiment which provides evidence of DNA as genetic material is the “Hershey and Chase experiment”.

Content: Hershey and Chase Experiment

  1. Definition of Hershey and Chase Experiment
  2. Hershey and Chase Experiment Steps
  3. Observation
  4. Conclusion

Definition of Hershey and Chase Experiment

Hershey and Chase Experiment is the experiment which has demonstrated the DNA is the genetic material where they have taken the radioactive T2-bacteriophage (Viruses that infect E.coli bacteria).

t2 bacteriophage used in hershey and chase experiment

T2-bacteriophage also refers as Enterobacteria phage T2 which belongs to the Group-I bacteriophage. The genome of  T2-bacteriophage comprises of linear, ds-DNA and categorize into the Myoviridae family. The body of  T2-bacteriophage comprises of basically three parts namely head, collar and tail. T2-bacteriophage resembles with the T4-bacteriophage and hence refers as T4-like virus which is having a tailed phage.

In the Hershey and Chase Experiment, the phage DNA and protein were labelled with radioactive phosphorus (P32) and radioactive sulphur (S35) respectively to know whether phage DNA or protein will infect the E.coli.

After labelling the Phage DNA and protein, Hershey and Chase performed a series of experiments like infection, blending and centrifugation. Finally, Hershey and Chase concluded that the phage DNA labelled with P32 had infected the E.coli by transferring its radioactivity.

Hershey and Chase Experiment Steps

Hershey and Chase gave the full proof evidence for the DNA being a genetic material by their experiments. To perform the experiment, Hershey and Chase have taken T-2 bacteriophage which are the invaders of E.coli bacteria. Hershey and Chase performed the following steps which include:

Radioactive Labelling of Bacteriophage

Hershey and Chase have grown T-2 bacteriophages in the two batches. In the batch-1, bacteriophages were grown in the medium containing radioactive sulphur (S35) and radioactive phosphorus (P35)  in batch-2.

After incubation, Hershey and Chase have concluded that the radioactive sulphur (S35) will tag the phage protein. The radioactive phosphorus (P35) will tag the phage DNA. Now there will be a question that why radioactive sulphur has labelled only the phage protein, why not DNA or vice versa.

The labelling of phage protein by S35 is because of the reason that sulphur being a structural element of protein will tag the Phage protein, not the phage DNA.
Hershey and Chase Experiment batch1 radioactive labelling
Whereas, the labelling of phage DNA by P35 is because of the reason that phosphorus is present in the structure of DNA as the phosphate backbone will tag the Phage DNA, not the phage protein.
batch2 radioactive labelling in Hershey and Chase Experiment

Infection

After radioactive labelling of the phage DNA and protein, Hershey and Chase infected the bacteria, i.e. E.coli by the radioactively labelled T-2 phage. In Batch-1, T-2 phage tagged with S35 and in batch-2 T-2 phage labelled with P35 were allowed to infect the bacterial cells of E.coli. After attachment of the T-2 bacteriophages, the phage DNA will inject into the cytoplasm of E.coli. The phage DNA will take up the host cell machinery. Degradation of the bacterial genome occurs by the T2-phage where they use the ribosomes for the protein synthesis to assemble capsid, tail fibres, base plate etc.

Blending

After infection, the bacterial cells subjects to the process of blending.  At the time of blending the bacterial cells are agitated for the removal of the viral coats. The method of blending also refers as the process of “Agitation”. In this stage, the bacterial cells along with the viral particles like capsid, tail fibres, base plate, DNA etc. agitates where we get a solution containing both bacterial cells and viral particles.

Centrifugation

After blending, the bacterial cells further subjects to the process of centrifugation for the separation of viral particles and bacterial cells. As a result of centrifugation, the heavier particles, i.e. bacterial cells will found as the “Pellets”. The lighter particle, i.e. viral particles will found in the “Supernatant”.
Experiment by Hershey and Chase

Observation

After the centrifugation step, observe the results to know the heritable factor. The phage DNA labelled with P35 will transfer the radioactivity in the host cell. Thus, the radioactive P35 will find inside the bacterial cells and remains as “Pellets”.

The phage protein tagged with S35 will not transfer the radioactivity in the host cell. Thus, radioactive S35 will not find inside the bacterial cell and remains as the “Supernatant” of the solution.

Conclusion

Therefore, the P35 labelled phage DNA will transfer its radioactivity to the host cell DNA. The P35 labelled phage DNA will present inside the E.coli cell even after blending and centrifugation. Hence, it is clear that the phage DNA will incorporate into the host cell genome and will produce its progeny by taking host cell machinery.

The S35 labelled phage protein will not transfer its radioactivity to the host cell DNA. After the process of blending and centrifugation, the P35 labelled phage protein will present as the supernatant of the solution. Thus, the P35 labelled phage protein will not infect the E.coli cells.

Therefore, Hershey and Chase Experiment prove that the DNA is the genetic material which transfers the radioactivity of the T2-phage to the host cell, i.e. E.coli.

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