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DNA and genetic information

HersheyBiographyand Chase, 1952: Bacteriophage genes are made of DNA

The background

This experiment was carried out seven years after the publication of AveryBiography's work. By then new techniques had been discovered. In particular, the use of bacteriophagesask Dr Chromo! as biological tools and radioactivity as a labelling tool were fully established.

  • Bacteriophage

    Bacteriophages (phages) are viruses that infect bacteria. A bacteriophage will inject its genetic material in a bacterium. The bacterium synthesises new bacteriophages, using the phages genetic material, and dies, releasing the new viruses in the culture medium.

  • Radiolabelling

    The use of radiolabelling allows protein and DNA to be unequivocally distinguished, and involves the incorporation of a radioactive atomask Dr Chromo!, or marker to the molecules.

      Proteins contain sulphurask Dr Chromo! that can be labelled with 35S,

      and DNA does not contain sulphur, but can be labelled with 32P which will not be incorporated into proteins.


    To label bacteriophages, put phages (a), and bacteria (b), and label (c) in a conical flask (d) and incubate for 8 hours. After that time, you will obtain large quantities of labelled phage. Bacteria are destroyed by the phage. This is illustrated in figure 1

    Once labelled, the protein and the DNA parts of the virus can be distinguished from each other since they are labelled with two different isotopes (DNA will be labelled with 32P and proteins with 35S). The two isotopes can be detected and differentiated.

    The experiment

    Hershey and Chase prepared a radioactive sample of a bacteriophage T2 in which the proteins were labelled with 35S, and the DNA with 32P, as explained above.

    The radioactive phage was used to infect a non-radioactive culture of E. coli (Figure 2, step 1). A few minutes after the inoculation of the culture with the phage, the tube is shaken violently using a blender, and the solution centrifugedask Dr Chromo! to precipitate the relatively heavy bacteria (Figure 2, step 3).

    The analysis of the labelling count shows that the pellet contains 80 % of the 32P and only 1 % of the 35S.

    Your Conclusion

    1. What is labelled with 32P and with 35S?
    2. What is the effect of violently agitating the culture?
    3. Can you describe the contents of the supernatantask Dr Chromo! after the centrifugation?
    4. What is your conclusion?

    The conclusion

    1) What is labelled with 32 P and with 35S?

    Proteins are labelled with 35S in their cysteine amino acids (if the protein concerned had no cysteine, the protein would not become labelled, but most proteins contain cysteines); DNA is labelled with 32 P.

    2) What is the effect of violently agitating the culture?

    Agitating the culture separates the phage from the surface of the cell. Just enough time is left to allow the genetic information to be injected into the bacteria.

    3) Can you describe the contents of the supernatant after the centrifugation?

    After the centrifugation, the supernatant contains light particles of phages. All the heavier particles, bacteria and some of the phages still attached to them, are in the pellet, at the bottom of the tube.

    4) What is the conclusion?

    Most of the 32P is in the pellet. Only DNA is labelled with 32P. Therefore only DNA has penetrated the bacteria (as shown in figure 3). Clearly, the genetic material is DNA and not proteins, as proteins do not penetrate the cell (1% 35S in the pellet represent phages that are still "stuck" on the bacteria).



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