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