In 1952 (seven years after Avery's demonstration that genes were DNA), two geneticists: A. D. Hershey and Martha Chase, provided further proof. They worked with a DNA virus, called T2, which infects E. coli (and so is a bacteriophage). Figure 5.2.1 shows the essential elements of the infective cycle of DNA bacteriophages like T2. The virions attach to the surface of their host cell (a). The proteins of the capsid inject the DNA core into the cell (b). Once within the cell, some of the bacteriophage genes (the "early" genes) are transcribed (by the host's RNA polymerase) and translated (by the host's ribosomes, tRNA, etc.) to produce enzymes that will make many copies of the phage DNA and will turn off (even destroy) the host's DNA.
As fresh copies of phage DNA accumulate, other genes (the "late" genes) are transcribed and translated to form the proteins of the capsid (c). The stockpile of DNA cores and capsid proteins are assembled into complete virions (d). Another "late" gene is transcribed and translated into molecules of lysozyme. The lysozyme attacks the peptidoglycan wall (from the inside, of course). Eventually the cell ruptures and releases its content of virions ready to spread the infection to new host cells (e).
Bacteriophages produced within bacteria growing in radioactive culture medium will themselves be radioactive. If radioactive sulfur atoms (35S) are present, they will be incorporated into the protein coats of the bacteriophages since two of the amino acids — cysteine and methionine — contain sulfur (Figure 5.2.2). However, the DNA will be nonradioactive because there are no sulfur atoms in DNA. If radioactive phosphorus (32P) is used instead, the DNA become radioactive — because of its many phosphorus atoms — but not the proteins.
Hershey and Chase found that when bacteriophages containing 32P (radioactive), were allowed to infect nonradioactive bacteria, all the infected cells became radioactive and, in fact, much of the radioactivity was passed on to the next generation of bacteriophages. However, when the bacteria were infected with bacteriophages labeled with 35S, and then the virus coats removed (by whirling them in an electric blender), practically no radioactivity could be detected in the infected cells. From these experiments, it was clear that the DNA component of the bacteriophages is injected into the bacterial cell while the protein component remains outside. However, it is the injected component — DNA — that is able to direct the formation of new virus particles complete with protein coats. So here is further proof that genes are DNA.