Alfred Hershey and Martha Chase conducted experiments on virus T2 bacteriophage that attacks the common bacterium Escherichia coli. The bacteriophage has two chemical components i.e., protein and DNA. Protein forms the external structures like head, sheath and tail fibres and a DNA molecule is in the head. The phage attacks E. coli by attaching with its tail fibres to the bacterial wall and injecting its genetic material into the bacterial cell to produce new phages.
Hershey and Chase labelled the DNA and protein components of the phage separately with specific radioactive tracers and then followed these components through the life cycle of the phage. They developed two strains of the virus, one with labelled protein and other with labelled DNA. Almost all proteins contain sulphur which is not found in DNA while all DNA molecules contain phosphorus which is not found in proteins. The T2 phages grown in the presence of radioactive sulphur (35S) has labelled proteins and T2 phages grown in presence of radioactive phosphorus (32P) had labelled DNA.
After developing these strains, Hershey and Chase allowed each strain to infect the bacteria. Soon after infection, the bacterial cells were gently agitated in a blender to separate the adhering phage particles. It was observed that only radioactive 32P was found in the bacterial cells and 32S was present only in viral coats in the surrounding medium and not inside the bacterial cells. When they studied the viral progeny for radioactivity, it was found that it had only 32P and no 35S .
The results clearly show that only DNA is the genetic material and not protein coat.
