Question

How did Hershey and Chase prove that DNA is the genetic material?

Answer 1

Alfred Hershey and Martha Chase conducted experiments on T${}_2$ bacteriophage that attacks the common bacterium Escherichia coli. The bacteriophage has two components-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 attacking 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 phosphorous which is not found in proteins. The T${}_2$ phages grown in the presence of radioactive sulphur (${}^{35}$S) 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 loosen and adhering phage particles. It was observed that only radioactive (${}^{32}$P) was found in the bacterial cells and ${}^{35}$S was present only 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 ${}^{32}$P and no ${}^{35}$S.
The result clearly shows that only DNA is the genetic material and protein coat does not even enter the bacterial cells and as such not protein but DNA is the genetic material.