Several experiments were conducted in the past to prove that the genetic material is made of proteins or DNA. Frederick Griffith conducted experiments with Streptococcus pneumoniae that cause pneumonia. It has two types of bacteria strains- the pathogenic S-strain and the non-pathogenic R-strain. When heat-killed S-strain bacterial cells were mixed with living R-strain bacterial cells, and were injected into healthy mice, they died of pneumonia. The bacteria isolated from dead mice were found to be of the living S-strain type. This proved that the R-strain bacteria had ‘transformed’ to the living S-strain bacteria.
Later Oswald Avery, Colin MacLeod and Maclyn McCarty repeated Griffith’s experiments and purified all three biochemicals such as proteins, DNA and RNA from heat-killed S-strain cells. They discovered that neither the proteases nor the RNases affected this transformation, whereas the DNases inhibited the transformation. This confirmed that the transforming substance was DNA.
Alfred Hershey and Martha Chase conducted experiments with viruses that attack bacteria. These viruses attach to bacteria and implant their viral genetic material inside the bacterial cell. In the experiment, viruses were grown on two different mediums: one with radioactive phosphorus and the other with radioactive sulfur.
The viruses grown in radioactive phosphorus had radioactive DNA and the viruses grown on radioactive sulfur had radioactive proteins. Thereafter, both these radioactive phages were allowed to infect E. coli bacteria. The bacteria infected with viruses containing radioactive proteins were not radioactive, while the bacteria infected with viruses containing radioactive DNA were radioactive. From this experiment, it was proved that it was the DNA, which entered from the virus to the bacteria and is the genetic material.