In the mid-twentieth century, scientists were still unsure as to whether DNA or protein was the genetic material of the cell

• Eukaryotic chromosomes were known to contain DNA and protein
  

• Viruses consisting solely of DNA and a protein coat could transfer their genetic material into hosts
  

In 1952, Alfred Hershey and Martha Chase conducted a series of experiments to prove that DNA was the genetic material

• Viruses (T2 bacteriophage) were grown in one of two isotopic mediums in order to radioactively label a specific viral component

  • Viruses grown in radioactive sulfur (35S) had radiolabelled proteins (sulfur is present in proteins but not DNA)

  • Viruses grown in radioactive phosphorus (32P) had radiolabeled DNA (phosphorus is present in DNA but not proteins)

• The viruses were then allowed to infect a bacterium (E. coli) and then the virus and bacteria were separated via centrifugation

• The larger bacteria formed a solid pellet while the smaller viruses remained in the supernatant

The bacterial pellet was found to be radioactive when infected by the 32P–viruses (DNA) but not the 35S–viruses (protein)

• This demonstrated that DNA, not protein, was the genetic material because DNA was transferred to the bacteria