Alfred D. Hershey and Martha Chase cultured bacteriophage in 2 different media one containing radioactive phosphorus (32P) and the other media containing radioactive sulfur (35S). Phosphorous is a component of the nucleotides which form the nucleic acid while sulfur is a component of amino acids methionine and cysteine which are among the 20 amino acids that form the protein. Now the two samples of bacteriophage were allowed to infect its host cell i.e., Escherichia coli. After infection, the samples were analyzed for the radioactivity in the host cells. For analysis, virus and bacteria were separated via centrifugation. Since the bacteria are large they form a solid pellet at the bottom after centrifugation and the smaller viruses remained in the supernatant. The bacterial pellet was found to be radioactive when infected by the 32P–viruses. While no radioactivity was found in the bacterial pellet when infected by the 35s–viruses. Thus, proving DNA is the genetic material.