Hershey, Alfred Day (1908-1997)

US biochemist who was awarded a Nobel Prize for Physiology or Medicine in 1969 for his work on the replication mechanism and genetic structure of viruses. He used bacteriophages (viruses that infect bacteria) to demonstrate that DNA, not protein, is the genetic material. His experiments demonstrated that viral DNA is sufficient to transform bacteria.

With Martha Chase he demonstrated that DNA is the genetic material, by studying the T2 bacteriophage, a virus that infects the bacterium Escherichia coli. It had been suggested by Roger Herriot in 1951 that a bacteriophage acted like ‘a little hypodermic needle full of transforming principles’; the virus as such never enters the cell; only the tail contacts the host and perhaps enzymatically cuts a small hole through the outer membrane and then the nucleic acid of the virus head flows into the cell.

Hershey and Chase tested this hypothesis by labelling the bacteriophage DNA with radioactive phosphorus and the protein with radioactive sulphur. A sample of E. coli was infected with the radiolabelled bacteriophage for a short incubation and then the two were separated by centrifugation. The bacteria were found to be full of radioactive phosphorus but practically devoid of radioactive sulphur. The bacteria were also fully competent to produce progeny virus. Hershey and Chase's conclusion was that ‘a physical separation of the phage T2 into genetic and nongenetic parts is possible’.

Hershey was born in Owosso, Michigan, USA. He graduated from Michigan State College and for the next 24 years worked at the Carnegie Institution in Washington to provide evidence for the nature of the material in genes.