Hershey and Chase Experiment on T2 Bacteriophage

Hershey and Chase worked with bacteriophage T2, a virus that infects the bacterium Escherichia coli. When T2 particles encounter a bacterium, they attach to its outer surface, inject some of their own material into the cell, and the infected bacterium eventually lyses (bursts) to release a large new generation of phage particles; the open question was whether the material injected into the bacterium, the material actually responsible for directing this new generation, was the phage's protein coat or its DNA. Because nucleic acids are rich in phosphorus but contain no sulphur, while proteins are rich in sulphur (in the amino acids cysteine and methionine) but contain essentially no phosphorus, Hershey and Chase realised they could label each component separately and unambiguously using radioactive isotopes: phage grown in a medium containing radioactive sulphur (35S) would have their protein coats specifically labelled, while phage grown in a medium containing radioactive phosphorus (32P) would have their DNA specifically labelled. They allowed each batch of labelled phage to infect ordinary, unlabelled E. coli, and then, before the bacteria had time to lyse, agitated the infected culture in a kitchen blender, a step gentle enough not to damage the bacteria but forceful enough to shear off any phage material still stuck to the outside of the cell wall. Centrifuging the mixture afterward let them separate the heavier bacterial cells (which pellet at the bottom) from the lighter, free phage debris (which stays in the supernatant liquid). The result was unambiguous: essentially all of the radioactive phosphorus (32P) was found inside the bacterial pellet, while essentially all of the radioactive sulphur (35S) stayed behind in the surrounding medium, never having entered the cell at all. Even the next generation of phage produced by these infected bac …