Phages have been widely used as tools by researchers in molecular genetics. In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2. This is one of many phages that infect Escherichia coli (E. coli), a bacterium that normally lives in the intestines of mammals and is a model organism for molecular biologists. At that time,biologists already knew that T2, like many other phages, was composed almost entirely of DNA and protein. They also knew that the T2 phage could quickly turn an E. coli
cell into a T2-producing factory that released many copies of new phages when the cell ruptured. Somehow, T2 could reprogram its host cell to produce viruses. But which viral component-protein or DNA-was responsible?
Hershey and Chase answered this question by devising an experiment showing that only one of the two components of T2 actually enters the E. coli cell during infection. In their experiment, they used a radioactive isotope of sulfur to tag protein in one batch of T2 and a radioactive isotope of phosphorus to tag DNA in a second batch. Because protein, but not DNA, contains sulfur, radioactive sulfur atoms were incorporated only into the protein of the phage. In a similar way, the atoms of radioactive
phosphorus labeled only the DNA, not the protein, because nearly all the phage’s phosphorus is in its DNA. In the experiment, separate samples of nonradioactive E. coli cells were infected with the protein-labeled and DNA-labeled batches of T2. The researchers then tested the two samples
shortly after the onset of infection to see which type of molecule-protein or DNA-had entered the bacterial cells and would therefore be capable of reprogramming them.
Hershey and Chase found that the phage DNA entered the host cells but the phage protein did not. Moreover, when these bacteria were returned to a culture medium, and the infection ran its course, the E. coli released phages that contained some radioactive phosphorus. This result further showed that the DNA inside the cell played an ongoing role during the infection process.
Hershey and Chase concluded that the DNA injected by the phage must be the molecule carrying the genetic information that makes the cells produce new viral DNA and proteins. The Hershey-Chase experiment was a landmark
study because it provided powerful evidence that nuclei acids, rather than proteins, are the hereditary material, at least for certain viruses.BY CELL,