In 1952, Alfred Hershey and Martha Chase published a convincing demonstration that DNA (not protein) was the genetic material. The Hershey–Chase experiment was carried out with a virus, called bacteriophage T2, that infects bacteria. Bacteriophage T2 consists of little more than a DNA core packed inside a protein coat. Thus, the virus is made of the two materials that were, at the time, the leading candidates for the genetic material.
The Hershey–Chase experiment was based on the biology of a virus, called bacteriophage T2, that attacks a bacterium. The virus consists of a DNA core packed inside a protein coat.
Part (but not all) of the virus enters the bacterial cell. About 20 minutes later, the cell bursts, releasing dozens of particles that are virtually identical to the infecting virus particle. Hershey and Chase deduced that the entry of some viral component affects the genetic program of the host bacterial cell, transforming it into a bacteriophage factory. They set out to determine which part of the virus—DNA or protein—enters the bacterial cell.
To trace the two components of the virus over its life cycle, Hershey and Chase labeled each component with a specific radioisotope. They used 32P to label DNA. The two strands of DNA have a sugar–phosphate backbone that contains phosphorus atoms. Phosphorus is not present in most proteins. They used 35S to label proteins. Proteins contain some sulfur, because sulfur is found in the amino acids cysteine and methionine. Sulfur is not present in DNA.
After growing the virus in the presence of either 32P or 35S, the two radioactive viral samples were added along with the host bacteria to fresh culture medium. The viruses were given enough time to infect the bacteria, after which the culture medium was agitated in a blender to detach the viruses from the bacterial cells.
Samples were then placed in tubes and centrifuged to force the bacterial cells to the bottom of the tube, forming a pellet. The supernatant fluid contains the viruses.
In each of these samples, the radioactive component will be in a different fraction, either the pellet or the supernatant. If DNA is, in fact, the genetic material, predict which of the two tubes—the one with 32P or the one with 35S—will have a radioactive pellet.
Hershey and Chase found that most of the 32P is in the pellet with the bacteria, while most of the 35S is in the supernatant fluid with the viruses. Because the 32P labeled the DNA, the scientists concluded that DNA, not protein, enters bacterial cells and directs the assembly of new viruses. That is, DNA is the genetic material.
In addition to the experiment described in this tutorial, Hershey and Chase performed similar but longer-term experiments, allowing the progeny (offspring) generation of viruses to reproduce in unlabeled bacteria. The resulting viruses contained almost no 35S and none of the parental viral protein. They did, however, contain about one-third of the original 32P—and thus, presumably, one-third of the original DNA. Because DNA was carried over in the viruses from generation to generation but protein was not, the logical conclusion was that the hereditary information was contained in the DNA.