The Hershey-Chase experiment

The second classical demonstration of DNA as the genetic material was an outgrowth of a group of molecular biologists who in the late 1940s and early 1950s started to use viruses that attack bacteria (bacteriophages--or literally "bacteria eaters") to study the nature of heredity.

This group, the "phage group", at Indiana University included the very young James Watson who would later propose a structure for DNA along with Francis Crick.

Alfred Hershey was one of the founders of this group. One of his students, Martha Chase, performed an experiment which clearly distinguished between DNA and protein as the genetic material

The phage group had identified a series of bacteriophage and shown that they consisted of a combination of only DNA and protein.

They had also shown that these viruses attacked cells from the outside, but that most of the virus never entered the cell.

Later work showed that the viruses are like microscopic hypodermic needles, injected their genome into the inside of the bacteria

Hershey & Chase used a simple radiolabling protocol to distinguish between the DNA and protein portions of the viruses being the genetic material

They knew that protein includes the atom sulfur (in two of the amino acids) but that DNA does not, and that DNA contains phosphorus while the proteins of the viruses did not.

They labeled viruses with either a radioactive form of sulfur (35S) or of phosphorus (32P) and used them to infect cells.

After allowing the viruses to infect the cells they put them through a blender-this knocked the viruses off of the cells without affecting the cells.

They found that the cells were still infected after this treatment.

They looked for the radioactivity of the 35S or the 32P and found that some of the 32P went with the cells, while all of the 35S did not.

This simple experiment clearly suggested that the nucleic acid, but not the protein, was the genetic material since the nucleic acid, and not the protein was transfered into cells when they were infected by viruses.