Hershey-Chase experiment

While DNA had been known to biologists since 1969, Some scientists believed that protein was the genetic material because protein appeared to have the necessary complexity to encode all the biochemical information in the cell's nucleus. Proteins are chains of twenty different and relatively simple organic molecules called amino acids. A protein can be made of ten amino acids or 1000 amino acids or 10,000 amino acids. The crucial element of any protein is the sequence of amino acids, that is, which amino acid follows which in the chain. This sequence of amino acids is as important to proteins as is the sequence of letters in a word ("ton" has a much different meaning than "not"). Protein supporters were of the opinion that the amino acid sequence in one protein serves as a model for constructing a new protein. Their outlook would be dealt a severe blow by the 1952 experiment of Hershey and Chase.

HERSHEY-CHASE EXPERIMENT

Dr. Alfred Hershey and Martha Chase

Alfred Hershey worked with Martha Chase at the Cold Spring Harbor Laboratory in New York. The pair studied bacteria and the viruses that multiply within those bacteria. In 1952, scientists knew that certain viruses use bacteria as chemical factories for producing new viruses (Figure 1), however, the actual mechanism was uncertain. Biochemists were also aware that bacterial viruses are composed of a core of DNA enshrouded in a protein coat. What they did not know was whether the nucleic acid or the protein (or both) directs replication of the virus. Hershey and Chase would answer that question and in so doing, they would establish the essential role played by DNA in cellular biochemistry and inheritance.

Figure (1)

Hershey and Chase made use of the observation that viral DNA contains phosphorus (P) but no sulfur (S). By contrast, the outer protein coat of the virus has sulfur (S) but no phosphorus (P). In their first experiments, Hershey and Chase cultivated viruses with the radioactive forms of phosphorus (32p) and sulfur (35S). They successfully prepared viruses whose nucleic acid was radioactive with 32p and whose protein was radioactive with 35S.

Now came the seminal experiments. Hershey and Chase mixed the radioactive viruses with a population of bacteria. Then they waited just long enough for viral replication to begin. At this point they used an ordinary household blender to shear away any viruses and debris clinging to the bacterial surface.

Then the analysis began. Hershey and Chase tested the bacteria and surrounding fluid to find out where the radioactivity was. This would enable them to develop a biochemical glimpse of viral replication. After experimentally bursting the bacteria, the researchers found most of the 32p within the contents of the bacterial cytoplasm. This finding indicated that viral DNA was entering the bacteria. Then they discovered that the 35S was largely in the sheared-away remains of the viruses and in the surrounding fluid. This observation indicated that the protein part of the viruses was remaining outside the bacteria. The results led Hershey and Chase to the inescapable conclusion that viral DNA enters the bacterium, whereas the viral protein remains outside (Figure 2 shows the process). Thus, DNA was the sole element responsible for viral replication. Protein had no place in the process.

Figure (2)

Certain experiments stand out as turning points in scientific history, and the experiments performed by Hershey and Chase are one such turning point. In retrospect, we can see how their results had substantial impact on the thinking of that era. Hershey and Chase clarified the important aspect of viral replication that nucleic acid goes inside the cell, whereas the protein coat remains outside. But in broader terms, their results strengthened the place of DNA in cellular biochemistry. Bacterial viruses, it should be remembered, are composed solely of nucleic acid and protein, and the Hershey-Chase experiments reinforced the concept that DNA, and only DNA, is involved in the synthesis of both nucleic acid and protein.