In 1917 the first bacterial viruses were discovered. Felix d'Herelle was experimenting with bacteria. His experiments led to the discovery of an organism capable of killing bacteria. He called these organisms "bacteriophage," meaning eater of bacteria. His hope was that this discovery would be useful in fighting disease. What he never anticipated was that it was crucial in establishing the identity of DNA (deoxyribonucleic acid) as the genetic material of all living things.
In 1952 Alfred Hershey and Martha Chase used this discovery as the basis for their experiment. They wanted to determine what the genetic material was of a bacteriophage. They knew that a bacterial virus is composed only of protein and DNA. Protein makes up the exterior of the virus and the DNA is in the inside. When a bacterium is infected by bacteriophage, the internal machinery falls under the control of the virus which uses the bacterium to produce more viruses. They wanted to understand which substance directed this take over - the DNA or the protein?
They took advantage the differences in composition of the protein and the DNA to conduct their experiment. Protein contains sulfur, DNA does not. Protein contains small amounts of phosphorus while DNA contains large amounts of phosphorus.
They used radioactive sulfur and radioactive phosphorus, added bacteriophage and grew these two cultures. The viruses grown in the culture with radioactive sulfur picked it up and incorporated it into their protein. This created a bacteriophage with a radioactive external protein coat. The culture grown with the radioactive phosphorus incorporated a little of it into the protein but most of it into the DNA. What this gave them was two types of bacteriophages: one with a radioactive external protein coat and another one with radioactive DNA. They used these in their experiment.
Each of these bacteriophages was introduced to separate cultures of bacteria. They were allowed to infect the bacteria, and then put into a kitchen blender which caused any part of the bacteriophages that had not gotten inside the bacteria to fall off. The cultures were then spun in a centrifuge, which separates the materials according to their weight. The heavier cells fell to the bottom and formed a pellet, the lighter ones remained liquid.
They wanted to know where the radioactivity was now. In the cultures infected with the radioactive sulfur, most of it was in the liquid. In the cultures with the radioactive phosphorus, the radioactivity was in the pellets. So, the protein had not entered the bacteria, but the DNA had.
The final proof that the DNA was the genetic material and not the protein, was provided by examining the children of the DNA (or phosphorus-labeled) bacteriophages. The radioactive DNA had passed down to them from their parents, but not the radioactive protein. What Hershey and Chase never realized was that this discovery was crucial in providing proof to the scientific community that DNA alone is the material of heredity. Their findings also served as motivation to Watson and Crick in their search to determine the structure of DNA.Mendel
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