Alfred Hershey and Martha Chase Experiment

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The experiment by Alfred Hershey and Martha Chase used bacteriophages, or viruses that contaminate bacteria and radioisotopes. Hershey and Chase already knew that viruses were composed mainly of DNA and protein; however, they did not know if DNA or protein was the genetic material. Hershey and Chase used radioisotopes to mark the DNA and protein. They used the radioactive isotopes phosphorus and sulfur because DNA contains phosphorus and proteins contain sulfur. Using these radioactive isotopes gave them the ability to distinguish between the DNA and the protein. They rationalized that if they allowed ample time for a bacteriophage to contaminate a bacterial cell that the genetic material would be discovered in the bacterial cell after the contamination. After allowing bacteriophages to infect the bacterial cells, they noticed that the radioactively labeled DNA was found inside the bacterial cell, and that the radioactively labeled protein was found outside of the bacterial cell. Hershey and Chase concluded that DNA was the genetic material that was introduced to the bacteria during contamination by a bacteriophage. Griffith worked with two different strains of Streptococcus pneumonia, a type S strain, and a type R strain. Type S bacteria were characterized by the existence of a polysaccharide, which allowed them to evade being attacked by the host cell's immune system; however, type R bacteria did not have such a polysaccharide capsule. Griffith injected type S bacteria into the mice. Due to the existence of the polysaccharide capsule, the type S bacteria were able to thrive in the mouse's blood stream. Therefore, the mouse died. Afterwards, Griffith injected type R bacteria into mice. Type R bacteria did not have the polysaccharide capsule so they were not able to elude the defenses by the host cell's immune system. Consequentially, the mouse still survived because the bacteria were destroyed by the immune system. Next, Griffith added the heat-killed type S...
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