summary
In order to determine whether protein or DNA is the genetic material that is necessary for replicating viruses inside of cells, Hershey and Chase used and followed the path of the radioactive elements Sulfur-35 and Phosphate-32 in the infection process. The scientists modified the T2 bacteriophage to have different radioactive elements based on different parts of the virus. In order to determine the transformation factor they infected bacteria cells, mixed them in a blender to knock off the elements of the phage that do not enter the cell, then spun the test tube to separate the pellet, or infected bacteria cell, from the other debris. By using radioactive elements, Hershey and Chase were able to determine whether the radioactivity was coming from the pellet or the sediment at the end of the experiment, ultimately proving which macromolecule contains the genetic material. Click here to watch a brief animation of the experiment.
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Phase 1: Protein Proteins consist of the element sulfur and DNA does not, therefore by using the radioactive element S-35 in the protein coat of the bacteriophage, Hershey and Chase were able to determine that after the phage attached to the bacteria cell, the DNA would enter the cell and reproduce the virus. The S-35 associated with the protein coat was found only in the growth medium outside of the bacterial cells, while the virus reproducing inside of the bacterium remained unaffected by the radioactive isotope. This indicates that protein is not the macromolecule containing genetic information that is transferred from generation to generation of bacteria cells.
Phase 2: DNA The backbone of DNA contains a phosphate group, so Hershey and Chase were able to use the radioactive element P-32 in the DNA and track its path in the process of the bacteriophage infecting the bacterium. They discovered that when using the P-32 isotope in the the DNA, the new viruses also had the P-32 isotope in the pellet, ultimately determining that DNA is the macromolecule that carries genetic material.
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The Data To the left is a data table from Hershey's publication Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage (Click here). This data proves that DNA is in fact the genetic material inside of cells. Because the test tubes were spun in a centrifuge, the bacteria cell was separated from the bacteriophage remnants. The bacteria cell is propelled to the bottom of the test tube, becoming the pellet or sediment fraction while the phage parts shaken from the cell in the blender are considered the supernatant fraction. The several sets of data tables ultimately come to the conclusion that "most of the phage sulfur remains at the cell surface and most of the phage DNA enters the cell on infection" (Hershey and Chase 7).
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Quality of the experiment
Hershey and Chase's experiment to determine which macromolecule provided hereditary information was of high quality because they practiced consistency in variables. For example, in his publication Hershey writes, "In all fractionations involving centrifugation, the sediments were not washed, and contained about 5 per cent of the supernatant. Both fractions were assayed,"(Hershey and Chase 2) ultimately demonstrating how from trial to trial, the exact same conditions were simulated.
An experiment that is inconsistent does not provide reliable data, however because Hershey and Chase were unvarying in their procedure, the data recorded is concrete proof that DNA is the genetic material in cells, pointing other scientists at the time studying hereditary material in the direction of DNA.
An experiment that is inconsistent does not provide reliable data, however because Hershey and Chase were unvarying in their procedure, the data recorded is concrete proof that DNA is the genetic material in cells, pointing other scientists at the time studying hereditary material in the direction of DNA.