Alfred Hershey and Martha Chase Experiment

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.

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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 bacteria into the mice. The bacteria were heat-killed preceding the injection into the mouse so the mouse survived. Finally, Griffith injected living type R bacteria and heat-killed type S bacteria into the mouse. Griffith discovered that the mouse died.

He concluded that the living type R bacteria were altered into the type S strain. Evidently, the type R bacteria had developed genetic material from the heat-killed type S bacteria; however, Griffith did not know what the genetic material was. Meselson and Stahl conducted experiments to determine whether or not DNA followed the semiconservative, conservative, or dispersive model of replication. The semiconservative model states that the two daughter molecules each consist of one old strand, from the parent, and one newly constructed strand.

This is the model that is currently accepted. The conservative model states that the parent molecule is preserved after DNA replication. Lastly, the dispersive model states that each of the four strands has a mixture of old and new DNA after replication. Meselson and Stahl’s experiments involved radioisotopes. They cultivated bacteria into a medium containing nucleotide precursors marked with Nitrogen-15. The bacteria combined the Nitrogen-15 into their DNA. The bacteria were then moved into a medium containing Nitrogen-14.

Any recently made DNA would appear lighter than the parental DNA made in the medium containing Nitrogen-15. The contents of the container were positioned into two separate tubes and centrifuged. One tube was centrifuged for 20 minutes and the other tube was centrifuged for 40 minutes. The first round of replication in the Nitrogen-14 medium produced hybrid DNA, which disregarded the conservative model. The second round of replication in the Nitrogen-14 medium produced both light and hybrid DNA. This rejected the dispersive model and reinforced the semiconservative model.

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