Structural Biochemistry/Nucleic Acid/DNA/Hershey-Chase Experiment

History

In 1944 Oswald Avery and colleagues did an experiment involving the use of pathogenic bacteria to determine the material that contained genetic information. Their experiment concluded that it was DNA and not proteins that is the hereditary material. Despite the findings, the popular and widely accepted conclusion remained that protein encoded genetic information, accounting for its diversity in function and much greater number compared to DNA. In order to gain more evidence on DNA scientists by the name Alfred Hershey and Martha Chase decided to perform a simple but effective experiment involving bacteriophages.

Bacteriophage

In order to understand the experiment that was performed we must examine first the vector used which played a crucial role in the experiment – Bacteriophages. Bacteriophages are types of viruses which infect bacteria such as Escherichia coli. They consist of a protein coat, collar, base plate, tail fiber and most importantly a head which houses the genetic material. They have a unique feature, which makes them the perfect candidate to prove whether DNA or proteins house the genetic information. They have an outer capsule of proteins, which surround an inner core of DNA. Bacteriophages, being viruses, are unable to proliferate on their own accord, as they lack the necessary system to do so. Viruses invade a host cell and inject their genetic material to the host’s own gene and allow the host to replicate the viral gene. Knowing this, Martha and Hershey Chase saw that if they labeled the bacteriophages they would be able to track what genetic information is passed on to the host bacteria - the labeled DNA or the labeled protein coat.

Experimental

A bacteriophage was taken and its encased DNA was labeled with radioactive ³²P and its protein coat was left nonradioactive. The bacteriophage was exposed to a sample of bacteria. The phage attached to the surface of the bacterial cell and injected the labeled DNA. The sample was then chilled to arrest growth. The sample was then shaken vigorously for several minutes in a Waring Blender. This process separated the phages coat from the surface of the bacteria. The sample was then centrifuged very quickly. The bacterial cells were at the bottom of the tube and the phage particles were in the supernatant. Hershey and Chase discovered that there was no disruption in the reproduction of viral phages inside the bacteria. A new generation of viruses had successfully propagated inside the host cell and these phages exhibited ³²P radioactivity in their own DNA.

Another set of Bacteriophage was then examined, this time nnznwith a radioactive protein coat ³⁵S and a nonradioactive DNA. The same procedure was followed and the phage attached to the bacterial wall and was allowed to inject its genetic material. Vigorous shaking of the bacteria caused the radioactive viral sheath to detach from the bacteria. Injection of the viral DNA into the bacteria still occurred and new phages were observed to have been produced. However, analysis of the new phages inside the bacteria showed that it had no radioactive properties; a property which should be present in the new phages, if proteins were in fact the genetic material that is passed on to new progeny. This experiment therefore illustrated that DNA, not protein, is the source of genetic information.

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Results

The first bacterial cell contained phages with observable radioactivity illustrating that the radioactive property present on the parent phage was passed on to the new phages. The second bacterial cell however showed no hint of 35S, showing that it was removed along with the protein coat and did not enter the bacteria. Hershey and Chase then deduced that the genetic material that is being passed on is DNA and not protein as previously accepted before.

This famous 1952 experiment allowed Hershey and Chase to demonstrate that it was DNA, not protein, that functioned as the T2 phage's genetic material. Viral proteins, labeled with radioactive sulfur, remained in the ouside of the host cell during infection. In contrast, the viral DNA, which was label with radioactive phosphorus, entered the bacterial cell. Concluding that the DNA is in fact the material within cells that contains useful genetic information.

Additional Information

An animated video of the Hershey and Chase experiment can be viewed by clicking on this link
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

The published papers of Martha Chase and Alfred Hershey can be viewed in this link
http://osulibrary.orst.edu/specialcollections/coll/pauling/dna/papers/hersheychase.html

Sources

Berg, Jeremy, John Tyzmozcko, Lubert Stryer. Biochemistry
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookDNAMOLGEN.html
http://www.accessexcellence.org/RC/VL/GG/hershey.php
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#
http://en.wikipedia.org/wiki/Hershey-Chase_experiment