The Hershey & Chase Experiment (DP IB Biology: HL): Revision Note

The Hershey & Chase Experiment

Which Biomolecule is the Heritable Material?

• DNA was identified in 1869 but many scientists assumed that protein was the heritable material

  • Owing to the fact that there are 20 amino acids and only 4 nucleotide bases

• In the 1950s, Alfred Hershey and Martha Chase showed that DNA, not protein, is a factor of heredity responsible for carrying genetic information from one generation to another

• Viruses that infect bacteria were used in their experiment as they only consist of DNA encapsulated by a protein coat

• This would allow the biomolecule of heredity (i.e. the one that caused bacterial cells to be used to produce viral progeny) to be easily determined

Analysis of results of the Hershey and Chase experiment

• Hershey and Chase used the T2 bacteriophage, a virus that infects the bacterium E. coli

• Their method relied on a chemical difference between the two biomolecules: DNA contains phosphorus but no sulfur, while proteins contain sulfur but no phosphorus

  • This allows a radioactive isotope of each element to label one biomolecule without labelling the other

Step 1: Making labelled viruses

• Bacteria were grown in media containing either radioactive sulfur (³⁵S) or radioactive phosphorus (³²P) , then infected with viruses

• The progeny viruses took up the radioactive atoms, giving either ³⁵S-labelled protein coats or ³²P-labelled DNA

• This step produced the labelled viruses; it is not the experiment itself

Step 2: Infecting unlabelled bacteria

• Fresh, unlabelled bacteria were infected separately with each type of labelled virus

• Whichever biomolecule entered the bacteria must be the heritable material, as it directs the production of new viruses

Step 3: Separating viruses from bacteria

• A blender detached the viral coats from the bacterial cells, and centrifugation separated them

  • Viruses are small, so remained in the supernatant; bacteria are larger, so formed a pellet

Results and conclusion

• With ³²P-labelled viruses, radioactivity was found in the pellet — viral DNA had entered the bacteria

• With ³⁵S-labelled viruses, radioactivity remained in the supernatant — the protein coats had stayed outside

• DNA, not protein, is therefore the hereditary material

Hershey and Chase experiment diagram

Hershey and Chase's experiment provided unequivocal proof that DNA is the heritable material

NOS: Technological developments can open up new possibilities for experiments - The availability of radioisotopes as research tools made the Hershey-Chase experiment possible

• Radioisotopes were made available to scientists as research tools at the end of the Second World War

• This enabled scientists in a variety of research fields, such as biochemistry and virology, to do experiments that were not previously possible

• Isotopes are particularly useful in studying chemical changes that occur during metabolic pathways or life cycles in organisms

• Without the availability of radioisotopes, Hershey and Chase would not have been able to label the different parts of a virus in order to determine that DNA is the heritable material in organisms