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First exams 2025

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The Hershey & Chase Experiment (HL) (HL IB Biology)

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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 took advantage of the chemical differences between DNA and proteins
    • DNA contains phosphorus but no sulfur
    • Amino acids (that make up proteins) contain sulfur but no phosphorus
  • Bacteria grown in separate media containing either radioactive sulfur (35S) or radioactive phosphorus (32P) were infected with viruses
  • The progeny viruses contained either 35S labelled proteins or 32P labelled DNA
  • Unlabelled bacteria were then infected separately with either type of virus
    • Bacteria would be expected to contain the heritable material following infection
  • A blender was used to remove attached viruses from the bacterial cells and centrifugation was used to isolate the bacteria
    • Viruses are small so remained in the supernatant in the centrifuge tube
    • Bacteria are larger so formed a pellet
  • Only the bacteria infected by 32P labelled viruses (DNA) were shown to be radioactive
  • This suggested that DNA (and not protein) was transferred to bacteria and is the hereditary (genetic) material

Hershey and Chase experiment diagram

Hershey and Chase experiment diagram 1
Hershey and Chase experiment diagram 2
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 

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.