Principles Module 46


  • Discuss the key experiments that demonstrated how we know that DNA is the heritable material

Key points

Morgan’s experiments on sex chromosomes demonstrated that genes were carried on chromosomes

Griffith’s transformation experiment

  • worked on pneumonia using mouse as a model system
  • worked with 2 strains of bacteria, virulent (S) and nonvirulent ®
  • experiment
    • inject with S strain, mice died
    • inject with R strain, mice lived
    • inject with heat-killed S strain, mice lived
    • inject with heat-killed S strain mixed with R strain, mice died
  • conclusion: something must be transferred from the dead S strain to the live R strain that transformed them into the S strain
Avery, McLeod, and McCarty experiment
  • used the same model system as Griffith
  • employed enzymes to digest either proteins, DNA, or RNA from the heat-killed S strain
  • transformation was inhibited when DNA was digested, but not protein or RNA
Hershey and Chase experiment
  • worked with bacteriophage virus (T2) that infects E. coli cells
  • this was a simplified model system because the virus only has protein and DNA
  • knew that phages injected genetic material into host cell in order to replicate
  • used radioactive P (in DNA) or S (in protein) to label molecules
  • infected bacteria, removed the virus particles, identified the radiolabel
  • E. coli cells contained radioactive P, not S, indicating that DNA was transferred from phage

In-class activities

Questions for Practice

  • In the Griffith experiment, what was the purpose of heating the S strain? Explain the result of mixing the killed S strain with the R strain.
  • In the Avery experiment, why was the killed S strain treated with enzymes?
  • What role did 35S and 32P play in the Hershey and Chase experiment?