Chapter 8: DNA and RNA

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Chapter 8DNA and RNA

• Section 8-1 Discovering DNA

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The Language of Genes

• In order to understand how genes are stored on
  chromosomes, biologists had to learn the molecule
  of heredity and the language to interpret the
  molecular code.

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Griffith and Transformation

• Frederick Griffith (Britain, 1928)
• Studying causes of pneumonia, isolated two types
  of bacteria from victims smooth and rough
• Injected mice with both types of bacteria those
  injected with smooth bacteria developed pneumonia

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Griffith and Transformation

• First major experiment Griffith heated a sample
  of smooth bacteria, injected it into mice
• Result no pneumonia, concluded heat killed all
  bacteria

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Griffith and Transformation

• Second experiment mixed heat-killed smooth
  bacteria with harmless rough bacteria
• Result mice developed pneumonia and died
• Samples taken from mice showed live, smooth
  bacteria

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Griffith and Transformation

• Conclusion Some molecule or group of molecules
  had changed harmless rough bacteria into deadly
  smooth bacteria.
• Process called transformation

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Avery and DNA

• Oswald Avery (Canada, 1944)
• Said key to finding molecule of heredity was
  studying transformation
• Hypothesized genes had been transferred in
  Griffiths experiments
• Conducted experiments in which heat-killed
  bacteria were treated with enzymes to destroy
  macromolecules

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Avery and DNA

• In all cases, transformation still occurred
  EXCEPT when DNA was destroyed
• Conclusion Genes are made of DNA

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Hershey-Chase Experiment

• Alfred Hershey and Martha Chase (USA, 1952)
• Studied bacteriophages, type of virus that
  infects bacteria
• Attach to outside of bacterium, inject something
  into bacterium that allow it to make more viruses
• Viruses only contain DNA and protein which ones
  contains genes?

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Hershey-Chase Experiment

• Used radioactive isotopes to label viral DNA and
  protein
• Phosphorus-32 used to label DNA, sulfur-35 used
  to label protein

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Hershey-Chase Experiment

• Bacteria infected and analyzed - only
  phosphorus-32 found in infected bacteria
• Conclusion Genetic material of bacteriophage is
  DNA, not protein.