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Chapter 8:
DNA and RNA
Section 8-1:
Discovering DNA
The Language of Genes
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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.
Griffith and Transformation
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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
Griffith and Transformation
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First major experiment – Griffith heated a
sample of smooth bacteria, injected it into
mice
Result = no pneumonia, concluded heat
killed all bacteria
Griffith and Transformation
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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
Griffith and Transformation
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Conclusion – Some molecule or group of
molecules had changed harmless rough
bacteria into deadly smooth bacteria.
Process called transformation
Avery and DNA
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Oswald Avery (Canada, 1944)
Said key to finding molecule of heredity was
studying transformation
Hypothesized genes had been transferred in
Griffith’s experiments
Conducted experiments in which heat-killed
bacteria were treated with enzymes to
destroy macromolecules
Avery and DNA
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In all cases, transformation still occurred
EXCEPT when DNA was destroyed
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Conclusion – Genes are made of DNA
Hershey-Chase Experiment
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Alfred Hershey and Martha Chase (USA,
1952)
Studied bacteriophages, type of virus that
infects bacteria
–
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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?
Hershey-Chase Experiment
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Used radioactive isotopes to label viral
DNA and protein
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Phosphorus-32 used to label DNA, sulfur-35
used to label protein
Hershey-Chase Experiment
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Bacteria infected and analyzed - only
phosphorus-32 found in infected bacteria
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Conclusion – Genetic material of
bacteriophage is DNA, not protein.