SBI4U
DNA: The Molecular Basis of Life
DNA: The Hereditary Material
 Is protein or DNA the hereditary material?
 Most scientists thought proteins carried genetic information because DNA had a much
simpler structure than proteins.
Friedrich Miescher (1869)
 He used pus cells to investigate the chemical composition of DNA.
 He discovered that the nuclei of cells contain large quantities of a substance that does not
act as a protein.
 He called this substance _____________.
Joachim Hammerling (1930)
 His experiments suggested that the hereditary material is located in _____________.
 He conducted experiments on a one-celled green __________________
Frederick Griffith's Experiment (1920) - The Discovery of Transformation
 He wanted to find a vaccine against pneumonia caused by Streptococcus pneumonia
 He used two varieties of streptococcus
o One variety of bacteria had a capsule (like a cell wall) – Type S
o The other did not have a capsule. – Type R
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SBI4U
DNA: The Molecular Basis of Life
Experiment
Injection with live Type S
Injection with live Type R
Injection with heat killed Type S
Injection with dead Type S and live Type R
*Note: Neither of these forms caused disease before, but
when placed together, something occurred to make the
living Type R (naked) bacteria virulent.
Result
Mice contracted
Mice lived, immune system destroyed
________________
Mice _____________________
Mice contracted
_____________________________
Conclusions
 Living bacteria acquired genetic information from dead bacteria - particularly the
instructions for making capsules, thus transforming the naked bacteria into encapsulated
bacteria.
 The transforming agent was discovered to be DNA. DNA was isolated and added to live
naked bacteria, and they were transformed into the encapsulated kind
 Some scientists still remained unconvinced.
The Hershey-Chase Experiment – Bacteriophage (1952)
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SBI4U
DNA: The Molecular Basis of Life
 Hershey and Chase conducted experiments using a virus (__________________) labeled
with radioactive phosphorus (P32) and sulfur (S35).
 Bacteriophages are composed of DNA and a protein coat.
 DNA contains phosphorous but not sulfur; proteins contain sulfur but not phosphorous.
(Therefore ________________________________________________________.)
 A bacteriophage infects a bacterial cell by attaching to the outer surface of the cell,
injecting its hereditary information into the cell and producing thousands of new viruses.
 Bacteria infected by phages containing radioactive protein did _____________________.
 Bacteria infected by phages containing radioactive DNA became ___________________.
Conclusions
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SBI4U
DNA: The Molecular Basis of Life
 It was the phosphorus-rich _________, not the protein, which was the molecule of
heredity.
………..By 1920:
 It was known that DNA consisted of three components:
1. ______________________
2. ________________________(negatively charged)
3. ________________________
Recall, the variation among the Nitrogen bases:
Homework:
p. 204 #1-4
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