Download Discovery of DNA Griffith`s Experiment Frederick Griffith (1928

Discovery of DNA
Griffith’s Experiment
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Frederick Griffith (1928) British medical officer
Studied Streptococcus pneumonia and was trying to develop a vaccine against a
virulent(disease causing) strain of the bacterium
Virulent bacterium is surrounded by a capsule made of polysaccharides that protect it
from the body’s defense systems
Virulent bacterium grow as smooth edged colonies called the S strain
Non-virulent strains grow as rough colonies called the R strain
In his experiment Griffith:
o Exp 1: Injected live S strain into mice and the mice died
o Exp 2: Injected live R strain into mice and the mice lived
o Exp 3: Injected heat killed S strain into mice and the mice lived
o Exp 4: Injected heat killed S strain and live R strain into mice and the mice died
Griffith concluded that heat killed virulent bacterial cells release a hereditary factor that
transfers the disease causing ability to the live harmless cells
The transfer of genetic material from one cell to another or from one organism to
another is called transformation.
Avery’s Experiment
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In the early 1940’s Oswald Avery and others set out to test whether the transforming
agent in Griffith’s experiment was protein, RNA, or DNA.
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They used enzymes to separately destroy each of the three molecules in the S cells
o Protease was used to destroy proteins
o RNA-ase to destroy RNA
o DNA-ase to destroy DNA
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They separately mixed the three experimental heat killed S cells with live R cells and
injected the mice.
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Results:
o The cells missing the protein and the RNA were able to transform R cells into S
cells and kill the mice
o Cells missing the DNA did not transform R cells into S cells and the mice lived
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They concluded that DNA is responsible for the transformation in bacteria.
Hershey-Chase Experiment
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1952 Martha Chase and Alfred Hershey tested whether DNA or protein was the
hereditary material viruses transfer when they enter a bacterium
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Viruses that infect bacteria are called bacteriophages or phages.
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They used radioactive sulfur to label protein and radioactive phosphorus to label DNA.
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They learned that all of the viral DNA and very little protein had entered the cells.
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They concluded that DNA is the hereditary molecule in viruses
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