Transcript The Discovery of DNA

The Discovery of DNA
1928 – Frederick
Griffith
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Transformation
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Change in genotype and
phenotype because of
assimilation of external
DNA by a cell
Used disease-causing
(pathogenic) and nondisease-causing
(nonpathogenic) bacteria
and mice
But what caused the
change in phenotype?? He
wasn’t sure…
1944 – Avery, MacLeod, McCarty
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Discovered that DNA is the
“transforming agent”
Their finding was not believed by
the scientific community because
proteins were believed to be better
candidates for the genetic material
1952 – Alfred Hershey & Martha Chase
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Is it protein or DNA
that is the genetic
material?
Used bacteriophages
(viruses that infect
bacteria) to show
that since DNA enters
the bacterial cells,
but protein doesn’t,
DNA must be the
genetic material
Hershey & Chase’s Experiment
Early 1950s – Erwin Chargaff
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Analyzed the base composition of
DNA
Set the stage for the base-pairing
rules
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# of adenines = # of thymines
# of cytosines = # of guanines
Known as Chargaff’s Rule
Early 1950s – Maurice Wilkins &
Rosalind Franklin
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Rosalind Franklin
took X-ray
crystallography
diffraction
photograph of
DNA
1953 – James Watson & Francis Crick
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Constructed model
of DNA as a
double helix
Purine +
pyrimidine for
consistent width
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C-G  3 hydrogen
bonds
A-T  2 hydrogen
bonds
Late 1950s – Matthew Meselson &
Franklin Stahl
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Semi-conservative
replication of DNA
Each new molecule
of DNA (after DNA
replication)
contains 1 old and
1 new strand