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DNA
Discovery and Structure
Timeline
Frederick Griffith
explores genetic
material with
mouse
experiments
Gregor Mendel
experiments
with pea plants
1866
1928
1903
Sutton
develops the
chromosome
theory of
inheritance
Griffith’s Experiment
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He studied Streptococcus pneumoniae, a
bacteria that causes pneumonia in mice
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He found that there were two types: S strain
and R strain
Griffith’s Experiment (cont.)
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In the first experiment, he
injected the R strain into the
mice
There was no change in the
mice
In the second experiment, he
injected the S strain into the
mice
The mice developed
pneumonia and died
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In the next experiment, he took the S
strain, heat-killed it, and injected it
into the mice
There was no change in the mice
In the last experiment, he took the
heat-killed S strain, combined it with
the R strain, and injected it into the
mice
The mice developed pneumonia and
died
Griffith’s Conclusion
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Griffith concluded that there was some factor
that was passed from the S strain to the R strain
that enabled the R strain to cause pneumonia
Griffith called this process transformation.
Also, since the ability to cause pneumonia was
transferred to bacterial offspring, the factor for
transformation must be a gene.
Timeline (cont.)
Erwin Chargaff discovered that in DNA
samples the amount of guanine was always
equal to the amount of cytosine and the
amount of adenine was always equal to the
amount of thymine.
1952
1952
Martha Chase and
Alfred Hershey
experiment with
bacteriophages to
prove that DNA is the
genetic material
Chase and Hershey’s
Experiment
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They worked with bacteriophages,
viruses that infect bacteria
Bacteriophages are made up of
Protein
only two substances: DNA and
protein
When a bacteriophage infects a
bacterial cell, part of the phage
enters the cell and part of it stays
outside
The part that enters the cell is what
takes control of the bacteria, so it
carries genetic information
DNA
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They wanted to find out if
the part that entered the
cell was DNA or protein
They grew viral cultures in
substrate containing
radioactive phosphate
isotopes (phosphate is
found in DNA but not
protein)
Radioactive DNA
They let the virus infect a
cell
Tests showed that the
bacteria became
radioactive
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Next, they grew viral
cultures in substrate
containing radioactive
sulfur isotopes (sulfur is
found in protein but not
DNA)
The bacteriophage infected
the bacteria
Radioactive Protein
Tests showed that the
bacteria did not become
radioactive
These experiments proved
that DNA was the genetic
material
Timeline (cont.)
Maurice Wilkins and
Rosalind Franklin
photographed DNA using
X-ray crystallography
1952
1952
Watson and Crick
develop the
double helix
model for the
structure of DNA
DNA Structure
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DNA is made up of nucleotides
Nucleotides are made up of three parts:
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Deoxyribose (a 5-carbon sugar)
Phosphate group
Nitrogenous base
Bases
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Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Adenine and Guanine are purines
Purines have a two-ring structure
Cytosine and Thymine are pyrimidines
Pyrimidines have a one-ring structure
Double Helix
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The shape of DNA is described
as a double helix
A double helix resembles a
“twisted ladder”
The “backbone” of the ladder is
made up of alternating sugar
and phosphate groups
The “rungs” of the ladder are
made up of the bases joined by
hydrogen bonds
Complementarity
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Bases that always pair with each other are
said to be complementary
Chemical bonds formed:
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A forms two
G forms three
T forms two
C forms three
So the only combinations possible are:
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A pairs with T
C pairs with G