DNA Part I
The History and
Discovery of the
Structure and Role of
DNA
DNA –How its structure was discovered
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Identifying DNA as a unique molecule.
1869- Friedrich Miescher was
a Swiss chemist and was the
first to identify DNA as a
unique molecule.
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Staining of DNA reveals somatic cells have
the same amount of DNA and half as much
as gametes.
1914 - Robert Feulgen, a German
chemist, found a staining technique
that stains more or less strongly
based in the amount of DNA
present (called Feulgen stain). He
found that all cells in an organism
had the same amount of DNA
except gametes, which had half the
normal amount.
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Staining of DNA reveals somatic cells have
the same amount of DNA and half as much
as gametes.
Cells stained with
Feulgen stain. It
is the DNA and
not the proteins
that are visible
under the
microscope.
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History of DNA
Fred Griffith demonstrated that
bacteria could be “transformed”
from one strain to another by
transferring genetic factor from one
organism to another. He used two
different strains of the same
bacteria. One could cause
pneumonia and the other could not.
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Griffith’s Experiment
The conclusion was that the bacteria had incorporated
heredity factor from a source and in doing so expressed a
new smooth trait.
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Extending Griffith’s Experiment and
Identifying DNA as the Transforming Factor
Avery, MacLeod and McCarty examined the
various molecules found in the S-strain
Pneumococcus cells to prove that DNA was
responsible for the transformation of the
bacterial cells.
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DNA is the Molecule of Heredity.
When various isolated
chemical components of
the S-strain
Pneumococcus cells
was mixed the R-strain
Pneumococcus cells, it
was shown that the
DNA from the S-strain
cells, that caused transformation.
Experiment of Hershey and Chase
Alfred Hershey and Martha Chase demonstrated the genetic
material is DNA by using viruses that infect bacteria. These
viruses only stay on the outside of the cell when infecting the
cells. Also viruses are composed of protein and DNA. It is
known that the virus injects its genetic material into the
bacterium which had to DNA or proteins.
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Experiment of Hershey and Chase
It demonstrated that DNA is the material that genes are made of and not
protein.
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Experiment Chargaff
Chargaff's Rule -> A+G=C+T=50%
Percentage of Various Nucleotides in Genome
Organisms
A
T
G
C
Humans
30.9
29.4
19.9
19.5
Wheat
27.3
27.1
22.7
22.8
Sarcina lutea
13.4
12.4
37.1
37.1
T7
26.3
26
23.8
23.9
Based on the observations above, two rules can be deduced
1.A+G=C+T=50%.
2.The percentages of the nucleotide vary for different species
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Work of Rosalind Franklin
Rosalind Franklin used x-ray crystallography to determine that DNA
was double stranded, a helix, phosphates were on the outside and three
distances, 2.0 nm, .34 nm, and 3.4 nm showed up in a pattern over and
over again in the diffraction pattern.
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Work of James Watson and Francis Crick
Based on the rules of
Chargaff and the
information from the
work of Franklin, James
Watson and Francis
Crick, determined the
structure of DNA by
making models.
1. Determined that the sugar and phosphates were on the
outside.
2. Determined that the nitrogenous bases were forming the
rungs of the ladder.
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Determining the Nitrogen Base Pairing
Based on the work Franklin’s
x-ray crystallography, Watson
and Crick found the bonding;
•two purines are too wide and
would overlap.
•two pyrimidines are too far
apart to form the hydrogen
bonds.
•a purine and a pyrimidine
however, are just right!
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Chargaff’s Snub
Chargaff felt there had been an
injustice done when he did not
receive the Nobel Prize in 1962
along with Watson, Crick and
Wilkins. Wilkins’ contribution to
the structure of DNA was to show
James Watson the work of Rosalind
Franklin without her permission.
Franklin did not share the Nobel
Prize as she passed away from
ovarian cancer in 1958 and
posthumous nominations are
forbidden.
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