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
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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
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pattern.
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 xray 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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Structure of a Nucleotide
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Sides of the Ladder
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Hydrogen Bonding and Nitrogenous Bases
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Hydrogen Bonding and Nitrogenous Bases
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Hydrogen Bonding and Nitrogenous Bases
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Hydrogen Bonding and Nitrogenous Bases
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Forming the Double Helix
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DNA Forming Chromosomes
Structure in eukaryotes.
•the DNA is wrapped around
proteins called histones
forming nucleosomes.
•This forms a fiber known as
chromatin.
•This forms a coil within a coil.
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