Discovery of the DNA molecule
Discovery of the DNA molecule
Students will be successful when they can……..
1. When they can summarize the experiments
leading to the discovery of DNA as the
2. Diagram and label the basic structure of
3. Describe the basic structure of the
The Fredrick Griffith Experiment
• Griffith used two different strains of a bacteria
that causes Pneumonia (Streptococcus
• One strain had a sugar coat (the S strain), and
caused pneumonia killing a mouse by
• the other did not have a coat and is referred
to as the rough strain (R strain). It did not kill
the mouse by injection.
• Griffith discovered that if he heated the S
strain it would die, and no longer kill the
• But, if he mixed the heated S strained with
some live R strain (non disease-causing) the R
strain would be transformed into the S strain,
killing the mouse.
• The Smooth strain was extracted from the
dead mouse and cultured. It was still the
rough strain, but had developed
characteristics of the disease causing strain.
• Griffith concluded that there had been a
transformation from the live R bacteria to Live
• This experiment set the stage for the search to
identify the transforming substance.
Oswald Avery’s Experiment
• Avery and Colleagues identified the molecule
that transformed the R strain of bacteria into
the S strain.
• He isoladed different macromolecules, such as
DNA, Protien, and lipids.
• When the live R strain was exposed to the
DNA of the S strains DNA, it was transformed
into the disease causing S strain.
Hershey and Chase Experiment
• Alfred Hershey and Martha Chase published
results of experiments that provided
definitive evidence that DNA is the
• Hershey and Chase used a new technique
called radioactive labeling using radio-active
Phosphorus(P32) and Radioactive sulfur (S35).
• Proteins take up sulfur not phosphorus, so the
proteins are labeled.
• DNA takes up Phosphorus, but not sulfur,
therefore labeling the DNA.
Viruses and Bacteria are Grown in Medium
Containing 32P, and another group of Viruses are
grown in a medium of 35S. And then Blended up
The Blender dislodges the viruses from the
bacteria cells, and the bacteria is seperated from
the liquid containing the viruses.
Results of the Hershey Chase
• Results: bacterial cells infected with the
radioactive label of 32P had incorporated 32P
into its cells.
• The bacteria infected with the 35S did not
incorporate it into itself, the 35S was left in the
DNA not Protein was the genetic material that
could be passed from generation to
generation in viruses.
II. DNA Structure
• After Hershey and Chase found that DNA was
the molecule responsible for inheritance a
scientific race had begun to discover the
structure of the DNA molecule.
What is a Nucleotide
The structure of Nucleotides includes: A
phosphate sugar backbone, with one of the 5
nitrogenous bases (ATCG,U).
The 4 Nucleotide bases
of DNA and Uracil (RNA)
Uracil (only found in bacterial RNA)
We’ve seen this structure before!!
(this is a molecular homology, and evidence for evolution)
The Brilliance of Erwin Chargaff
• Chargraff analyzed the numbers of each
nucleotide (GCTA) in the entire genomes of
different species. Here’s what he found!
Well, for starters it implies that the Adenine and
Thymine are related some how, as well as
Cytosine and Guanine?
Like, maybe they’re totally complementary base
• Rosalind Franklin took a new
job at King’s college in
London working with a
technique called X-ray
• X-ray diffraction shoots Xrays at molecules and
measure how the energy
waves are broken up.
Allowing photographs of
molecules to be taken.
• Photograph 51 showed a
pattern describing a double
• This photograph along with
other data was taken without
her knowledge by her Mentor
and shared with two other
James Watson and Francis Crick
• Both were working at Cambridge
University (London) when they saw
• They soon realized that structure of
the DNA was a double helix, and
built their famous model.
The DNA Model that won them
the Nobel Prize
1. Two outside strands consist of alternating
deoxyribose and phosphates.
2. Cytosine and guanine bases pair to each
other by three hydrogen bonds
3. Thymine and adenine bases pair to each
other by two hydrogen bonds
Orientation of the Two Strands
The top strand of DNA is the 5’ (5 prime) strand
and at the end of the top rail is the 3’ (3
prime) and said to be 5’ to 3’ orientated.
The bottom rail is the opposite, 3’ to 5’.
This is referred to as antiparallel.
In 1953 Watson and Crick surprised the scientific
community by publishing a one-page paper in
the journal Nature suggesting a structure for
the DNA molecule as well as a hypothesized a
method of replication.