DNA
Discovery and Structure
What is DNA?
• Deoxyribonucleic acid
• DNA is the molecule
of genes.
• It is found in the
chromosomes.
• It holds the
instructions to making
all the proteins in an
organism.
• Proteins make up the structures of cells.
• Proteins, in the form of enzymes, also
control all cell processes.
• DNA carries the instructions from mother
to daughter cells, from parents to children.
Therefore it is the molecule of heredity.
mitosis
sexual reproduction
Who discovered DNA?
• Deoxyribonucleic acid (DNA) was first isolated in
1869 by the Swiss scientist Friedrich Miescher.
He called the white, slightly acidic chemical that
he found in cells "nuclein." He identified DNA as
part of a chromosome in 1871.
Did DNA or Protein makeup a
molecule of heredity?
By the 1920s it was thought that genes were
made of protein, the other main ingredient of
the chromosome. Many scientists at the time
thought DNA was too simple to carry genetic
information. ---They were wrong! thought
that proteins were molecules of heredity.
How did scientists discover that
DNA was the hereditary molecule?
Scientists such as the Griffith’s bacteria
experiment in 1928, and Avery’s
bacteria experiment in 1944, pointed
toward DNA.
Oswald Avery
The famous Hershey-Chase
experiment finally ended
the debate.
Hershey-Chase: The “Waring Blender”
experiment – 1952.
Alfred Hershey and Martha Chase
• They experimented with bacteriophages,
viruses that infect bacterial cells.
• These viruses are basically composed of
protein surrounding DNA.
• Bacteriophage viruses force bacteria cells to
make new virus particles.
• The experiment would see if the virus’
proteins or the virus’ DNA caused the
bacteria to produce the new virus particles.
• Hershey and Chase grew the viruses in a mixture of
radioactive isotopes of elements found mainly in either
the DNA (phosphorus-32) or in the Protein (sulfur-35).
• The viruses were mixed with bacteria. After a few
minutes, the viruses were separated from the bacteria.
• They used a blender to do this which is why it is
sometimes referred to as the “Waring Blender
Experiment.”
The bacteria were then tested for phosphorus-32
(32P) and sulfur-35 (35S).
Nearly all the radioactivity in the bacteria was
phosphorus-32 (32P), the isotope that
marked the DNA!
Conclusion: The genetic material of the
bacterophage viruses was not the protein
but DNA.
DNA is the molecule of heredity!
Rosalind Franklin and Maurice Wilkins
examined DNA in the laboratory to
determine its structure.
X-Ray Diffraction (X-Ray Crystallography)
photograph of DNA, taken by Rosalind Franklin
was very important to the scientists who
eventually discovered the structure of DNA.
“photo 51”
B form of DNA from Calf Thymus Gland
DNA structure was discovered by
James Watson and Francis
Crick in 1953.
ORIGINAL DNA MODEL
BUILT BY WATSON & CRICK
Watson & Crick in their Cambridge office c. early1950s
Watson, Crick and Wilkins shared the Nobel
Prize in Physiology or Medicine in1962.
Unfortunately, Franklin had died of breast cancer
prior to the award.
MMMMM!
Rosalind Franklin, seen
Evaporating Dish
Coffee!
here serving coffee in
evaporating dishes at her
Parisian laboratory in the
late 1940s, went on to
publish an impressive
array of scientific papers
in the decade she had
remaining to her.
What is DNA’s Structure?
• DNA is a biological polymer made up of
subunits called nucleotides.
• DNA Nucleotides are made up of:
a) 5-carbon deoxyribose
sugar
b) phosphate group
c) nitrogen-containing base
By the way ---the sugar and base alone make up a
nucleoside
There are two classes of nitrogen bases:
• Purines (double ring structure)
adenine (A) guanine (G)
• Pyrimidines (single ring structure)
cytosine (C) thymine (T)
Mnemonic Device
• All that is Good is Pure.
Adenine
Guanine
purines
• C (see) The pyramids.
Cytosine
Thymine
pyrimidines
Another discovery that helped Watson & Crick
Erwin Chargaff - demonstrated that
guanine and cytosine are always in equal
amounts (as well as thymine & adenine)
Rosalind Franklin’s X-Ray evidence
showed, among many other things, that
DNA has the shape of a double helix or
“twisted ladder.”
Summary of DNA’s Structure:
1) Alternating sugars & phosphates on the
sides of the ladder. (Covalently-bonded)
2) Nitrogen bases in the rungs
A-T , G-C (base-paring)
Note: A & T are not labeled
correctly in the animation
Mnemonic Device
AT the Grand Canyon
A to T
G to C
3) Weak bonds between bases (Hydrogen
bonds)