Discovering the Structure
of DNA
What is DNA?
• DNA = deoxyribonucleic acid
• Stores, transmits and copy all information
• Located in the cell’s nucleus
What do you know about DNA?
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•
•
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Codes for proteins
nucleic acid
Monomer is a nucleotide
The three parts of a nucleotide:
– 1. Phosphate group
– 2. Sugar (deoxyribose)
– 3. Nitrogen base
Nitrogen bases
• nitrogen two types:
purine- 2 carbon rings
pyrimidine. -1 carbon ring
More about nitrogen bases
• DNA has 4 nitrogen
bases:
– Thymine (T)
– Adenine (A)
– Cytosine (C)
– Guanine (G)
• A G - purines
• C T - pyrimidines.
YouTube:
DNA
Structure of DNA
A collaborative effort!
• Early 1900s
– known: information is passed from cell to cell.
– Unknown: what carried the information?
• scientists thought a protein
• others that it was a nucleic acid.
• Three major experiments helped shows nucleic
acid carried cell information:
– Griffith
– Avery
– Hershey-Chase
Frederick Griffith
• studied pneumonia
bacteria
• 1928
• isolated two strains of
pneumonia
• injected them into mice
Griffith got lucky?
– Something
transferred from
heat-killed bacteria
– What happened?
• Griffith’s conclusions:
• Transformation = process by
which one strain of bacteria
changes the gene(s) of another
bacteria
Avery
• DNA was transforming bacteria
• 1943
• Uses pneumonia ( just like
Griffith)
• Showed gene is made of DNA
• Scientists were slow to accept the
results
Hershey and Chase
• bacteriophages to see
if information is
carried on proteins or
DNA
• 1950
• Further supported
Avery’s experiment
that genetic material
is DNA
http://www.accessexcellence.org/RC/VL/GG/images/HERSHEY.gif
Discovery of the structure of DNA
• Many scientists contributed to determining
the structure of DNA
– Erwin Chargaff
– Rosalind Franklin
– James Watson &
Francis Crick
Erwin Chargaff
• Base pairing rule
• 1950
• percentages of bases in
DNA
• 10% A = 10% T
• 40% C = 40% G
• This is Chargaff’s Rule!
Rosalind Franklin
• x-ray photography to try to
find DNA structure
• 1952
• Her “Photo 51” revealed
DNA’s structure
• Died of cancer in 1958
Watson and Crick
http://teachers.sduhsd.k12.ca.us/lolson/im
ages/watson_crickjpg
• Credited with finding
the structure of DNA)
• 1953
• Watson got a sneak
peak at Franklin’s x-ray
photos and used them
with other evidence
• Did not give credit to
Franklin
DNA structure
• Twisted ladder
•
made of nucleotides
• Sides- phosphate/sugar
• Rung- nitrogen bases
What bonds with what?
– A bonds with T
– G bonds with C
– Bonds
sides- covalent- strong
rungs - hydrogen - weak
Your turn...the structure of DNA
• On the
diagram:
– Circle and
label a
nucleotide.
– Label the
sugar and
phosphate
molecules.
– Label the
bases
– Label a base
pair.
– Label the
sugarphosphate
backbones.
– Label the
hydrogen
bonds.
Sugar /P
backbone
Base pair
Sugar /P
backbone
A
Hydrogen bonds
C
A
T
G
P
S
A
T
G
C
G
nucleotide