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DNA
Structure &
Function
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• Isolated two slightly different strains of
pneumonia bacteria from mice
• Only one of the strains caused
pneumonia = smooth (S) colonies
• Harmless strain = colonies with
rough (R) edges
• Used mice and discovered deadly
bacteria could “transform” harmless
bacteria
Frederick Griffith and
Transformation (1928)
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Transformation• Process: one strain of bacteria is changed
into another strain of bacteria
• Griffith hypothesized since ability to cause
disease was inherited by transformed
bacteria's offspring, the transforming factor
might be a gene (genetic material)
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Oswald Avery’s experiment (1944)
1. Made extract (juice) from heat killed bacteria
2. Used enzymes to destroy
biomolecules & RNA
1. Transformation still occurred
3. Repeated experiment using
DNA destroying enzyme
1. Transformation did NOT occur = DNA must be the
genetic material
Discovered nucleic acid DNA stores & transmits
genetic info from one generation to the next
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Alfred Hershey & Martha Chase
Experiment (1952)• Used bacteriophages (“bacteria eater”)
• Goal: find out which part of the phage (DNA
or protein) produced new phages
• Grew bacteriophages
with radioactive markers
– phosphorus-32 (32P)
only in DNA
– sulfur-35 (35S)
only in Protein
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DNA• Long molecule of units called
nucleotides.
• Nucleotides made of three basic components:
• 5-carbon sugar (deoxyribose)
• a phosphate group
• a nitrogenous base
• 4 kinds of nitrogen bases found in DNA
• adenine (A), guanine (G) = double ringed (purines)
• cytosine (C), thymine (T) = single ringed (pyrimidines)
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Erwin Chargaff
• American biochemist, 1940s
• Discovered percentages of guanine [G]
and cytosine [C] bases are almost equal in
any sample of DNA
– Later found that [A] = [T]
• Scientists had NO idea what this was…
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Chargaff's Rules
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Rosalind Franklin &
Maurice Wilkins (1952)
• Used X-ray diffraction to get info about structure of DNA molecule
• X-shaped pattern showed strands in DNA are twisted around each other,like coils of a spring
– shape known as a helix
– X suggests there are two strands in structure
– Other clues suggest nitrogenous bases are near center of the molecule
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James Watson & Francis Crick
• Trying to understand structure of DNA
– Built 3-D models
• 1953: Shown picture of Franklin’s x-ray &
immediately knew the structure:
• double helix - 2 strands wound around each other
(twisted ladder/spiral staircase)
• Discovered Hydrogen bonds form between
certain nitrogenous bases – base pairing
• Provide just enough force to hold the two strands
together
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Genetic Code
• Genome: organism’s complete set of DNA
(genetic code)
• All living things have a genetic code
– Bacteria – several hundred to several thousand
genes
– Human – ~ 30,000 genes
– Yeast – 6,000 genes
*about 1/3 of yeast genes related to human genes
• Nitrogenous bases hold key to this code
– Order of bases determines what protein is made15
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