Topic 7 The Discovery of DNA

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Topic 7The Discovery of DNA Its Roles

• October 7-14, 2005
• Biology 1001

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Introduction to DNA

• DNA is deoxyribonucleic acid, a polymer of the
  four nucleotide monomers adenine, guanine,
  cytosine, thymine
• Arranged as a double helix
• DNA is the molecule of heredity
• DNA is precisely replicated during cell division
• Each cell in the organism has a copy and a copy
  is transmitted from parent to offspring
• DNA encodes the amino acid sequences of all the
  proteins in an organism
• All biochemical, anatomical, physiological, and
  behavioural characteristics of organisms are at
  least partially determined by DNA

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Science as a process

• A series of classic experiments elucidated the
  molecular basis of inheritance
• Griffith (1928) The genetic material transforms
  bacteria
• Avery MacLeod (1944) The transforming agent
  is DNA
• Hershey Chase (1952) DNA is the hereditary
  material of viruses
• Watson Crick (1953) Modeled the 3D structure
  of DNA
• Meselson Stahl (1958) DNA replication is
  semi-conservative

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7.1 The Genetic Material Is DNA

• A. Griffiths (1928) Experiment
• Worked with two strains of Streptococcus
  pneumoniae, a pathogenic one that causes
  pneumonia (S) and a nonpathogenic one (R)
• R bacteria mixed with heat-killed S bacteria
  became pathogenic
• The pathogenicity was inherited by the offspring
  of these R bacteria
• Griffith reasoned that the R bacteria were able
  to take up a chemical from the environment

Figure 16.2!
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7.1 The Genetic Material Is DNA

• Griffith called this process transformation, but
  he did not know what the transforming chemical
  was
• Avery and colleagues spent 14 years testing
  various chemicals from the S bacterial remains to
  see which would transform nonpathogenic bacteria
  into pathogenic ones (R into S)
• Only DNA worked
• Their discovery was met with considerable
  skepticism because it was widely held that
  proteins were a better candidate for the genetic
  material
• The question than became whether this finding was
  generalizable to other organisms, or specific to
  bacteria

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7.1 The Genetic Material Is DNA

• B. The Blender Experiment of Hershey and Chase
  (1952)
• Studied the T2 bacteriophage that infects E. coli
• T2 is a virus comprised of DNA protein
• Infected E. coli produce new viruses viral DNA
  OR protein is responsible
• Separately radiolabelled each component DNA with
  32P and protein with 35S
• Allowed infection to proceed, then blended
  centrifuged the bacteria
• 32P was found in the pellet, 35S in the
  supernatant
• DNA is the hereditary material in viruses

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7.2 The Discovery of the Model of DNA

• What was known to Watson Crick
• DNA is the hereditary material
• DNA is a polymer of nucleotides
• Nucleotides contain a pentose sugar
  (deoxyribose), a phosphate group, and a
  nitrogenous base (A,C,G, or T)
• A G are purines, C T are pyrimidines
• The sugars and phosphates form the backbone of
  the polymer and give it directionality
• The proportion of each base varies from species
  to species first evidence of diversity of DNA
• But in each species, the proportion of adenine
  equals that of thymine, and the proportion of
  guanine equals that of cytosine
• These are Chargaffs rules AT and GC

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7.2 The Discovery of the Model of DNA

• The challenge was to devise a 3D structure that
  would account for DNAs role in inheritance
• Watson had also seen Rosalind Franklins X-ray
  diffraction data suggesting that the molecule was
  helical and wide enough to accommodate two
  strands of DNA a double helix
• Franklin reasoned that the sugar phosphate
  backbone faced outward, allowing the hydrophobic
  bases to occupy the interior of the molecule
• What remained to be determined was the specific
  pairing of bases holding the two strands together

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7.2 The Discovery of the Model of DNA

• In 1953, Watson and Crick reported their model in
  a one page paper to Nature entitled Molecular
  Structure of Nucleic Acids
• Its key feature was the arrangement of bases
  between the two strand strands of the helix
• Watson and Crick proposed that adenine paired
  with thymine, and guanine with cytosine, with
  hydrogen bonds between bases holding together the
  two strands of the helix

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Figure 16.7 The Structure of DNA
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Review - The 3D Structure of DNA

• Features of the model
• Two strands of DNA arranged in a double-helix,
  antiparallel with the bases inward and the sugar
  phosphate backbone outward
• The two strands are held together by hydrogen
  bonding between base pairs, A with T and C with G
• The helix is right-handed with a 3.4 nm spacing
  between adjacent turns of the helix, and a 0.34
  nm spacing between adjacent base pairs
• The model fits the X-ray diffraction data and
  Chargaffs rules