The History of DNA

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The History of DNA

• Mark Mayo
• Cypress College

Last update 9/16/13
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Transformation Frederick Griffith (1923)

• Used healthy mice
• Mice were injected with either R(rough) strain of
  Streptococcus pneumoniae. The mice live and
  their immune system kills R bacteria. No live
  bacteria
• Mice injected with the S (smooth) strain of
  Streptococcus pneumoniae. The mice die. The
  dead mice have live S bacteria.
• Mice injected with heat-killed S strain. Mice
  live with no live bacteria found in mice.
• Mice injected with mixture of live R strain and
  heat-killed S strain. Mice die and live S strain
  bacteria are found in the dead mice.
• Heat does not destroy the active factor that is
  responsible for heredity (DNA).
• It is said that the bacteria were transformed by
  this active agent.

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Transformation Frederick Griffith (1923)
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Transformation II Avery, McCarty, Macleod (1944)
Think re-mix of Griffith

• Repeated Griffiths work, but knew that DNA was
  the substance of transformation
• Separated classes molecules from s cell debris
• Tested each fraction for transforming ability,
  one at a time
• Only DNA transformed r cells into s cells
• To provide r cells with s DNA is to provide r
  cells with s genes

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Transformation II Avery, McCarty, Macleod (1944)
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DNA or Protein as Active Agent Alfred Hershey and
Martha Chase (1950s)

• Used radioactive labels on bacteriophage
  components to decide if DNA or protein was the
  transforming factor
• Label viral protein with S35
• Label viral DNA with P32
• Allow infection
• Wash viral particles (with blender!)
• Check for label after subsequent infection into
  new bacteria
• Found only P32
• Hence DNA is the transforming factor

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DNA or Protein as Active Agent Alfred Hershey and
Martha Chase
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Chargaffs RulesErwin Chargaff (1949)

• He studied the relative amounts of each nucleic
  acid base in a great variety of plant and animal
  species
• Roughly found that AT and GC, but not exactly
  due to errors in the technology!
• Purines are exactly equal to pyrimidines
• His methodology for the time was good, but now we
  get exact amounts
• He could not make the connection (Watson and
  Crick used his data however)

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Chargaffs Rules Erwin Chargaff (1949)
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Alpha HelixLinus Pauling (1948-1950)

• Worked with proteins and determined that collagen
  has a helical arrangement for its polypeptides
• He called the helix an alpha helix
• He suspected that DNA might also have a helical
  arrangement, but could not get it to compute with
  a single strand
• Pauling suggested DNA had a triple helix, but had
  no proof
• Watson and Crick heard his idea about a helix

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Alpha HelixLinus Pauling (1948-1950)
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X Ray DiffractionMaurice Wilkins and Rosalyn
Franklin

• Used Xray diffraction to study proteins and other
  molecules
• DNA was very difficult to crystallize and a tough
  candidate for Xray diffraction
• Rosalyn Franklin was a very talented graduate
  student in the lab of Wilkins
• She was successful at crystallizing DNA in two
  forms A and B
• The forms on an X was seen indicating some kind
  of helix
• She could measure the distances between repeating
  units on the molecule
• She could also measure the diameter of the
  molecule
• Wilkins sent her unpublished data to Watson and
  Crick without her permission
• She died before the Nobel prize or she might have
  shared it

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X Ray DiffractionMaurice Wilkins and Rosalyn
Franklin
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Semi-conservative DNA Replication Matthew
Meselsohn and Frank Stahl

• They used two isotopes of Nitrogen 14N and 15N
• 15N is heavier than 14N
• They grew bacteria for several generations in
  heavy 15N (all DNA would be heavy!)
• Abruptly changed the medium to lighter 14N for
  one or two generations
• Used density-gradient ultracentrifugation to
  separate the DNA strands by weight
• After one generation all DNA was medium between
  heavy and light (thus SEMI-CONSERVATIVE)
• After two generation DNA had two bands medium
  and light.

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Semi-conservative DNA Replication Matthew
Meselsohn and Frank Stahl
This one would be one heavy and one light band
Half light and half heavy medium weight
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Discontinuous DNA ReplicationReiji Okazaki

• He knew about DNA polymerase
• It moves from 5 to 3 only on the leading strand
• He searched for a second polymerase that worked
  in the reverse direction
• After unsuccessfully searching he used his
  brilliance
• Found numerous small fragments and also long
  segments as DNA was replicated
• Finally decided that the short segments were from
  the lagging strand
• DNA polymerase worked on both sides continuously
  on the leading strand and in several places on
  the lagging strand
• DNA ligase connects the small portions (now
  called Okazaki fragments) on the lagging strand

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Discontinuous DNA ReplicationReiji Okazaki