DNA

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DNA
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Early Experiments

• Griffith (1928)
• Used Streptococcus pneumoniae
• S-strain (pathogenic)
• R-strain (not pathogenic)

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Griffiths Experiments

• Transformation
• Some factor was transferred from the dead
  S-strain bacteria to the live R-strain bacteria.
• The newly transformed R-strain was virulent in
  further generations.
• What was the factor?...lots of work to find out.

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The Hereditary Factor

• Molecule of Inheritance.
• Early on both Proteins Nucleic Acids were
  candidates for encoding the genetic material.
• Proteins were both specific and variable.
• Not much was known about Nucleic Acidsuntil

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Hershey Chases Experiments

• Used a Bacteriophage (phage)
• A bacteria-infecting virus.
• Viruses Protein Nucleic Acid.
• Used Escherichia coli (E. coli)
• Used radioactive isotopes to label Protein DNA.
• Sulfur for Protein
• Phosphorous for DNA

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Hershey Chases Experiments
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Hershey Chases Experiments

• DNA from the Virus was the factor that infected
  the bacteria.
• DNA was the information molecule the
  Hereditary Molecule.

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Additional Data

• Chargaffs Data
• Adenine Thymine
• Guanine Cytosine
• Wilkins Franklins Data
• X-ray diffraction

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Deoxyribonucleic Acid

• Watson Cricks model
• Double Helix connected by N-bases.

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DNA Replication

• Copying the genetic material Duplication.
• Providing blueprints for future generations of
  cells!
• Suggested by its very structure!

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DNA Replication

• Separation of the double helix Helicase.
• Unzipping of the ENTIRE DNA Molecule.

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DNA Replication

• Semiconservative Replication.
• Each parent strand provides a template for the
  addition of complimentary bases.
• DNA Polymerase.

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DNA Replication

• Resulting in two molecules, each identical to
  the parent, and to each other.

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How does it begin?

• Initiation DNA replication is initiated at
  specific sites specific nucleotide base
  sequences along the parent DNA strand.
• Numerous points of initiation are established
  along a DNA strand.
• Helicase (the unzipper).
• Topoisomerase (the reliever of pressure).
• Single-strand binding proteins (SSBs)
  (stabilizers).

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How does it proceed?

• Elongation new nucleotides are added by DNA
  polymerases.
• Actually, addition of nucleoside triphosphates
  occurs.

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Antiparallel Elongation

• DNA polymerase works from the 3 to 5 end of the
  parental strand of DNA.
• DNA polymerase adds new nucleotides to the
  free-floating 3 end of the newly-forming DNA
  strand only.

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Antiparallel Elongation

• The LEADING STRAND is the fork that elongates
  continually from 5 to 3.
• The LAGGING STRAND is the fork that must also
  elongate from 5 to 3 but in the opposite
  direction!

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Antiparallel Elongation

• The LAGGING STRAND must, therefore, elongate AWAY
  from the replication fork.
• This results in the formation of small segments
  of double-stranded DNA Okazaki fragments.

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Antiparallel Elongation

• DNA Ligase responsible for connecting
  (ligating) the Okazaki Fragments.

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OK, An even closer look atInitiation

• PROBLEM
• DNA Polymerase can only add new nucleotides by
  attaching them to the 3 end of another
  nucleotide.

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OK, An even closer look atInitiation

• SOLUTION
• A Primer is needed (segment of complimentary RNA)
  is attached out of the blue.
• Primase is the enzyme responsible.
• Once enough bases are in place, DNA Polymerase
  takes over.
• (by adding bases to the
• 3 end NOW there)

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OK, An even closer look atInitiation

• In Leading Strand
• This all happens once.
• In Lagging Strand
• A different DNA Polymerase replaces each Primer
  (RNA).
• Later, Ligase connects the 5 and 3 ends of the
  two Okazaki fragments.

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Summary of DNA Replication
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Proofreading

• Mistakes do occur.
• Proofreading of the newly-formed DNA is
  accomplished by other DNA polymerases.
• Can occur AFTER replication has finished.
• In this case a Nuclease enzyme cuts out a
  segment containing the damaged DNA, which is then
  replaced by DNA Polymerase and Ligase.

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Animation

• http//www.stolaf.edu/people/giannini/flashanimat/
  molgenetics/dna-rna2.swf

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DNA Replication shortens DNA

• DNA Polymerase can only add to the 3 end.
• Once a primer is removed, nothing can be attached
  to the exposed 5 end.

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Telomeres

• Caps of non-coding DNA at the ends of Eukaryotic
  DNA (chromosomes).
• Repeating segments
• TTAGGGTTAGGGTTAGGGTTAGGG

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Telomeres

• Postpone the Protein-encoding parts of the
  chromosome from being eroded after successive
  replications.
• Eventually, they get shorter and shorterwhich
  may contribute to cell senescence (no more
  dividing).
• Many proteins are responsible for keeping the
  cell from activating self-destruct modes.

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What about Germ Cells?

• Stem cells sex cells give rise to more and more
  cells (blood cells, gametes, etc.)
• Erosion of genetic material on these cells would
  be bad.
• TELOMERASE enzyme responsible for maintaining
  Telomere length.

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Assignment for Tuesday

• 1 paragraph.
• What is a Thymine Dimer?

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Thymine Dimer
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VERY INTERESTING Assignment

• Read about Telomeres p. 306.
• Segments of Eukaryotic DNA that do not contain
  genes.

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