DNA REPLICATION AND REPAIR

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DNA REPLICATION AND REPAIR

• SBI 4UI 4.3

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Reproduction

• A crucial property of organisms
• Ability to reproduce
• Reproduction requires DNA replication
• Mitosis
• Daughter cell has exact copy of parents DNA

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DNA Replication

• DNA has a double-helical structure
• How might it replicate most easily?
• Hydrogen bonds break between bases
• Each strand acts as a template
• Resulting in 2 identical DNA molecules

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Semiconservative Replication

• Each DNA molecule consists of a parent strand and
  a newly synthesized strand
• Meselson and Stahl
• Does E. Coli use semiconservative replication?
• Used a centrifugation process to separate DNA of
  successive generations

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Semiconservative Replication

• Found that parent DNA remained intact through
  several generations
• Other scientists soon proved the same thing for
  eukaryotic cells

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Process of DNA Replication

• Replication begins at a specific site known as
  the replication origin
• Prokaryotic DNA has only one, eukaryotic has
  several
• To replicate DNA must be first unwound and
  separated

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Process of DNA Replication

• DNA helicase
• Unwinds double helix by breaking H bonds between
  complementary bases
• Single-stranded binding proteins
• Binds to exposed DNA to block hydrogen bonds from
  reforming

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Process of DNA Replication

• DNA gyrase
• Relieves tension from unwinding DNA

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Replication Fork

• Junctions where DNA still joined is called the
  replication fork
• In eukaryotes
• Multiple sites of origin
• Therefore, many replication forks
• Thus, rapid replication of DNA

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Replication Rules

• DNA is always synthesized in a 5 to 3 direction

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Building Complementary Strands

• DNA Polymerase III
• Adds nucleotides in a 5 to 3 direction
• RNA primer
• Sequence of RNA bases used to initiate DNA
  replication
• Primase the enzyme that build RNA primers

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Leading Strand

• Uses 3 to 5 template as a guide
• Built towards replication fork
• Continuous

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Lagging Strand

• Built away from replication fork
• Discontinuous
• DNA polymerase III
• Builds using short fragments called Okazaki
  fragments

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Building Complementary Strands

• DNA polymerase I
• Removes RNA primers from both strands
• Replaces them with appropriate nucleotides
• DNA ligase
• Joins Okazaki fragments together
• New strand automatically twists into helix

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Quality Control

• DNA polymerase III and I
• Proofread
• Exonuclease
• Either enzyme can delete mistakes and replace
  them with appropriate nucleotides

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Questions?