Leading and Lagging strand

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Leading and Lagging strand

• Leading strand
• Continuous DNA synthesis
• Lagging strand
• Nucleotides are added one section at a time,
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• Okazaki fragments
• Fragments are joined by ligase

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Getting started priming

• DNA polymerase can only extend an existing DNA
  molecule, cant start one
• A short RNA primer is built first on the strand
  by enzyme primase
• RNA primer later removed by DNA polymerase I

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The Replication fork
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For Review

• Video overviewCD

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Enzymes A Review

• Leading Strand
• Helicase unwinds
• Primase puts an RNA primer
• DNA polymerase III synthesizes new DNA strand
• DNA polymerase I removes primer and replaces it
  with DNA

• Lagging strand
• Synthesizes a primer for each Okazaki frag.
• Elongates each fragment
• Replaces each primer with DNA
• Ligase joins each Okazaki fragment

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Proofreading and Repair

• DNA replication must be very accurate! Overall
  errors only 1/10 billion nucleotides
• More errors initially occur but are repaired by
• DNA polymerase I also proofreads, cuts out
  mistake and fixes with help of
• Nuclease
• Ligase
• Xeroderma pigmetosum

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Problems at the end

• Ends of chromosomes are eroded with each
  replication
• Primer is removed but cannot be replaced at ends
  where no 3 end is available

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Telomeres

• Expendable non-coding sequences at the end of DNA
  molecules
• Short sequences of bases repeated 1000 times
• TTAGGG in humans
• Telomerase an enzyme in humans that lengthens
  telomeres, compensating for shortening during
  replication