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DNA Replication

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  1. DNA Replication

  2. Review • Why is the process of DNA called a semiconservative process? • Let’s review DNA structure: • List all characteristics you can recall about DNA molecule

  3. A closer look • Today we will learn about some incredible stuff happening in your body right now! (p.313)

  4. Getting Started • DNA replication begins @ particular sites called origins of replication • Proteins attach and begin copying DNA strand • Forms replication bubble  could be multiple in single DNA helix = speeds up replication process • At each end of bubble = replication forks: the y=shaped region where parts of parental DNA being unwound DRAW pic. on pg. 313

  5. Proteins involved in unwinding • Helicase: untwists/unzips DNA at forks =opens up template strands • Single –strand binding proteins: bind to unwound parental strands, keeping them from re-pairing • Topoisomerase: helps relieve strain on unwound DNA while unwinding is occurring  so DNA does not break permanently


  6. Replication Initiation • Enzymes that add nucleotides need help to start… can’t initiate process on their own • initial nucleotide chain is added = a short RNA chain called a primer created by enzyme called primase -usually 5-10 nucleotides long *new DNA strand will start from 3’ end of primer

  7. Adding Bases • Main protein involved = DNA polymerase  Specifically DNA polymerase I (DNA pol I) and DNA polymerase III (DNA pol III) • After primer added, polymerase will begin adding nucleoside triphosphates (dATP) to the strand • Similar to ATP except sugar: ATP = ribose, dATP = deoxyribose  When added to template, 2 phosphates break off (b/c unstable) = exergonic reaction that drives polymerization

  8. Antiparallel Elongation • DNA strands/replication has directionality (like 1 way street)  compliment strands are antiparallel How does the antiparallel arrangement of the double helix affect replication? (HINT: think back to the rule of the primer)

  9. Answer: • DNA pol can only add bases to 3’ end of primer .. So a new DNA strand can ONLY elongate from 5’-3’ direction Let’s take a closer look at replication forks of the bubble to see how this works!

  10. Antiparallel Elongation • 1st strand, DNA pol III can synthesize complimentary strand continuously as DNA unwinds from 5’ – 3’ direction = LEADING STRAND • Only 1 primer needed • 2nd strand, DNA pol III must work in opposite direction (away from replication fork) to continue replication in 5’ – 3’ direction = LAGGING STRAND  Synthesized discontinuously in segments called Okazaki fragments

  11. DNA Replication

  12. What do we do with the primer? • When segment replication is complete, primer must be removed • DNA pol I comes in and removes RNA primer and fills hole with DNA nucleotides • An enzyme called DNA ligase comes in and patches two segments of lagging strand together Let’s see it in action!

  13. Review: • For the following enzymes/proteins, list their roles in DNA replication: • DNA polymerase III • DNA helicase • DNA ligase • Primase • Single-strand binding proteins • Topoisomerase • DNA ligase