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DNA Replication

DNA Replication . Needed only when a cell is about to divide S phase of cell cycle prior to mitosis or meiosis. NO OTHER TIME!!!. Semi-Conservative Replication. Thought most likely mode contrast w/ conservative and dispersive Meselson and Stahl density-gradient centrifugation. Cesium

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DNA Replication

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  1. DNA Replication • Needed only when a cell is about to divide • S phase of cell cycle prior to mitosis or meiosis NO OTHER TIME!!!

  2. Semi-Conservative Replication • Thought most likely mode • contrast w/ conservative and dispersive • Meselson and Stahl • density-gradient centrifugation Cesium chloride

  3. Meselson and Stahl • Produced E. coli with “heavy” DNA • nitrogens were 15N (extra neutron) • Allowed E. coli to replicate using normal 14N • Examined how chromosomes separated in gradient

  4. x 15N alone 14N alone semi cons. cons. dispers. x

  5. x 15N alone 14N alone semi-conservative dispersive x

  6. 15N alone 14N alone 1st replication 2nd replication

  7. DNA Replication • Bi-directional • must add to 3’ end • Discontinuous • Okazaki fragments • (Multiple origins of replication - euk.) • Enzyme catalyzed

  8. Phosphodiester bond “nicked” nuclease Helix unwinds helicase (topoisomerase) torsion stress causes H bonds between bases to break 2 strands separate single strand binding proteins

  9. RNA primer need free 3’ end RNA primase removed later DNA polymerase III catalyzes phosphodiester bonds between new nucleotides proofreads DNA polymerase I removes/replaces RNA primers DNA ligase seals Okazaki fragments

  10. Okazaki fragment 5’ DNA polymerase helicase A U 5’ 5’ LAGGING STRAND T A G G C A A C A T C G G A C A A C T A G C C T G T T G A G A T C RNA primer T G G G C C T LEADING STRAND A 3’

  11. 5’ lagging strand discontinuous replication 3’ 5’ Okazaki fragments 5’ leading strand continuous replication 3’

  12. 5’ Polymerase I; ligase 3’ 5’ Okazaki fragments 5’ 3’

  13. 5’ 3’ 5’ 5’ 3’

  14. Leading vs. lagging... leading lagging 5’ 3’ 5’ 3’ leading lagging

  15. Two identical chromatids

  16. Replication • One DNA molecule replicates to form two identical molecules • One chromosome  two identical chromatids

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