Ch13DNAOct16 - DNA Replication or Synthesis BIOLOGY DNA...

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1 10-Oct-09 Gardiner 1 DNA Replication and Repair B I O L O G Y 12 2 3 10-Oct-09 Gardiner 2 Introduction • The specific pairing of nitrogenous bases in DNA was the flash of inspiration that led Watson and Crick to the correct double helix. • The possible mechanism for the next step, the accurate replication of DNA, was clear to Watson and Crick from their double helix model. 10-Oct-09 Gardiner 3 During DNA replication, base pairing enables existing DNA strands to serve as templates for new complimentary strands • In a second paper Watson and Crick published their hypothesis for how DNA replicates. • Essentially, because each strand is complementary to each other, each can form a template when separated. • The order of bases on one strand can be used to add in complementary bases and therefore duplicate the pairs of bases exactly. 10-Oct-09 Gardiner 4 DNA Replication or Synthesis It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. 10-Oct-09 Gardiner 5 When a cell copies a DNA molecule, each strand serves as a template for ordering nucleotides into a new complimentary strand. One at a time, nucleotides line up along the template strand according to the base-pairing rules. The nucleotides are linked to form new strands. 10-Oct-09 Gardiner 6 The original Watson and Crick model required that while one strand would be copied in a 5’ b 3’ direction, the other would have to be copied in a 3 ’ b 5’ direction
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2 B I O L O G Y 12 2 3 10-Oct-09 Gardiner 7 •Watson and Crick’s model, semiconservative replication , predicts that when a double helix replicates each of the daughter molecules will have one old strand and one newly made strand. •Other competing models, the conservative model and the dispersive model, were also proposed. 10-Oct-09 Gardiner 8 Matthew Meselson and Frank Stahl Demonstrated that the two strands of the double helix separate during DNA replication Used 15N to differentially label the parental and daughter DNA strands Separated heavy from light DNA in cesium chloride density gradients 10-Oct-09 Gardiner 9 Experiments in the late 1950s by Matthew Meselson and Franklin Stahl supported the semiconservative model, proposed by Watson and Crick, over the other two models. In their experiments, they labelled the
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Ch13DNAOct16 - DNA Replication or Synthesis BIOLOGY DNA...

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