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DNA Replication and repair

SBI 4U November 5 th , 2012. DNA Replication and repair. Thanks to Watson & Crick (and other scientists), scientists now understood the shape, size and chemical composition of DNA  but how did it replicate so well? Especially with such a low error rate?. DNA REPLICATION IS SEMICONSERVATIVE.

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DNA Replication and repair

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  1. SBI 4U November 5th, 2012 DNA Replication and repair

  2. Thanks to Watson & Crick (and other scientists), scientists now understood the shape, size and chemical composition of DNA  but how did it replicate so well? Especially with such a low error rate?

  3. DNA REPLICATION IS SEMICONSERVATIVE • Semiconservative replication: Separating the two parent strands and building a new, complementary replacement strand for each

  4. The Meselson-Stahl Experiment • Grew E.coli with either a ‘heavy’ isotope of nitrogen or light isotope of nitrogen • Bacteria grown for 17 generations • Equal volume of light DNA was mixed with the heavy DNA  What do you expect will happen to the next generation’s DNA?

  5. The Meselson-Stahl Experiment, Continued … • Determining density of DNA  placed in a centrifuge tube  tube contained mixture that produces a density gradient • Originally, the heavy DNA sunk to the middle  but the intermediate DNA was in the middle  proving semiconservative replication

  6. DNA REPLICATION: THE PROCESS 3 steps: • Parental strands of DNA separate • Complementary DNA strands are assembled • New strands are proofread and repaired

  7. Step 1: Strand Separation • Replication origins: act as starting points for replication • Helicase binds to the replication origins and unwinds the 2 strands  hydrogen bonds • Replication fork is formed • DNA replicates from 5’ to 3’

  8. Step 1: Strand Separation • Single-strand binding protein: Prevent rejoining of the base pairs

  9. Step 1: Strand Separation • As the 2 replication forks proceed in opposite directions, the space between them is filled with newly replicated DNA is called a replication bubble

  10. Step 2: Building Complementary Strands • New nucleotides are bound by DNA polymerase very specialized in eukaryotes • DNA polymerase can only add nucleotides to the 3’ end of an existing DNA strand  therefore assembly is from 5’ to 3’

  11. Step 2: Building Complementary Strands • DNA polymerase uses nucleoside triphosphates to make the new strand of DNA • Nucleoside triphosphate very similar to the finished nucleotides. Contain deoxyribose sugar, bonding to 1 of the 4 bases and a group of phosphate molecules

  12. Step 2: Building Complementary Strands • Nucleoside triphosphate hydrolysis reaction cleaves 2 phosphates from nucleoside triphosphate  energy released to drive DNA synthesis

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