ch13im - Chapter 13DNA Replication And Repair DNA...

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C hapter 13DNA Replication And Repair DNA Replication: Background Information and Overview I. A. Organisms duplicate by asexual or sexual reproduction B. Cells duplicate by cellular division C. Genetic material duplicates by DNA replication – replication machinery is also used to repair the genetic material after it has sustained damage II. Capacity for self-duplication is presumed to have been one of first critical properties to have appeared in the evolution of the first primitive life forms A. Without propagation, any primitive assemblage of biological molecules would be destined for oblivion B. First carriers of genetic information were probably RNA molecules that could self-replicate C. Later RNA was replaced by DNA as the primary storehouse of genetic information 1. 2. DNA contains information for its own duplication, but cannot perform the activity itself like RNA III. A. Suggested that replication occurred by gradual double helix strand separation via successive breakage of H bonds, much like the separation of the two halves of a zipper B. Since each strand is complementary to the other, each has the information needed to construct the other; once separated, each strand can serve as template to direct the formation of the other strand. IV. Semiconservative nature of replication - Watson & Crick predicted that new DNA should consist of one old A. Possible types of replication 1. 2. Conservative - 2 original strands stay together after serving as templates for 2 new strands that also stay together; one contains only "old" DNA, the other only "new" DNA 3. Dispersive – integrity of both parental strands disrupted; new duplex strands made of old & new DNA; neither the parental strands nor the parental duplex is preserved B. Of the suggested mechanisms, the third is the most unlikely, but it is the only one that avoided the seemingly impossible task of unwinding the 2 intertwined DNA duplex strands during replication C. Matthew Meselson & Franklin Stahl (1957, Caltech) - grew bacteria in media with 15 NH 4 Cl as sole nitrogen source for many generations; DNA bases contain "heavy" nitrogen 1. Wash out 15 NH 4 Cl; put bacteria in 14 NH 4 Cl ("light"); remove samples at intervals over several generations; use 14 N & 15 2. Extract DNA & subject it to CsCl equilibrium density-gradient centrifugation to find buoyant density 3. Mix DNA with concentrated CsCl solution, centrifuge until double-stranded DNAs reach equilibrium according to their density; density of DNA is directly related to percentage of
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ch13im - Chapter 13DNA Replication And Repair DNA...

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