MeselsonWiegle - c +, and + mi ) results in a different...

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The Meselson and Wiegle Experiment Experiments beginning in the 1940s where E. coli B strain bacteria were coinfected with two rII mutant bacteriophage resulting in progeny that could produce plaques on a lawn of E. coli K provided the first genetic evidence for recombination. The first physical evidence of recombination came from experiments by Matthew Meselson and Jean Weigel in 1961 (see slide 4 of the Recombination slide set: Griffiths fig 19-2). They used two strains of a bacteriophage called lambda ( λ ) which contains a linear DNA genome. One strain was wild type for the genetic markers c and mi and the other had mutations at both of these sites. They grew the mutant c mi phage in media containing “heavy” carbon ( 14 C) and nitrogen ( 15 N), while the + + wild type phage was cultured in media consisting of “light” isotopes of these elements. Both wild type and mutant phage of these strains produce plaques but each combination (+ +,
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Unformatted text preview: c +, and + mi ) results in a different plaque morphology. After coinfecting E. coli with both strains of λ , progeny phage were isolated from plaques and separated according to their varying densities using a cesium chloride centrifugation. When the centrifugation was completed, a smear of phage bands was observed in the centrifuge tube (a single band is normally observed when bacteriophage of the same size/density are centrifuged through a cesium chloride gradient). Intermediate bands were carefully removed by inserting a fine needle through the centrifuge tube. After several washing steps to remove all of the cesium chloride, phage from individual intermediate bands were used to infect E. coli . Some of the resulting plaques had morphology consistent with a c + genotype, indicating that a physical exchange of light and heavy DNA had occurred in the formation of the recombinant phage....
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This note was uploaded on 05/17/2011 for the course MCDB 101A taught by Professor Thrower during the Spring '08 term at UCSB.

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