Difference between revisions of "20.109(S09):Bacterial amplification of DNA (Day3)"
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Bacteria can take up foreign DNA in a process called transformation, during which a single plasmid enters a bacterium and, once inside, replicates and expresses the genes it encodes. Most bacteria do not exist in a transformation-ready state, but can be made permeable to foreign DNA by chemical treatment or other means. Cells that are capable of transformation are referred to as competent. Competent cells are extremely fragile and should be handled gently, i.e., kept cold and not vortexed. Bacterial transformation is efficient enough for most lab purposes, resulting in as many as 10<sup>9</sup> transformed cells per microgram of DNA, but even with highly competent cells only 1 DNA molecule in about 10,000 is successfully transformed. Thus we need a way to identify transformed cells, which is usually accomplished with antiobiotics. For example, the plasmid carrying inverse pericam (calle [https://catalog.invitrogen.com/index.cfm?fuseaction=viewCatalog.viewProductDetails&productDescription=615 pRSET]) also carries a gene that leads to ampicillin-resistance. Consequently, a transformed bacterium will grow on ampicillin-containing agar medium, while untransformed cells will die before they can form a colony (see figure above right). Given the low concentration and nicked structure of your DNA to begin with, you should perform your transformations today with great care. | Bacteria can take up foreign DNA in a process called transformation, during which a single plasmid enters a bacterium and, once inside, replicates and expresses the genes it encodes. Most bacteria do not exist in a transformation-ready state, but can be made permeable to foreign DNA by chemical treatment or other means. Cells that are capable of transformation are referred to as competent. Competent cells are extremely fragile and should be handled gently, i.e., kept cold and not vortexed. Bacterial transformation is efficient enough for most lab purposes, resulting in as many as 10<sup>9</sup> transformed cells per microgram of DNA, but even with highly competent cells only 1 DNA molecule in about 10,000 is successfully transformed. Thus we need a way to identify transformed cells, which is usually accomplished with antiobiotics. For example, the plasmid carrying inverse pericam (calle [https://catalog.invitrogen.com/index.cfm?fuseaction=viewCatalog.viewProductDetails&productDescription=615 pRSET]) also carries a gene that leads to ampicillin-resistance. Consequently, a transformed bacterium will grow on ampicillin-containing agar medium, while untransformed cells will die before they can form a colony (see figure above right). Given the low concentration and nicked structure of your DNA to begin with, you should perform your transformations today with great care. | ||
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[[Image:Be109agarosegelEM.jpg|thumb|250px|left| Scanning EM image of agarose polymer]] | [[Image:Be109agarosegelEM.jpg|thumb|250px|left| Scanning EM image of agarose polymer]] | ||
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− | Although all DNA molecules travel in the same direction during gel electrophoresis, they do so at different rates: larger molecules get entwined in the matrix and retarded, while smaller molecules wind through the matrix more | + | Although all DNA molecules travel in the same direction during gel electrophoresis, they do so at different rates: larger molecules get entwined in the matrix and retarded, while smaller molecules wind through the matrix more quickly and thus travel further from the well. Ultimately, fragments of similar length accumulate into |