Chapter 1 : Molecular Nature, Conjugal Transfer, and Replication of Extrachromosomal Elements, 1961 to 1973

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Molecular Nature, Conjugal Transfer, and Replication of Extrachromosomal Elements, 1961 to 1973, Page 1 of 2

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This chapter reviews some of Stan Falkow's early contributions relative to the molecular nature, conjugative transfer, and replication of extrachromosomal elements, particularly R factors. The earliest equilibrium gradients were run in a Spinco model E analytical ultracentrifuge which occupied nearly half of a wall in the laboratory and was affectionately referred to as the "gray elephant"; later, for both isopycnic and rate zonal gradients, the Spinco model L preparative ultracentrifuge was used. Initial studies in which the Freifelder technique was used involved an examination of the labeled DNA after 60 min of mating in neutral sucrose gradients. Pulse-chase experiments were used to demonstrate that the ccc form of the plasmid was the end product of replication and that the oc form was an intermediate in the replication process and not just the result of nicking of some of the ccc molecules during cell lysis and manipulation. The kinetics of appearance of cell structure-bound (presumably membrane-bound) DNA were very similar to the kinetics of appearance of 44S (linear monomer) DNA seen in the pulse-chase experiments. As the membrane-bound fraction decreased, there was a sequential appearance of free 50S (oc) and then 75S (ccc) molecules. It had been shown that a bacterial strain harboring a sex factor or an R factor could not accept the same, or a closely related, extrachromosomal element by conjugation, a phenomenon referred to as "exclusion," "entry exclusion," or "surface exclusion".

Citation: LeBlanc D, Silver R. 1994. Molecular Nature, Conjugal Transfer, and Replication of Extrachromosomal Elements, 1961 to 1973, p 3-16. In Miller V, Kaper J, Portnoy D, Isberg R (ed), Molecular Genetics of Bacterial Pathogenesis. ASM Press, Washington, DC. doi: 10.1128/9781555818340.ch1

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Bacterial Genetics
Sodium Dodecyl Sulfate
DNA Synthesis
Genetic Elements
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