Detection of satellite DNAs was stimulated by the development of density gradient ultracentrifugation methods. Equilibrium ultracentrifugation in the density gradient suggested by Meselson and co-workers (Meselson et al. 1957) is one of the most valuable methods of DNA fractionation and characterization. The method is based on the equilibrium distribution of the macromolecular material in the density gradient by sedimentation of a low molecular substance solution in a constant centrifugal field. At equilibrium, a stable concentration gradient of the low molecular substance is formed under the effect of two opposite actions: sedimentation and diffusion and, as a result, a gradual increase of density is observed towards the direction of the centrifugal force. The initial concentration of the low molecular substance, the centrifugal field, and the height of the liquid column can be so selected that at equilibrium the macromolecules will be pooled into the density gradient zone. The centrifugal field directs the macromolecules to a region where the sum of the forces acting on a given molecule is equal to zero. This tendency is opposed by the Brownian motion, and as soon as equilibrium is reached, the macromolecules form a zone in the density gradient, with a width inversely proportional to the molecular weight (Fig. 1).
KeywordsSedimentation Fractionation Macromolecule CsCl
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