Equilibrium Density-gradient Centrifugation on:
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Buoyant density centrifugation (also isopycnic centrifugation or equilibrium density-gradient centrifugation) uses the concept of
Buoyant density centrifugation (also isopycnic centrifugation or equilibrium density-gradient centrifugation) uses the concept of buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
to separate molecules in solution by their differences in density.
Implementation
Historically acaesium chloride
Caesium chloride or cesium chloride is the inorganic compound with the formula Cs Cl. This colorless salt is an important source of caesium ions in a variety of niche applications. Its crystal structure forms a major structural type where each ...
(CsCl) solution was often used, but more commonly used density gradients are sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
or Percoll
Percoll is a tool for efficient density separation in Cell biology that was first formulated by Pertoft and colleagues. It is used for the isolation of cells, organelles, and/or viruses by density centrifugation. Percoll consists of colloidal si ...
. The sample is put on top of the solution, and then the tube is spun at a very high speed for an extended time, at times lasting days. The CsCl molecules become densely packed toward the bottom, so even layers of different densities form. Since the original solution was approximately the same density, they go to a level where their density and the CsCl density are the same, to which they form a sharp, distinctive band.
Isotope separation
This method very sharply separates molecules, and is so sharp that it can even separate different molecular isotopes from one another.DNA separation
Buoyant density of majority of DNA is 1.7g/cm3 which is equal to density of 6M CsCl solution. Buoyant density of DNA changes with its GC content. The term " satellite DNA" refers to small bands of repetitive DNA sequences with distinct base composition floating above (A+T rich) or below (G+C rich) the main component DNA.See also
*Isopycnic
An isopycnic surface is a surface of constant density inside a fluid. Isopycnic surfaces contrast with isobaric or isothermal surfaces, which describe surfaces of constant pressure and constant temperature respectively. Isopycnic surfaces are so ...
* Satellite DNA
References
Further reading
* * Separation processes Laboratory techniques {{Chem-stub