Buoyant density centrifugation (also isopycnic centrifugation or equilibrium density-gradient centrifugation) uses the concept of

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

to separate molecules in solution by their differences in density.

Implementation

Historically a cesium chloride (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 ...

Percoll Percoll is a reagent consisting of colloidal silica particles used in cell biology and other laboratory settings. It was first formulated by Pertoft and colleagues, and commercialized by Pharmacia Fine Chemicals. Percoll is used for the isolatio ...

. This application requires a solution with high density and yet relatively low viscosity, and CsCl suits it because of its high solubility in water, high density owing to the large mass of Cs, as well as low viscosity and high stability of CsCl solutions. 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 a continuous gradient of layers of different densities (and CsCl concentrations) 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. CsCl density gradient centrifugation

Isotope separation

This method very sharply separates molecules, and is so sharp that it can even separate different molecular isotopes from one another. It has been utilized in the Meselson-Stahl experiment.

DNA separation

Buoyant density of the majority of DNA is 1.7g/cm³ which is equal to the density of 6M CsCl solution. Buoyant density of DNA changes with its

GC content In molecular biology and genetics, GC-content (or guanine-cytosine content) is the percentage of nitrogenous bases in a DNA or RNA molecule that are either guanine (G) or cytosine (C). This measure indicates the proportion of G and C bases out of ...

. The term "

satellite DNA Satellite DNA consists of very large arrays of tandem repeat, tandemly repeating, non-coding DNA. Satellite DNA is the main component of functional centromeres, and form the main structural constituent of heterochromatin. The name "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.

Implementation

Isotope separation

DNA separation

See also

References

Further reading