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 ...
to separate molecules in solution by their differences in density.
Implementation
Historically a
caesium chloride (CsCl) solution was often used, but more commonly used density gradients are
sucrose 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
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 ...
. The term "
satellite DNA Satellite DNA consists of very large arrays of 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 the ...
" 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
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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 s ...
*
Satellite DNA Satellite DNA consists of very large arrays of 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 the ...
References
Further reading
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Separation processes
Laboratory techniques
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