To separate DNA based on density, DNA is mixed with CsCl and centrifuged at very high speeds (e.g., 50,000 rpm) in an ultracentrifuge for many hours. As the tubes spin, the opposing forces of sedimentation and diffusion produce a stable, linear gradient of CsCl with the lightest density at the top and the heaviest density at the bottom. As the CsCl gradient forms, the DNA comes to equilibrium in the gradient where its density equals the density of the surrounding CsCl. If DNA of only one density is present, the result will be a single band of DNA. If two DNAs are present with different densities, the result will be two bands of DNA. Click on the "Animate" button to see a simulation of the formation of a CsCl gradient and the formation of a DNA band. As you watch the animation, note the CsCl gradient forming (change in grey shading) and the formation of a band of DNA.