These cells can be extracted from whole blood using ficoll, a hydrophilic polysaccharide that separates layers of blood, and gradient centrifugation, which will separate the blood into a top layer of plasma, followed by a layer of PBMCs and a bottom fraction of polymorphonuclear cells ( such as Basophils are sometimes found in both the denser and the PBMC fractions.
12.
The method, equilibrium density gradient centrifugation, was sufficiently sensitive that Meselson and Stahl were able to separate DNA containing the heavy isotope of nitrogen, 15 N, from DNA made of the lighter isotope, 14 N . In their classic experiment, described and analyzed in a book by science historian Frederic L . Holmes, they grew the bacterium " Escherichia coli " for many generations in medium containing 15 N as the only nitrogen source and then switched the bacteria to growth medium containing 14 N instead.
