ABSTRACT
A typical eukaryotic genome might contain 50,000 different genes and it is extremely difficult to detect a single GC-rich gene by density gradient centrifugation, since the GC-rich gene would constitute only 1/50,0000 of the total DNA loaded onto a cesium chloride density gradient. Purification of the circular DNA molecules present in extrachromosomal elements, including bacterial plasmids and phage, eukaryotic viruses, and organellar DNA present in mitochondria and plastids, is somewhat more complicated and takes advantage of a difference in the physical properties of a circular and a linear DNA molecule. Thus, a linear DNA molecule can accomodate more twisting stress than can a covalently closed circular DNA. Because closed circular DNA is physically constrained and cannot relieve as much twisting strain as can linear DNA, circular DNA cannot bind as much dye as can a linear DNA of the same size.
