ABSTRACT
Except for the use of sucrose gradients to separate proteins and ribonucleo-proteins on the basis of sedimentation rate and the use of KBr gradients for lipoproteins, macromolecules and macromolecular complexes have generally been purified using self-generated gradients of heavy metal salts. A major attraction of the use of Nycodenz or iodix-anol for macromolecules and macromolecular complexes is that any subsequent analysis of the fractionated products can be carried out without the need to dialyze away the medium. For many years, self-generated gradients of heavy metals salts and of CsCl in particular were used as the standard media for the fractionation of nucleic acids. There are two types of interactions between nucleic acids and proteins that are important in the control and regulation of transcription and translation - ionic and sequence-specific. Perhaps one of the most important modern uses of density gradients is the purification of plasmid DNA..
