Electrophoresis of DNA and other polyelectrolytes: Physical mechanisms

The dramatic recent advances in molecular biology, which have opened a new era in medicine and biotechnology, rely on improved techniques to study lar ge molecules. Electrophoresis is one of the most important of these. Separa tion of DNA by size, in particular, is at the heart of genome mapping and s equencing and is likely to play an increasing role in diagnosis. This artic le reviews, from the point of view of a physicist, the mechanisms responsib le for electrophoretic separation of polyelectrolytes. This separation is m ainly performed in gels, and a wide variety of migration mechanisms can com e into play, depending on the polyelectrolyte's architecture, on the electr ic fields applied, and on the properties of the gel. After a brief review o f the thermodynamic and electrohydrodynamic principles relating to polyelec trolyte solutions, the author treats the phenomenology of electrophoresis a nd describes the conceptual and theoretical tools in the field. The reptati on mechanisms, by which large flexible polyelectrolytes thread their way th rough the pores of the gel matrix, play a prominent role. Biased reptation, the extension of this model to electrophoresis; provides a very intuitive framework within which numerous physical ideas can be introduced and discus sed. It has been the most popular theory in this domain, and it remains an inspiring concept for current development. There have also been important a dvances in experimental techniques such as single-molecule viodeomicroscopy and the development of nongel separation media and mechanisms. These, in t urn, form the basis for fast-developing and innovative technologies like ca pillary electrophoresis, elechophoresis on microchips, and molecular ratche ts.