The study of the behaviour of DNA when subjected to electric fields poses several intriguing problems of fundamental physico-chemical importance. Electric field (Kerr effect) orientation of DNA in free solution as well as migration of DNA in gel electrophoresis are two well-established, but so far rather separate, research fields. Whereas the first one has been generally concerned with basic structural and dynamical properties of DNA (Charney, 1988), the second is closely related to techniques of molecular biology (for a review on DNA electrophoresis, see stellwagen 1987).

Photoreceptor cells are special neurons which convert light to an electrical signal. The absorption of a photon by a rhodopsin molecule triggers a sequence of chemical processes leading to a change in the membrane voltage of the photoreceptor. The mechanism of this light-induced voltage change is different in vertebrates and invertebrates. Light causes a hyperpolarization in vertebrates, but a depolarization in invertebrate photoreceptors.