Symmetries in biology

Abstract

This topic being extremely large, this presentation is only a key to literature, with some indications on recent trends in the study of symmetries and symmetry breakings in biological morphogenesis. Symmetry problems are essential in condensed matter physics and, for instance, in research on solid and liquid crystals. Living matter can be considered as a mosaic of solids, of liquids and of a large series of intermediate states, which often are liquid crystals or close analogues of liquid crystals.

Is it possible to develop symmetry studies on biological systems, as do physicists In their own field? This question was given a positive answer at the molecular level by Louis Pasteur in the nineteenth century and all further studies have confirmed this pioneer work.

The problems considered here concern higher levels of organization and morphogenesis of structures elaborated by considerable sets of cells. For instance, the shapes of organs and of individuals are elaborated mainly by the production of fibrous networks made of various biopolymers. Most classical examples of these networks are found in the integument, in the connective tissue and in the skeletal system. Morphogenesis of such networks results from the activity of cells secreting polymers and from a self-assembly mechanism, resembling a transition from an isotropic state to a liquid crystal In concentrated solutions of these polymers. These ordered secretions are stabilized either by chemical cross-linking between polymers or by microcrystals forming within the liquid crystalline phase. This gives solid or supple systems, showing In their organization most structures and symmetries of liquid crystals.

Liquid crystals contain several types of singular points and lines, whose distribution is often regular and this leads to the differentiation of characteristic textures and shapes. Such architectures also exist in the biological counterpart of liquid crystals. Chiral components and helical polymers are essential In the formation of highly elaborated morphologies of liquid crystals and this is probably one reason why enantiomers rather than racemates or non active components are adopted In living systems.