The handedness or chirality of biological structure at the molecular and at higher levels of structural organisation

Macromolecular asymmetry

The helix is the most common and the most readily recognized example of an enantiomorphic structure. Helical proteins and DNA are good examples of structures where a clear explanation can be provided as to why they adopt one hand or the other. Proteins and DNA are composed of chiral building blocks, amino acids and nucleotides, respectively. Only the L-amino acids occur in proteins; this uniformity of handedness is a prerequisite for helix formation and thus, one could argue, for the development of higher life forms. Helical proteins form higher order helical structures, from collagen and viral capsids to cotton fibres.

Reflections on the ambivalent helix

The helix is nature's favourite shape. Because of its elementary geometry and distinctive appearance it is also the clearest instance of an enantiomorphic object--a helix and its mirror image are identical in all respects except their screw sense. This is a distinction that can be ignored from the points of view of pure geometry and pure group theory but any helical structure is actually available as either or both hands. Whether in nature helices do occur as just one hand, or both, is one of the best--perhaps the best--puzzles of the science of form. In this short review I look at a few examples of naturally occurring helices, some where only one hand is found, some where both are commonly found, and perhaps the most interesting examples in biological terms--those where both are found but one hand is very much rarer than the other. I review what mechanisms--physico-chemical, genetic, evolutionary--underlie the different manifestations of left- and right-handedness.