Helical Imperative: Paradigm of Growth, Form and Function
Published Online: 17 JUN 2010
Copyright © 2001 John Wiley & Sons, Ltd. All rights reserved.
How to Cite
Galloway, J. 2010. Helical Imperative: Paradigm of Growth, Form and Function. eLS.
- Published Online: 17 JUN 2010
Helices appear at every anatomical level across the nine (or so) orders of magnitude that span the range of size between molecules and the biggest organisms. They provide solutions to any number of the challenges of growth and form, structure and function including significantly movement, that evolution has thrown up. This essay explores the helix both as an abstract mathematical idea, with its stark elegance, simplicity and economy, and the ‘real’ helical structures that contribute to the richness and complexity of the living world – and the relationship between them. Helical structures are so pervasive that the helix can perhaps be regarded as providing a unifying and even necessary structural principle for life. The helical idea goes a long way to explain why life at its most fundamental level of genes and proteins depends on two classes of small enantiomeric molecules, amino acids and nucleotides, significantly the molecules in each of the two classes all possessing the same hand. Life's apparent requirement for helical symmetry at the deep molecular level forces the uniformity.
Bilateral or ‘mirror’ symmetry is the most obvious symmetry at the level of whole animals, but helical symmetry is far commoner overall and at every anatomical level.
Helical symmetry lies at the heart of the structural molecular biology – the result of simple repetitive algorithms of growth based on thermodynamic ‘equivalence’.
The huge range (tens of thousands) of extant and extinct molluscan (and other) shell-types shows the scope of achievable variations on a simple parameterised structural theme, the conical helix or concho-spiral.
Thought of in engineering terms, living things are best thought of as tension structures, held together and stabilised by fibres. Most fibres are typically helices. It is a good question whether life is conceivable in any other way.
Because of its simple but striking geometry, a helix is a particularly clear example of an enantiomorphic object. The two mutual mirror image forms are not superposable. Whether in the real world both forms actually occur, or not, turns out to be a rather deep question.