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Symmetry

24 25 enAntiomorphy left and righthandedness Amongst other things, our dorsiventral bodyform gives us a pair of hands that are similar in most respects, except that they are mirror reversed. The same is true of our feet, of course, and of horns and butterfly wings and many other animal features 1. But the possibility for a figure, or an object, to exist in two distinct forms in this way is not limited to the mirrorsymmetries adopted by living organisms. Any spiral, for instance, has to choose whether to go clockwise or anti clockwise on the page 2, and similarly, all helices can appear in one or other of two different ways in 3dimensions 3. In fact the possibility of alternate forms applies to any object, animate or inanimate, that has a twist in its structure. Mollusk shells are found as both lefthanded and righthanded types some species opt for a particular handedness, in others the choice appears to be random 4. There is a somewhat similar situation in the familiar twisting habits of vines and other climbing plants the majority opt for righthandedness, but a substantial minority are lefties. In chemistry this phenomenon is known as chiralitythe most common mineral with this trait being quartz 5. Chirality is of particular importance in the field of organic chemistry, since many biological molecules are homochiral, that is to say, are of the same handedness, including amino acids which are the components of proteins, and DNA 6 . This, in effect, means that the entire chemical basis of life itself is chiral. At some early stage in the origins of life on Earth the earliest molecules to master the art of selfreplication opted for a particular stereochemical profile, and in so doing determined the entire, righthanded, course of evolution. lemon orange sweet bitter spearmint caraway Above Flemings left and righthand rules for motor and generator. Below A selection of stereoisomers which smell different in left and right handed forms. Right The righthanded DNA helix. 2. The Coupe du Roi A sphere or apple is cut into either two left or two right handed shapes. Note that a left right pair cannot reform the sphere A pair of left and righthanded fans in a square. 3. Spirals and helices can either be left or righthanded 1. 4. 5. 6.
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