Helices turn right or left

By Vic Arcus 29/10/2009

I was in the garden this weekend and I saw the new tendrils on our passion-fruit vine winding their way outwards from the main stem of the vine. The tiny passion-fruit leaders form helices which grasp onto, and wind their way around the wire which we attached to the wall. The new spring growth in the garden has lots of examples of these beautiful helical forms. There are the new spirals of the black mamaku pongas and the beans spiral their way up the netting. There is a fascinating property of helices which is worth describing. If you look at an old fashioned cork screw (which is a helix) you’ll notice that it goes in one direction. I checked our cork screw and found that it was right handed. That is, you must screw it clockwise to get it to go into the cork (in the time before screw top wine bottles). If you turn it anticlockwise, nothing happens. If the cork screw wire was straight, it wouldn’t have this “handedness” about it. But as soon as you wind it into a helix, it must be either right handed or left handed (I wonder if anyone makes left handed cork screws which go anticlockwise into the cork?). The term for “handedness” in biology is called chirality. And chirality abounds in nature. Our amino acids have chirality, our DNA has chirality. Indeed, we have left and right hands! So our bodies are chiral.

But back to the garden. I checked the tendrils on the passion-fruit vine and found that those which branch off one side of the main stem are mostly left-handed helices and those which branch off the other side of the stem are mostly right-handed. On the other hand, the beans seem to climb up the wire in spirals or helices of just one direction.

This reminded me of a wonderful picture that appeared on the front cover of the September 2009 issue of the EMBO Journal (the Journal of the European Molecular Biology Organization). This is a picture of a bacterium called Bacillus mycoides which grows in spiral shaped colonies and can be isolated from many soils. The cells themselves are slightly curved and link head to tail to form these filamentous helices. In all cases of Bacillus mycoides, the filaments grow in the same direction. But there are different strains which grow either exclusively right handed or exclusively lefthanded. The strains are like the cork screw – strain A of Bacillus mycoides always grows clockwise and strain B always grows anticlockwise (for more cool pictures down a microscope see here). This means that it is genetics which determine the way the bacteria spiral. However, scientists have yet to track down which genes are responsible for chiral growth in Bacillus mycoides. If you were an aspiring microbiologist, this would be a problem which would potentially have an impact on many, many forms of life – from bacteria to plants to people!