As a kid did you ever try to force a vine to climb a post or string in the opposite direction? Carefully you unwrapped a portion of the bean, pea, or morning glory vine, and wrapped it around its suport in the opposite direction. The next day you saw that the growing tip of your recalcitrant plant had alredy rejected your orders and started climbing in its original direction.
When a vine climbs a post it winds its way making a helix - the the threads on a bolt or corkscrew. These windings coulb be either "left-handed" or "right-handed" turns. They 'could' but they always choose one way or the other. In the figure to the right are sketches of the two sorts of windings. Cleverly, the left-handed helix is on the left!). How can you tell which is which? Hold up your left hand with your four fingers slightly curved and your thumb pointing up. Pretend that you are going to slide your fingers around the grooves of the left figure. Note that your fingers tend to rise - in the direction that thumb is pointing. Try your right hand on the left figure. The fingers want to go down - not the direction your right thumb is pointing. But if you use your right hand on the figure to the right, everything works. It must be a right handed helix. (No, don't try turning the two-dimensional figure over! Get a real three-dimensional bolt or screw: that one you can flip upside down and it is still the same handed. Need more practice? Go to your nearest stairwell: which type of helix is it?
Returning back to the insistant vine, an idea ought to immediately pop into your mind: "There must be something genetic in this!" Indeed there is! Even DNA itself is composed of interlacing right-handed helices. While chemists have made DNA into left-handed helices to show that it is possible to exist that way, it NEVER does in nature.
Pretend for a moment that you are the inventor of the idea of screws and bolts. Inherently, there is not advantage of threads going in one direction or the other. As the inventor wanting to put food on your family's table, you want to go into production with your newfound idea. You consider that were you to make make nuts and bolts of BOTH directions, you would need equipment to manufacture both types - two sets of expensive equipment. What is more is that you would have to keep the products well separated, otherwise your customers would get unhappy as they would spend half their time trying unsuccessfully putting mismatched nuts on bolts. How much cheaper and easier if you were merely arbitrarily to choose one direction, buy one set of manufacturing equipment, ignor sorting, and have happy customers.
Apparently nature saw it that way also! Having only one direction of DNA requires only one direction of complementary proteins to fit with it to make duplicate copies, make messenger-RNA, and to stick to it as obstructing regulators (turning the gene off). Nature evolved the principles of economics!
Now let's turn to protein structure - specifically to the alpha-helix. Is it always left-handed? Or only right-handed? Or both? Find out this information on your own.* Make your own helix in the correct handedness using the strips of "primary structure" polypeptides and tape or paper clips. (Be sure to load your printer with legal sized paper for this print-out.)
Question: In terms of the time scale of evolution, discuss an approximate time when this choice of right-handed DNA was made.
FIELD TRIP! While out walking through the woods with your date, take a look at the tree trunks as you stroll by. Many are gently twisted into a form of helix. Which handedness? Is it species specific? Well, one thing this author can say is that you will indeed look strange as the two of you are standing there on the trail holding your hands like you want to hitch a ride! But you might just as well have a good time while studying.
* "Is it better to give a hungry person a fish to eat, or show the person how to fish?" College education is aimed at giving you few "fish", but giving all the tools for catching your own "fish." In plain language, college is designed to teach you how to find out your own information. The spoon feeding of information ended at about 5th grade.
