Depicting DNA: when left isn't entirely right
As visual science communicators, all too often we find ourselves wincing in distress when we come across poor representations of DNA, especially from medical or science groups that really should know better.
So, what’s with our gripe? To the untrained eye, DNA appears simply as a double helix with things between the strands that loosely resemble the rungs of a ladder. To a trained eye, however, the infamous left-handed DNA helix (termed Z-DNA), is instantly recognisable and brings immediate suspicion upon the credibility of the source or affiliated article, or at the very least a deflated "Oh c'mon, guys".
You see, the double helix structure of DNA has a ‘handedness’ based on the direction that the anti-parallel strands twist around each other. In the vast majority of cases, DNA is found in the B form which has a right-handed helical spiral. While is does exist in cells, left-handed DNA is rare and is often associated with stretches of alternating purines and pyrimidines, e.g. GCGCGC. The right-handed helix owes its structure to the interactions between adjacent base-pairs, known as stacking interactions, such that they favour a more stable energy state. Right-handed DNA is certainly the most common form found in cells and therefore the form that should be most often depicted in science illustrations.
Right handed B-DNA (source: Wikimedia Commons)
Now, before we get too far ahead of ourselves, we must acknowledge the freedom afforded by artistic licence in visual representations of DNA. The DNA double helix is an incredibly complex structure defined by some serious molecular forces, and to most people a simple double helix is sufficient. But for those who want to impress their peers and avoid embarrassing face-palm moments, you should probably stick with the most common and accurate DNA representation.
Handy tips for handedness - staircases and karate chops
There are two fool-proof ways to identify the handedness of DNA in illustrations:
Picture the helix as a spiral staircase. As you descend the staircase, note the direction that you would be turning as you navigate the twist of the staircase. If you turn to the right as you walk down, the DNA is right-handed, and vice versa. Too many mental gymnastics?... See method 2.
Identify the orientation of the stands as they cross to the front of the image (closest to the viewer and therefore in front of the complementary strand). If the ‘highest’ point of these sections of the spiral begin on the right side of the strand and travel to the bottom left (like the motion of a right-handed karate chop across the body), the helix is right-handed. If they begin on the left side, you guess it, left-handed DNA. This method is no doubt the quickest and simplest way to recognise the handedness of DNA in illustrations, but if you prefer to walk in small circles down an imaginary staircase then that is an altogether-acceptable method.
Get your groove on
In addition to the handedness of DNA, each form differs slightly in the spaces between turns of the double helix (termed groves). Normal B-DNA has uneven spacing such that there exists distinct minor and major grooves along the length of the double helix. These play a vital role in determining what and where things can bind to the DNA (proteins, drugs, dyes, etc.) and mustn’t be overlooked. And if you really want to impress your peers, remember that there are approximately 10-10.5 base pairs (ladder rungs) per full turn of the DNA.
If the composition of the right-handed DNA in your image is not ideal, flipping the image either vertically or horizontally will change the helix into the left-handed form, which is often an unwanted side effect of improving the composition of the image (note: flipping can therefore correct a left-handed helix). Rotate images until your heart is content as this will obviously not change the handedness of the DNA.
Image flipping will change the handedness of DNA. Left: L-handed, Right: R-handed (adapted from Wikimedia Commons)
So if you are a science illustrator or have commissioned illustrations of DNA, be sure to check on these key features that will help to improve the accuracy of the image, and therefore the reputation of all involved. Below are just a few examples of groups that should know better….
DN...eh? The left-handed double helix strikes again (source: Twitter)