The DNA double helix is one of the icons of our time. You’d wish people would draw it right.
You’d think getting it wrong would make the artist feel like a complete mug, as if they’d drawn the Statue of Liberty holding up a lump of coal instead of a flaming torch.
But the DNA helix in all sorts of places is simply wrong.
You don’t need to be a TV drama-smart science geek to know which artists have screwed up.
Let me show you an easy way. (And impress your non-scientist friends!)
Does it turn the right way?
Look at the DNA with the ‘rungs’ of the ladder horizontal and the helix standing vertically.
Apply this rule:
From the right downwards, means the helix is right-handed. (Shown to the right.)
From the left downwards, means the helix is left-handed – which is wrong. (Shown to left.)
(Modified from original image at Georgia Perimeter College webpage. Some mobile phone users with smaller screens may see the left image above the right image.)
So, this is right-handed,
but these are left-handed,
Flip it over
One way to correct a left-handed DNA cartoon is simply to flip the picture over to it’s left-right mirror image.
No excuses for artists not to put their errors right – that’s click-of-a-button stuff.
A plague of wrongness
Leftie DNA is all over the place, especially in commercial art sites.
Unfortunately most commercial artists insist that you pay for their work, even tiny images on-line. So we’ll have to make do with links. The free one to the right is from photosxc.com. Here are more:
I could go on, but you get the picture.
As a more complex example, this pair of chromatids (ID 275311, turbosquid.com) has a mixture of left- and right-handed DNA that the artist has likely created by taking mirror-image of an original.
As I remarked to a reader, I was “a bit disappointed at the DNA” in the logo for Watson and Crick. (That’s not the DNA model they’re famous for!)
It’s even wrong on a web page that aims to show people how to draw the DNA double helix!
Some of these illustrations are wrong in other ways too.
To make things confusing you can get left-handed DNA, it’s just not the stuff our genes are made of. There’s a slightly fancier way of telling what is left- or right-handed too.
Let’s try a little more detail.
A little more about DNA
Here’s a model of DNA:
This model is made from data giving the position of the atoms in a crystal of DNA using X-ray crystallography – one way to take a ‘picture’ of molecule. It’s the real deal, in detail; we can see where every atom is.
There are several forms of DNA. The best-known are A, B and Z.
The ‘classical’ form, the one that most of DNA our genes are made of is in, is B-form DNA like that in the animated model above and the illustration to the left.
You can see that DNA is made of two helical chains with the DNA bases in the middle.
(The phosphates of the deoxy-ribose sugar-phosphate backbone are orange and red in the rotating model above and black in the illustration to the left.)
B-form DNA has two grooves of different sizes: the major and minor grooves, as marked in the illustration to the left.
Reminds you of keys in music, right?
The major groove is the wider ‘valley’ between the phosphate backbones.
The minor groove is the narrow one.
Other common mistakes
Making B-form DNA left-handed isn’t the only common error you’ll see.
No major / minor grooves (or, grooves the same size)
Some illustrators draw DNA with two equal-sized grooves, like in the example to the right.
Too many or too few bases per helical turn
B-form DNA typically has 10 base pairs per full helical turn.
There’s a wee trick to this.
If you watch the rotating model shown earlier you’ll see that one base pair gets hidden behind the point where the two helical backbones cross.
So, looked at the right way, for each half helical turn you’ll see four base pairs side on, with the fifth obscured.
Too skinny or too fat
Very roughly, the length of a full helical turn should be about one and a half times the width of the double helix.
(Alternatively, the width should be about two-thirds of the height of a full helical turn. B-form DNA is roughly 2 nanometers wide for each roughly 3.4 nanometer turn of the double helix.)
So that ‘DNA’ to the right looks like alien DNA, right?
Left-handed – check.
Far too many base pairs per turn – check.
No major / minor grooves – check.
Way too svelte – check!
Even in scientific journals…
You might forgive creationists for getting the structure of DNA wrong, like on the cover of a creationist take on epigenetics to the left. Having gotten much else of the science wrong it’s not so startling to see the DNA helix wrong in that context.
Embarrassingly there are also examples of left-handed DNA representations in scientific journals. Almost all of these are works of illustrators. Nevertheless, that’s not really much excuse.
There’s good a comment by scientific illustrator Kalliopi Monoyios in her article, The DNA Hall of Shame, on her blog, Symbiartic, at Scientific American blogs that images may not get the same scrutiny as text despite that they probably should. Looking at the examples in Tom Schneider’s pages you’d have to agree.
These goofs aren’t limited to the ‘lesser’ journals either. (In fact, I wonder if a contributing reason that they occur in the more prominent journals is simply that these publications are more able to afford to outsource illustration work – ?)
This month (July 2013) Nature Methods’ cover shows the work of a freelance designer, featuring a stylised blue-and-gold left-handed helix. (The illustration is highlighting an article, Simultaneous DNA amplification and detection using a pH-sensing semiconductor system, describing the development of a DNA-detecting chip – “an integrated chip for real-time amplification and detection of nucleic acid using pH-sensing complementary metal-oxide semiconductor (CMOS) technology”.)
Another example is in PLoS Computational Biology, in an illustration intended to be a “representation of different elements related to bioinformatics” for the cover of the June 2013 issue.
(Detail shown to left; full image below, taken from the cover information page.)
Yet another example is in an older Nature Genetics cover (September 2005). It’s a lovely illustration, but the interwound ribbons representing DNA are left-handed.
Tip for illustrators
It’s perhaps best to get a hold a model of real DNA (ideally from the crystal structure databases) as a template to stylise rather than try roll your own. In addition to getting the handedness right, you’ll also get other aspects of the geometry right this way.
To see a really nice—we could even say sexy—animated model of DNA, try the solid model at wikimedia. I’d put it up here if it weren’t that it’s 2.5Mb in size.
An excellent source of older examples is in Tom Schneider’s Left Handed DNA Hall of Fame.
This article was inspired by noting that one of the logos for famous scientists—for Watson and Crick—features a DNA helix with a problem. Can you see what the problem is?
(Lest anyone forget, the alpha-helix of proteins is also right-handed!)
1. A political satirist might, but that’s another story.
2. I’ve given the names of the sites rather than the artists; I’m not trying to hit on anyone here, but illustrate that this is widespread.
3. I would like to have used this as the example helix, but I haven’t time to sort out if the illustration is free for others to use.
4. I’m simplifying here, this isn’t a textbook!
5. More accurately, x-ray crystallography gives a map of electron density that guides the modelling of the atomic structure.
6. The major and minor grooves are on opposite sides of the DNA base pairs that form the ladder rungs between the two strands.
7. Actually it’s often slightly more that 10 base pairs per helical turn, but that’s getting fussy for cartoons.
Other articles on Code for life: