PHASE ONE: APRIL 1953
James Watson and Francis Crick published their description of the structure of DNA in Nature on 25 April 1953. Their paper included a drawing of the molecule, complete with the sugar-phosphate chains of the double helix and the base pairs that connect the two chains.
The artist who created the drawing was Odile Crick, wife of Francis. In the words of James Watson, "Francis can't draw, and I can't draw, and we needed something done quick.... [The drawing] showed the essence of the structure".
There is something distinctive about that drawing: DNA has a vertical line running up the center of its structure. Why? I suggest two possible reasons. The first I derive from a famous photo taken on 21 May 1953. It shows Watson and Crick observing their three-dimensional metal model of DNA. In the photo there is a vertical rod running up the center of the model. The 3D model could not support itself with the parts that represented the sugar-phosphate double helix. The four bases of the model are clamped horizontally to the vertical rod, and the two sides of the double helix are attached to the bases.
This is ironic. Ordinarily the double helix is supposed to be the stable structure: It consists of strongly bonded sugars, phosphates and bases, whereas the base pairs are the more unstable part in the sense that they break when the molecule comes apart to generate RNA and when it is reproduced in polymerase chain reaction. Mutations arise when insertion, deletion or substitution disrupts the sequence of bases. It is rare for this to happen when the double helix is intact.
The three-dimensional model is older than the photo, so it might be the basis for Odile Crick's drawing.
There is another possible antecedent for the vertical line. On 15 March 1953, Crick wrote to his son about what he and Watson had discovered. The letter includes a rough sketch of DNA, including the double helix and the base pairs. This sketch has a vertical line running up the center of the molecule. Imagine that Odile Crick's professional drawing was a refined version of Francis Crick's sketch.
Odile Crick's drawing seems to be either an artifact of the rod supporting the three-dimensional model or an improved version of Francis Crick's sketch.
After I made the point about the vertical line in the 1953 drawing on Brigitte Nerlich's blog, this comment came from Kindra Crick, granddaughter of Francis Crick: "Interesting interpretation. I believe the central line was intended to call attention to the three dimensional shape. A reference line similar to the arrows which very simply indicate that the [strands] run in opposite directions". Commenting on the blog, Peter Ellis added: "There's no mystery here. In the caption of the figure in the original paper, it states, 'The vertical line marks the fibre axis.' ... the line does not represent anything physical and is simply a guide to the underlying symmetry".
The caption says, "This figure is purely diagrammatic". And then, "The two ribbons symbolize the two phosphatesugar chains, and the horizontal rods [symbolize] the pairs of bases holding the chains together". If "purely diagrammatic" means nonrepresentational, then the ribbons and the rods are representational, and the reader could infer that the line is just as representational as the ribbons and the rods.
And there is a problem with the "underlying symmetry": The base pairs are symmetrical one way and asymmetrical another way. Adenine is wider than thymine, and guanine is wider than cytosine. All four base pairs have the same total width, namely, 2.0 nanometers (which Watson and Crick described as 20.0 Ǻ), so the DNA molecule is externally symmetrical. But the hydrogen bonds connecting each pair of bases are off-center.
Anyone can imagine whatever internal symmetry they want to, but it seems to me misleading to depict the internal symmetry of a molecule that is internally asymmetrical. The vertical line represents a nonexistent symmetry.
This raises another point. The model, the sketch and the drawing all depict DNA as a vertical object. But DNA itself can be vertical, horizontal or diagonal within a genome. I conclude that it is an aesthetic convention, not an empirical conclusion, to depict DNA as vertical. Indeed, the recent illustrated edition of Watson's The Double Helix shows DNA horizontally on the dust jacket.
This reminds us that it is often difficult to depict a natural object in models and drawings. This can be especially problematic for nanoscale objects, e.g. molecules, because they cannot be seen with optical microscopes. As best as I can tell, the vertical line disappeared from illustrations of DNA not long after the paper of 25 April 1953. It still appears in some three-dimensional models because those models need a vertical rod to support them. Yet if a two-dimensional depiction of DNA included it today, implying that it was representational, then scientists would object that DNA includes no such thing.
PHASE TWO: THE GOLDEN AGE of DNA ILLUSTRATIONS
As the importance of the discovery by Watson and Crick became known to scientists and nonscientists, there arose a great variety of depictions of DNA. Some variations are these:
• The sugar-phosphate chains can be depicted as flat ribbons, round rails or a series of sugar and phosphate molecules.
• The base pairs can be indicated as one horizontal piece or as two different parts of a horizontal piece. In the latter case, the parts may be labeled as A, T, G and C.
• The bases can be depicted as symmetrical pairs or as asymmetrical pairs.
• DNA can be depicted in monochrome, or its different parts can be shown in different colors.
• In most cases, DNA is shown vertically, but it could also be shown horizontally or diagonally.
There is a limit to these variations, namely the underlying structure of DNA. Even so, these variations prompt questions: Why would an illustrator choose one combination of aesthetic features while a different illustrator chooses a different combination? What does the illustrator want the audience to know about DNA?
Here I speculate about those questions. There are two principal populations that consume visual depictions of DNA. One comprises scientists and science students, plus physicians and medical students. When they see DNA in their textbooks, it is helpful that visual depictions provide precise details: that the sugar-phosphate chains look different from the base pairs, and that each base has its own unique signal, whether color or labels of A, T, G or C. This is important information to the population in question.
Another population consists of consumers who need little detail in the depictions they see: It is enough to perceive only that DNA embodies a double helix and four base pairs. Why is this?
I suggest that simpler depictions of the molecule have minimal didactic value because they serve audiences that have little need to know what the parts are. It is sufficient for this audience that a stripped-down picture of DNA tells the viewer that if a product, a vision or a brand includes a picture of DNA, then it is "scientific." Before April 1953, one might have claimed that something is scientific by showing the outline of an optical microscope, or by showing a two-dimensional image of an atom with electrons orbiting a nucleus.
Now we are in a different epistemic moment. A common way to say that something is scientific is to associate it with a simple depiction of DNA, and for this purpose there is no reason to differentiate the physical details of DNA with aesthetic details.
Between those two audiences and their needs are other kinds of audiences. A notable one constitutes nonscientists who are curious about science, e.g. those who read the science section in the New York Times or the Guardian. What does this audience expect of illustrations of DNA? Perhaps they need more empirical detail than the simpler depictions but not the same detail that one finds in medical textbooks.
My comments may sound elitist: Scientists need many empirical details while nonscientists are satisfied with few. But I emphasize that there is a spectrum of consumers. It includes scientists, physicians, science teachers, medical students and more. There is no single formula for how to depict DNA because there is no single audience to depict it for.
The golden age of depicting DNA continues. Illustrations in genetics textbooks may become more detailed for the purpose of communicating more empirical knowledge, but the simpler pictures remain useful for conveying a simpler message that here is something scientific. I add two more phases below.
PHASE THREE: GENETIC ENGINEERING
Genetic engineering is a technology that uses recombinant DNA (rDNA) to transfer the genotype of a trait from one organism to another. A desirable trait is identified; the genes that code for that trait are located; rDNA captures the relevant DNA from the source organism and then inserts it into the genome of a target organism, which can be from a different species. When this works properly, the trait manifests itself in the target organism. Genetically modified foods are vivid proof that genetic engineering is real. The ultimate genetically modified organism is a creature that has acquired bioluminescence, e.g. kittens that glow in the dark because genes for luminescence have been inserted in vitro into their genomes.
How then is DNA depicted in genetic engineering? The core purpose of visual depictions of DNA in genetic engineering is to show that a segment of DNA is extracted from one organism and inserted into another.
But if scientists are doing genetic engineering, then it is desirable to show them doing so. Depictions of genetic engineering often show a person wearing a white lab coat (because this says that the person is a scientist), and they show that the person is in the process of moving a segment of DNA from a source organism to a target organism.
The DNA molecule is much too small to be seen by an optical microscope, let alone the human eye. Were the scientist depicted at a scale making their humanity visible, a piece of DNA could not be depicted at the same scale.
The solution is to exaggerate the size of the DNA segment. The genetic material may seem to be about 20 centimeters in length, which is to say that the illustrator depicts humans at a human scale but depicts DNA as something millions of times larger than it really is. If one is going to illustrate a nanoscale process like genetic engineering, while also putting scientists into the illustration, then it is necessary to distort the scale of the DNA. Didactic intent determines this aesthetic decision. Here Google helps us. Search for "genetic engineering" and select "images." One can see how the human scale and the molecular scale are made to fit into one image.
Phase three does not represent an end to phase two. On the contrary, the two can occupy the same time period, because different illustrators have different didactic intentions.
PHASE FOUR: SYNTHETIC BIOLOGY
Synthetic biology is a sophisticated variation of genetic engineering. Again, a desirable trait is identified, and the genes that code for that trait are located. But instead of deriving the DNA from a living organism, one can assemble it in a chemistry lab where there are supplies of A, T, G and C, plus sugar molecules and phosphate molecules. Synthetic biology then acquires a sense that it is a process of design and construction, like civil engineering. Depictions of DNA sequences emphasize that they have been designed, constructed and delivered to the right places, like the pieces of a building or a bridge. Engineering practices govern the visual representation of DNA.
When the Woodrow Wilson Center first launched its Synthetic Biology Project more than 10 years ago, the opening page on its web site showed cranes, construction workers in hard hats and other signs that synthetic biology builds its own DNA according to its own preferences. How quaint it seems for classical genetic engineering to rely on organic sources of DNA. More recently, however, that image has been deleted in favor of an emphasis on building muscles with the benefit of synthetic vitamins and supplements.