May 16, 2018, by Brigitte Nerlich
The vertical rod in the center of the DNA molecule
A while ago I read this tweet by Kindra Crick: “Odile Crick, my grandmother, drew the first published diagram of #DNA It was 65 years ago #onthisday, as a diagram in the first of the three papers published in @nature on the structure of DNA. (Watson & Crick, Wilkins et al, Franklin & Gosling) #SciArt ✨ #Scicomm #DNADay”. I posted the tweet on a ‘list’ I keep (scireps) and Chris Toumey, a long-standing science visualization friend, emailed back and told me there was more to be told about this. So here is his story.
Guest post by Chris Toumey (Toumey@mailbox.sc.edu)
When James Watson and Francis Crick published their description of the structure of DNA (Nature, 25 April 1953), their paper included a drawing of the molecule, complete with the sugar-phosphate strands of the double helix, and the four base pairs connecting the two strands to each other.
It has been said that the artist who created the drawing was Odile Crick, wife of Francis; she was trained in visual art, and her talent was most useful in this case. Recently her granddaughter, Kindra Crick, confirmed via Twitter that Odile was indeed the artist.
When we think of the many hundreds or thousands of visual images of DNA that we have seen over many years, there is something unique about Odile Crick’s drawing: the double helix has a vertical rod running up the center of the structure. Today no one believes that the DNA molecule has any such feature. Why is it in that drawing? (And in the 1953 drawing by Francis Crick himself in his letter to his son)
I think I know why. There is a famous photo of Watson and Crick admiring their three-dimensional model. Actually there are two shots of that scene, with minor differences between them. But in both photos, Francis Crick sits in the lower left-hand corner and looks up to his left to observe the model. James Watson stands on the right, holding a short pointer to draw the viewer’s attention to a part of the model. By the way, in the background one can see a drawing, presumable by Odile, pinned to the wall (see Martin Kemp, 2012).
The model was made of metal parts. And it has – guess what? – a vertical rod running up the center. This probably means that the model could not support itself with the metal parts of the double helix. It appears that the base pairs are clamped to the vertical rod, and then the two sides of the double helix are attached to the base pairs. Without the central rod, the three-dimensional model could not have stood vertically.
One can construct a model of DNA which lies horizontal and needs no central rod. And today there are probably some vertical three-dimensional models which need no such feature. But remember that DNA itself is neither vertical nor horizontal as it exists within a genome. A vertical model, whether in metal or plastic or a drawing, makes it easier to see the parts of the structure of DNA. So be it.
I suspect that the vertical rod disappeared from models and drawings of DNA not long after the paper of 25 April 1953. If a model or a drawing included it today, tens of thousands of scientists would object that DNA includes no such thing.
In April 1953, the metal three-dimensional model needed the vertical rod, and the drawing was faithful to the model. Let this remind us that it is often difficult, and sometimes impossible, to perfectly depict a natural object in models and drawings. The 1953 model was scientifically elegant, and the drawing by Odile Crick was truly valuable, but our visual knowledge of the structure of DNA was imperfect in both cases. Not because James Watson and Francis Crick were wrong, and not because Odile Crick was unable to draw the model. Instead, the vertical rod in the center of the molecule is an artefact of the need to support the three-dimensional model.
PS: Kindra Crick just tweeted a different interpretation: “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 stands run in opposite directions.” Interesting!
Image: Piece of the world’s longest DNA model, Wikimedia
There’s no mystery here. In the caption of the figure in the original paper, it states, “The vertical line marks the fibre axis”. That to me makes it clear that the line does not represent anything physical and is simply a guide to the underlying symmetry