Some of you Twitter monsters like myself may have cheered with more of less enthusiasm the change in Twitter policy that the limit of characters per tweet is now doubled to 280 characters as opposed to the previous 140.
To test how effective this change could be for science, I decided to conduct the experiment of tweeting the 1953 structure of DNA paper published in Nature by Watson and Crick. The only rule I imposed myself was that the chunks I tweeted had to be paragraphs or complete sentences. I only tweeted the main text body. In total, it took me 28 tweets, which I now reference below for your perusal.
Probably this is the first time this landmark paper has been ever shared via this medium.
(1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other.
(2) Some of the van der Waals distances appear to be too small. Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds.
This structure as described is rather ill-defined, and for this reason we shall not comment on it. We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid. This structure has two helical chains each coiled round the same axis (see diagram).
We have made the usual chemical assumptions. namely, that each chain consists of phosphate diester groups joining ß-D-deoxyribofuranose residues with 3’,5’ linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis.
Both chains follow righthanded helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions. Each chain loosely resembles Furberg’s2 model No. 1; that is, the bases are on the inside of the helix and the phosphates on the outside.
The configuration of the sugar and the atoms near it is close to Furberg’s standard configuration’, the sugar being roughly perpendicular to the attached base. There is a residue on each chain every 3-4 A. in the z-direction.
One of the pair must be a purine and the other a pyrimidine for bonding to occur. The hydrogen bonds are made as follows: purine position 1 to pyrimidine position 1; purine position 6 to pyrimidine position 6.
If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms (that is, with the keto rather than the enol configurations) it is found that only specific pairs of bases can bond together.
These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine). In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine.
The sequence of bases on a single chain, does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain, is given, then the sequence on the other chain is automatically determined.
Some of these are given in time following, communications. We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereo-chemical arguments.
We have also been stimulated by a knowledge of the general nature of the unpublished experimental results and ideas of Dr. M. H. F. Wilkins, Dr. R. E. Franklin and their co-workers at King’s College, London.
So, here we go! The whole body text of Watson & Crick’s paper in 28 tweets.
Which one is your favourite? Mine Tweet #24.