deoxyribonucleic acid (DNA)
When the structure of DNA was first published by Crick and Watson in 1953, much of it was based on limited experimental data and guesswork -very good and accurate guesswork, as we now know. The experimental data hey had available came from the diffraction of DNA fibers. Although this technique uses X-rays just like crystallography does, the information obtained by fiber diffraction is much more limited. This information was obtained by Rosalind Franklin who died too young to share the Nobel Prize with Crick and Watson.
Here we see not the structure of DNA, but the structure of a fragment of DNA. Altogether this fragment contains 12 base pairs, enough to form slightly more than one turn of the DNA double helix. This structure was obtained from X-ray diffraction of crystals of the DNA fragment, so it is much more accurate than the model Crick and Watson first proposed. It does of course confirm that their initial model was correct and that DNA does indeed look as they suggested. We see the sugars, linked by phosphate groups, forming the backbone of the DNA and located on the outside of the helix. Since this exposes the highly charged phosphate groups, early models of DNA had tried to put them inside - and failed. The revolutionary idea, put forward by Crick and Watson, to put the backbone outside and the bases inside, is of course correct, but that was not at all clear 50 years ago. We also see the interior of the DNA, with the bases stacking nicely like the steps of a spiral staircase and forming hydrogen bonds between opposing bases from the two strands. Everything is just as Crick and Watson told us 50 years ago, but this time not based on incomplete fiber data and model building, but accurately measured by X-ray diffraction.
Protein Data Bank (PDB)
auther: Arno Paehler