Direct Structural Information
Franklin and I convinced ourselves that we had so much direct structural information in the intensity and position of the diffracted beams, nearly a 100, that we should calculate a Patterson Function from the data. This would show helical or asymmetric chains without the need to build models.
At this time we were summoned to Cambridge to see a proposed structural model that Watson and Crick had built. This showed the Na/P ionic atoms forming a central core and the sugar rings coming out to the bases on the outside. Franklin demolished this suggestion with gusto! Mainly because we knew the Na/P complex must be on the outside of the structure because the water went in and out so easily. As Crick remarked later, it was the only time he had seen Watson at a loss for words!
With hindsight it is easy to see that the structure they had built was so rigid that it must be wrong. It is know that there are 3 billion base pairs in the human genome. At 34Å thick, this would mean a length of 102 cm, thus requiring great flexibility to allow folding of the structure to fit into the cell nucleus.
So we returned to Kings and started with renewed vigor on our intensity measurements, correction factors, and Patterson function calculations. For us, model building was out. Bragg forbade Watson and Crick from any further model building. The DNA problem, it was agreed, belonged to Kings!
However some months later, on learning that Pauling was actively thinking of DNA structure, Bragg lifted his ban on model building, and Watson and Crick began building models again, this time using our data as to helical radius 10Å, and to the number of nucleotides along the fiber axis per repeat interval, 10 in 34Å, and the space group of structure A, Monoclinic C2. This permitted Crick to rightly postulate two chains per lattice point related by a diad axis, i.e., one going up and one going down.
When they added to this data, the specific pairing of the bases all fell into place. We were summoned once more to Cambridge. This time we were convinced. It was so elegant, it must be right. On our return to London Franklin and I examined our Patterson data and everywhere we looked we could see indications of a double helix.
I think it is very sad that Franklin was never able to see the exponential mushrooming of molecular biology resulting from her work. I would love to have heard her opinion of the current controversy on the cloning of stem cells.
The laws of the physical universe will always apply, and it is only our interpretation and use of the data that may be wrong. However, when experimenting with living material, even the protocol of the experiment may be questioned.
I believe Franklin would have advocated a global approach to the regulation of such work. We are now facing direct manipulation of the evolution of our species and the creation of new transgenic life forms. A problem probably more important than global warming.
Do you think Rosalind Franklin has gotten enough credit for her role in discovering the double-helix structure of DNA?
Who’s Rosalind Franklin?