data Franklin proposed a double helix structure with precise measurements for

Data franklin proposed a double helix structure with

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data, Franklin proposed a double-helix structure with precise measurements for the diameter, the separation between each of the coaxial fibers along the fiber axis direction, and the pitch of the helix (3). The resolution of the B-form photograph #51 shown allowed Franklin to determine that each turn of the helix in the B form is 34 Å long and contains 10 base pairs separated by 3.4 Å each (3), in accordance with
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Rosalind Franklin and the Double Helix - Physics Today - 3 less precise data obtained by William Astbury and Florence Bell in 1938. (4) Wilkins, photographing living sperm cells in 1952, obtained an X-shaped B-form diffraction pattern similar to Franklin's. Her photograph, though, showed much more detail. Additional contributions The cylindrical Patterson map calculations that Franklin applied to the A-form of DNA were the first such calculations applied to any molecule. They confirmed her suggestions that the hydrophilic sugar phosphates form the external backbone of the DNA molecule and that the hydrophobic base pairs are protected inside that backbone from the cell's aqueous environment. The calculations also allowed her to deduce that the A-form helix has two antiparallel chains. With Gosling, Franklin provided details of the physical distortion accompanying the dehydration transformation from B-form to A-form DNA (5). She also showed that the bases of the A form are tilted and curved slightly, and that 11 pairs of bases are compacted within a repeat distance of 28.1 Å. In her section of the 1952 King's Medical Research Council (MRC) report, Franklin gave quantitative measurements for the interphosphate distances and discussed the external placement of the phosphates. Her presentation was instrumental in getting Watson and Crick to abandon their earlier attempts at placing the bases on the outside of their model. Initially, they (and Linus Pauling, too) mistakenly thought that the bases would have to be externally accessible in order to pass genetic information. In May 1952, Franklin presented her clearest evidence of the helical backbone, with her diffraction photograph #51. Although she did not yet realize how the nitrogenous bases are paired or that the helical backbone rails of B-form DNA are antiparallel, her notebook entries starting in January 1951 clearly show that she was making significant progress toward solving those two final aspects of DNA structure. After reading an article by June Broomhead (6), and studying other related papers, she had used the keto configuration for at least three of the four bases. She was aware both of Jerry Donohue's work concerning tautomeric forms of bases and of Chargaff's work. Astbury and Bell's earlier, less clear diffraction photographs and later data of Wilkins suggested some of the data that Franklin derived from her photograph #51. But Franklin's results were much more precise than the Astbury and Bell data, which showed neither an X pattern nor layer lines. Astbury and Bell themselves described their results as "still rather obscure." After Oxford crystallographer Dorothy Hodgkin
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