DNA Structure Puzzle #5 - X-Ray Diffraction from DNA Fibres
clivedelmonte at c-i-delmonte.freeserve.co.uk
Sun Oct 31 03:41:11 EST 1999
Rosalind Franklin's Deduction from X-Ray Diffraction from DNA Fibres
Aaron Klug (11, page 373) has reported that Rosalind Franklin was working on
a sugar-phosphate helical diameter of 1.1 nm, deduced from her Pattersons,
at the time that Watson & Crick formulated their idea of a double helix.
Franklin's value of 1.1 nm resonates with the values of 1.2 nm to 1.3 nm
outer helical diameters recorded, or deducible, for example, in Puzzle
#1(ref. 1 and other AFM &STM refs.), Puzzle 2 (ref. 4), Puzzle 3 (refs. 5 &
6) and Puzzle 4 (refs. 8-10).
Most importantly, and unknown to Franklin at that time,1.1 nm is also the
width of the Watson-Crick base pair. Supposing that base pairs are connected
across the two (antiparallel) helices recorded by Lee et al. (Puzzle 1, ref.
1), then the geometry requires each sugar-phosphate helix to adopt the same
diameter as the base pair width. This leads us directly to dimensions for
the duplex of about 1.2 nm x 2.2 nm.
Wilkins et al. (12) analysed the diffraction pattern of B-DNA:
"...The strong equatorial reflection....suggests that the helices have a
maximum diameter of approximately 2.0 nm....On the sixth, seventh and eighth
layer lines the maxima correspond to a helix of diameter approximately 1.2
and, further, Wilkins et al. write (13):
"...In particular, it appears that the helix, when viewed along the fibre
axis, may not be quite round in section but elliptical."
So Wilkins' group finds a helical diameter of 1.2 nm in an elliptical
section of major diameter approximately 2.0 nm, which reinforces closely
similar, elliptical values of 1.2 nm and 2.2 nm.
The dimensions found by Wilkins' group accord with a true side-by-side model
(please visit the website for the diagram).
11 The Path to the Double Helix; R. Olby: Macmillan
12 Molecular Structure of DNA; M.H.F. Wilkins, A.R. Stokes and H.R. Wilson;
Nature Vol 171 (1953) 738 - 740
13 Helical Structure of Crystalline DNA; M.H.F. Wilkins, W.E. Seeds, A.R.
Stokes & H.R. Wilson; Nature Vol 172 (1953) 759 - 762
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