Rosalind Franklin, a chemical physicist (1920–1958), used x‐ray diffraction to determine the structure of DNA. What exactly could she read out from her x‐ray pattern, shown in Fig. 1?1 In lecture notes dated November 1951, R. Franklin wrote the following: “The results suggest a helical structure (which must be very closely packed) containing 2, 3 or 4 co‐axial nucleic acid chains per helical unit, and having the phosphate groups near the outside.”2 This was 16 months before J. D. Watson and F. Crick published their description of DNA, which was based on R. Franklin's x‐ray photos. How they gained access to her x‐ray photos is a fascinating tale of clashing personalities and male chauvinism.2,3
REFERENCES
1.
R.
Franklin
and R. G.
Gosling
, “Molecular configuration in sodium thymonucleate
,” Nature
171
,740
–741
(April 25, 1953
).2.
A. Sayre, Rosalind Franklin and DNA (Norton & Company, 2000).
3.
J. D. Watson, The Double Helix: A Personal Account of the Discovery of the Structure of DNA (Athenium, New York, 1968; Weidenfeld and Nicholson, London, 1981).
4.
D. T.
Crouse
, “X‐ray diffraction and the discovery of the structure of DNA
, “J. Chem. Educ.
84
(5
), 803
–809
(2007
).5.
A.
Lucas
, Ph.
Lambin
, R.
Mairesse
and M.
Mathot
, “Revealing the backbone structure of B‐DNA from laser optical simulations of its x‐ray diffraction diagram
,” J. Chem. Educ.
76
(3
), 378
(1999
).6.
We used a red laser pointer and also a 632.8‐nm red laser, but green laser pointers also can be used (available for $8–12 at www.amazon.com or www.ebay.com)
7.
E. Hecht, Optics, 2nd ed. (Addison‐Wesley, 1974). In order to convince students that this is the case, it is useful to show first the diffraction pattern of a slit and then the pattern of a wire that has a similar thickness as the slit.
8.
A similar setup for use in spatial filtering is described in
A.
Eisenkraft
, “A closer look at diffraction: Experiments in spatial filtering
,” Phys. Teach.
15
, 199
–211
(April 1977
).9.
We do not know the details of Franklin's setup. The x‐ray wavelength and the distance to the film are based on typical data for x‐ray. We think this is close enough to the actual apparatus to illustrate the point.
10.
M. H. F.
Wilkins
, A. R.
Stokes
, and H. R.
Wilson
, “Molecular structure of deoxypentose nucleic acids
,” Nature
171
, 738
–740
(April 25, 1953
).11.
F. Wang, www.maplesoft.com/applications/view.aspx?SID=4902&view=html.
12.
C.
Kittel
, “X‐ray diffraction from helices: Structure of DNA
,” Am. J. Phys.
36
, 610
–616
(July 1968
).13.
When you unscrew the retractable pens from various brands and take out the helical springs, you will discover that the springs vary in pitch, thickness of wire, and diameter. Different sized springs can be used to prevent all groups having the same values. In addition, some pen springs are right handed, while others are left handed. Although the handedness of the helix does not play a role in our simple analysis of the diffraction pattern, it is good to point out the difference, and to note that the DNA helix is right handed, although left handed can also exist in vivo. See for example
Jaworski
et al., “Left‐handed DNA in vivo
,” Sci.
6
, 773
–777
(Nov. 1987
).
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2011
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