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Here's Rosalind Franklin's famous Photo 51, the X-ray diffraction image of DNA from which Watson and Crick deduced its structure:

Rosalind Franklin's Photo 51

My understanding is that it depicts a short segment of DNA shown from the side (so the axis that the strands wind around would run up and down through the center of the photo). Like this double-helix diagram from Watson and Crick's paper:

Double helix diagram

I have a lot of trouble connecting the photo to the diagram.

What am I actually seeing in the photo? There are dark blobs at the top and bottom. A diamond shape of dark lines. A concentric circle (or rounded diamond) inside the outer diamond. An X formed from two intersecting rows of 7ish mostly horizontal short lines or blobs. Which of these features correspond to what parts of the DNA molecule?

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    $\begingroup$ The interpretation of this image is not as easy as you think, but it took me less than a minute to find this. $\endgroup$ – another 'Homo sapien' Apr 26 '17 at 7:55
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    $\begingroup$ I just gave an example link. It was just to tell that its not very difficult to find such papers, seeing that @GerardoFurtado has found a couple more links ;) $\endgroup$ – another 'Homo sapien' Apr 26 '17 at 8:03
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    $\begingroup$ @GerardoFurtado Why? You digged out the relevant paper, so I think you deserve the credit for this. $\endgroup$ – Chris Apr 26 '17 at 9:02
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    $\begingroup$ Your opening statement is incorrect - "Here's Rosalind Franklin's famous Photo 51, the X-ray diffraction image of DNA from which Watson and Crick deduced its structure" - Watson and Crick did not deduce the structure from the picture alone. They were able to do so because they already had a host of information about the structure and had been working on the problem for some time. If it were a matter of simply looking at the picture, Franklin - who was far more skilled at this kind of crystallography - would doubtless have rapidly identified the structure. $\endgroup$ – Jack Aidley Apr 26 '17 at 12:56
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    $\begingroup$ Like Gerardo, I thought this was down the long axis - I also did some re-reading into the issue thanks to this question and discovered the photo was actually Ray Gosling's, not Rosalind Franklin's. I can't even confirm whether she was (formally) his supervisor; it sounds from another source (with better references than Wikipedia) that she wasn't officially eligible to supervise students because of the position she held at the time. I'll probably put together a proper question about this later when I have time. $\endgroup$ – arboviral Apr 28 '17 at 8:05
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Well, you said that you...

...have a lot of trouble connecting the photo to the diagram.

And that's quite excusable: interpreting that X-ray image is actually very complicated .

All the quotes and images in this answer, except the bulleted list further down, come from this paper (Lucas, 2008), which explains that historic picture in details.

Full disclosure: I always though that that image depicted the X-ray going longitudinally (that is, along the main axis) through the DNA. However, it goes transversely:

When filamentous macromolecules are packed in a fibre along a fixed direction, the X-ray intensities diffracted by the fibre fall on the observation screen along approximately straight and equidistant lines, the so-called layer lines, perpendicular to that direction. This important concept was introduced by Michael Polanyi in 1921 for the X-ray study of cellulose. (emphasis mine)

These are the so called layer lines (I'm keeping the original paper's legend in all images):

Fig.2 Principle of the geometrical construction of the hyperbolic layer lines in the X-ray diffraction pattern of a linear array of point scatterers (dark circles) with period P. ES: Ewald Sphere; LP: Layer Planes; LL: Layer Lines. The shaded regions are cones whose intersections by the screen generate the layer lines.

Then, still according to the paper, Crick and others started to hypothesise how would be the X-ray diffraction in a monoatomic helix:

In 1952, Cochran, Crick and Vand developed an analytical theory for X-ray diffraction by a monoatomic helix. The immediate interest of their theory was to give a transparent, analytical expression of these amplitudes at a time, in 1952, when computers, if at all available, were barely capable of a brute force calculation of the total diffraction intensity.

This is the X-ray diffraction pattern in a monoatomic helix, which is quite important to understand the famous DNA diffraction image later on:

Fig.3 Computer simulated pattern of X-ray diffraction by a phosphorus helix of period P, radius r and atomic repeat p_a. Notice the central St Andrew's cross, the Bessel oscillations along the layer lines and the diamond pattern (highlighted by broken lines) with nearly empty North and South diamonds. The structural parameters of the helix (P, r, p_a) are readable from the geometrical parameters of the pattern (2pi/P, alpha, 2pi/p_a).

Thus, with that theoretical background, we can understand our famous image:

Fig.4 The historic X-ray fibre diffraction patterns (a) of A-DNA and (b) B-DNA plotted on the same scale. The 8th layer line of pattern (a) occurs at about the 10th layer line of pattern (b) reflecting a 20% increase in the DNS helix period. The A-DNA pattern shows crystalline spots on the first few layer lines.

In this image, a X-ray diffraction image of A-DNA (left) and the more common B-DNA (right) show a periodic pattern in the layer lines.

The diffraction patterns can be interpreted as follows (source):

  • The layer line separation reveals the value of the polymer repeat period. The decrease of over 20% of the layer line spacing when going from A to B implies an increase of that much in the period: P = 2.8 nm for A-DNA and 3.4 nm for B-DNA.
  • Look at the sharp, discrete spots observed near the center of the diffraction pattern for A-DNA along the first few layer lines, these suggest the crystalline order in the fibre. In the high-humidity B-DNA pattern, these crystalline spots are absent, suggesting that the extra water molecules must have invaded the space between the DNA molecules, freeing them from being locked into crystallites.
  • The thick arcs at the top and bottom of the B-DNA pattern are found at approximately 10 layer line intervals from the center, implying that B-DNA had 10 repeating units within one period of 3.4 nm. These are produced by the scattering of X-rays by the equidistant, nearly horizontal flat bases separated by 0.34 nm. The A-DNA pattern lacks these big smears, suggesting that the bases in A-DNA are not horizontal, and the number of base pairs per helical period is closer to 11.
  • The central cross in B-DNA represents the Saint Andrew cross expected from a helical molecule. The large radius r (1 nm, indicated by the meridian angle of the cross) and the absence of intensity in the meridian diamonds indicates that the phosphate backbone is at the periphery of the helix. This cross appears to be absent in A-DNA, however, this is due to destructive interference from some of the inclined base pairs.

Finally, this is an image better relating the double strand structure of the DNA (both A and B DNAs) with the X-ray diffraction image:

P11. Optical simulation of the X-ray diffraction by an A-DNA fibre. The motif on the left is arranged on a 2-D lattice in panel P11 of the slide. The strong features on the 6 to 8th layer lines of both the X-ray picture and the simulated pattern arise from the inclined base pairs seen edge on and forming a zigzagging double slit grating. P12. Optical simulation of the X-ray diffraction by a B-DNA fibre. The strong streaks on the 10th layer lines of both the X-ray picture and the simulated pattern arise from the horizontal base pairs seen edge on.

Reference:

Lucas, A. (2008). A-DNA and B-DNA: Comparing Their Historical X-ray Fiber Diffraction Images. Journal of Chemical Education, 85(5), p.737.

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    $\begingroup$ IIRC, in The Double Helix (a controversial source to be sure), Watson claimed that the major impact of the image was that it allowed a particular angle to be measured, which allowed them to turn their idea of a double helix into an actual atomic structure (since the angle was only made possible by the bond angles of that particular molecular arrangement). Rosalind Franklin’s image was published in the same journal as Crick and Watson, though at the time she (and the editors) believed it was an independent corroboration of their structure, rather than a crucial piece in determining it. $\endgroup$ – KRyan Apr 26 '17 at 14:07
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    $\begingroup$ Terrific answer. Thank you for such a thorough and understandable explanation. $\endgroup$ – Robert Apr 26 '17 at 19:23

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