Photo 51

Plate 100 speaks for itself: “An X on film revealed the double helix.”

The Physics

Franklin and Gosling's X-ray photograph of B-DNA shows an X of spots, the signature of a helix, with layer lines spaced by its pitch.

P = 10 h

• P: helical pitch along the fibre axis, 3.4 nm
• h: rise per base pair, 0.34 nm
• 10: base pairs per turn, dimensionless

The X of spots means a helix. Layer lines repeat every 1/P: fibre-axis P = 3.4 nm.

Honest caveat: About 10 base pairs per turn is the fibre value; DNA in solution is closer to 10.5.

Sources

• R. E. Franklin, R. G. Gosling, Nature 171, 740 (1953).
• J. D. Watson, F. H. C. Crick, Nature 171, 737 (1953).

On the poster

Plate 100, “Photo 51”: the pixel-art plate with its slogan “An X on film revealed the double helix.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.