Answer: The X-shaped pattern observed in X-ray diffraction images of B-DNA corresponds to the helical structure of DNA. This characteristic X-shaped pattern, often called the "X-ray fiber diffraction pattern of B-DNA," was first described by Rosalind Franklin and Raymond Gosling in 1952.
Explanation: Horizontal Lines: The horizontal lines in the X-pattern represent the regular spacing between the bases along the length of the DNA molecule.
Vertical Lines: The vertical lines represent the helical repeat distance, which is the distance between one helical turn of the DNA and the next.
This X-shaped diffraction pattern provided crucial evidence for the helical structure of DNA. James Watson and Francis Crick, who were working on building a model of DNA, used Franklin's X-ray data (which she produced through her X-ray crystallography experiments) along with other experimental data to propose the famous double-helix structure of DNA in 1953. Franklin's data were critical in elucidating the structure of DNA, although her contributions were often overlooked initially.