Double Strand Helix Structure of DNA

In the early 1950s, Rosalind Franklin and Maurice Wilkins used X-ray diffraction to unravel the structure of DNA. The DNA fibres they examined produced a characteristic diffraction pattern: the central, X-shaped pattern indicated a helical structure, while the heavy black arcs at the top and bottom of the diffraction pattern revealed the regular spacing of the stacked bases along the helix.

Working from this and other data, the structure elucidation of DNA by Watson and Crick in 1953 was a momentous event in science. They postulated a three-dimensional model of DNA consisting of two antiparallel helical DNA chains wound around the same central axis to form a right-handed double helix. In this model the hydrophilic backbones -- made of alternating deoxyribose sugar and phosphate groups -- sit on the outside of the double helix, facing the surrounding water, while the purine and pyrimidine bases of both strands are stacked on the inside of the helix, close together and oriented perpendicular to the long axis, which reduces the electrostatic repulsion between the negatively charged phosphate groups on the backbone. Because the two strands pair up with a slight offset rather than lying exactly opposite one another, the surface of the resulting duplex shows a major groove and a minor groove. …