double helix

Picture a ladder that someone has grabbed at both ends and twisted, so the whole thing spirals like a corkscrew. That twisted ladder is the double helix: the shape DNA takes when its two strands wrap around a common axis. The image, made famous in 1953, is now one of the most recognizable in all of science.

The two long rails of the ladder are sugar-phosphate backbones, one from each strand. The rungs are pairs of nitrogenous bases reaching in from opposite sides and meeting in the middle, held together by hydrogen bonds. Adenine always pairs with thymine, and guanine always pairs with cytosine, so the two strands carry complementary information.

The helix described by James Watson and Francis Crick — building on X-ray data from Rosalind Franklin and Maurice Wilkins — is the common right-handed B-form. Its tidy, predictable geometry immediately suggested how DNA could be copied: pull the two strands apart, and each can act as a template to rebuild its partner.

DNA can adopt other shapes under different conditions (such as the A-form and the left-handed Z-form), so the classic double helix is the dominant form, not the only one. Still, for understanding heredity, the B-form double helix is the picture to keep in mind.