Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid

Overview
Watson and Crick proposed a three-dimensional model for deoxyribonucleic acid (DNA) that explained how genetic information could be stored and replicated. Published in Nature in 1953, the brief report described DNA as two polynucleotide chains wound around a common axis to form a right-handed double helix. The model synthesized data from X-ray diffraction, chemical analyses, and base composition studies to present a coherent structural picture.

Structural Description
The double helix consists of two complementary polynucleotide strands with sugar-phosphate backbones on the outside and the nitrogenous bases stacked inside the helix. The strands run in opposite directions, making them antiparallel, and the bases project inward, forming a regular helical array. The geometry imposed by the sugar-phosphate backbone and base stacking produced a uniform structure with a repeating pitch and consistent major and minor grooves.

Base Pairing and Complementarity
A central feature is specific pairing between purine and pyrimidine bases: adenine pairs with thymine, and guanine pairs with cytosine, each pair stabilized by hydrogen bonds. This complementary pairing maintains a constant helix width because a purine always pairs with a pyrimidine. Chargaff's empirical findings on base ratios are naturally accommodated: the amounts of adenine and thymine are equal, and the amounts of guanine and cytosine are equal, reflecting the pairing rules.

Implications for Genetic Information and Replication
The sequence of bases along one strand encodes genetic information, with the complementary strand providing a template for accurate copying. The pairing mechanism implies a straightforward method for replication: separation of the two strands would allow each to serve as a template for assembling a new complementary strand, producing two copies each containing one old and one new strand. The authors famously noted that their specific pairing "immediately suggests a possible copying mechanism for the genetic material," linking structure to function in genetics.

Evidence and Acknowledgments
The model drew on multiple lines of evidence, including X-ray diffraction images that suggested a helical form and chemical data on nucleotide composition and connectivity. Watson and Crick acknowledged that the model depended on unpublished and published experimental results from colleagues working on DNA chemistry and X-ray analysis. They emphasized that the structure satisfied known chemical constraints and provided clarity where earlier models had failed.

Consequences and Legacy
The double helix provided a conceptual foundation for molecular biology by supplying a physical basis for heredity, mutation, and the transmission of biological information. It opened experimental pathways to investigate the mechanics of replication, transcription, and genetic coding. The concise 1953 report catalyzed rapid follow-up work to elucidate how sequences of bases specify proteins and how genetic information is faithfully transmitted across generations, reshaping biological research and enabling advances in genetics, biotechnology, and medicine.