= Nucleic acid double helix
{wiki}
A <nucleic acid double helix> consists of two <nucleic acid> strands winding around a common axis. In the familiar <DNA> arrangement, <sugar>–<phosphate> backbones lie outside and paired <nucleobases> lie inside, stacked along the axis. The strands are <antiparallel nucleic acid strands>. The flattened <base pairing> ladder above displays partners and direction; winding that ladder into a <helix> adds spatial organization. <RNA> can also form double-helical paired regions, so the geometry is not exclusive to <DNA>. See https://www.ncbi.nlm.nih.gov/books/NBK26821/[Alberts et al., Molecular Biology of the Cell: DNA structure].
The useful surprise is that a structure explains a copying principle. Complementary <base pairs> allow each <DNA> strand to specify the other's <nucleic acid sequence>; separate strands can each act as templates in <DNA replication>. This is a structural constraint, while the <enzymes> and stages of <DNA replication> supply the actual process. A <nucleic acid sequence> specifies letter order without, by itself, giving every detail of the three-dimensional <helix>.
On 25 April 1953, James Watson and Francis Crick published a model with two oppositely directed chains, outside <phosphate> groups, and specific <base pairs>. Their short paper explicitly connected this pairing to a possible copying mechanism, while describing the model as requiring further experimental tests. See the original https://doi.org/10.1038/171737a0[Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid, Nature 171, 737–738]{ref}, with an accessible https://inters.org/files/watsoncrick1953.pdf[reprint of the original paper]{ref}.
Rosalind Franklin and Raymond Gosling's paper in the same issue had a different role: it presented <X-ray> <diffraction> evidence from <DNA> fibres and its structural interpretation, including the helical character of the wetter B form and evidence for <phosphate> groups on the outside. They distinguished forms produced at different water contents and assessed compatibility with the Watson–Crick model. Read their original https://doi.org/10.1038/171740a0[Molecular Configuration in Sodium Thymonucleate, Nature 171, 740–741 (1953)]{ref}. The model and the experimental analysis are distinct contributions; a <diffraction> pattern is evidence constraining a <molecular geometry>, rather than a direct picture of letter-by-letter <base pairing>.
Back to article page
