Other scientists in the field have already put forth several models for the structure of deoxyribose nucleic acid (DNA), but there are errors in their models. Some errors are the lack of hydrogen bonding and the repulsion of the phosphate groups.
Watson and Crick propose a model for DNA structure.
In the model, DNA consists of two chains coiled in a helix around one axis. The adjacent phosphates and sugars are connected by 3’,5’ linkages. The sequences on each chain run in opposite directions of each other. The bases are on the inside and the phosphates are on the outside of the helix. The helix turns at precise and uniform angles. The distances between nucleotides are uniform as well. Less water content in the molecule can make DNA more compact.
Two base pairs on different chains that lie side by side form hydrogen bonds. They bind in a direction perpendicular to the long axis. One of the bases in the pair must be a purine, and the other base must be a pyrimidine. Thus, only certain base pairs can be together. Adenine (purine) must pair with thymine (pyrimidine). Guanine (purine) must pair with cytosine (pyrimidine). Along one chain, the nucleotides can appear in any order. In this case, the nucleotide order on the other chain will be automatically determined due to the specific base pairing.
If ribose sugar replaces deoxyribose in the backbone to form RNA, a helix would not form. The extra oxygen atom instead of an -OH would create too much repulsion. The specific base pairing suggests a mechanism in which genetic material can be copied during DNA replication.