The first description of the DNA (deoxyribonucleic acid)chain was done by Watson and Crick in 1953, the nucleotides that constitute the DNA are four: cytosine (C), guanine (G), timine (T) and adenine (A). When doing a model is essential to remember that cytosine ONLY joins guanine and timine only link adenine, this is due to the size and chemical properties of each molecule. Later, another related and similar and molecule was discovered, the ribonucleic acid or RNA, which also is constructed by nucleotides.

In both cases, the nucleotides are compound of 3 main components: a nitrogen base, a pentose and a phosphate. When the molecule lacks the phosphate group, is called nucleoside. Depending on the chain ( DNA or RNA) the nitrogen base derives from purine (adenine and guanine) or pyrimidine (cytosine or timine) just in the RNA instead of timine is uracil (U)

The union is established between adjacent molecules through the phosphate, while among each other through the nitrogen base-remember C=G and T or U=A. The core of each molecule is the pentose. Therefore the best representation of the molecule is attached

References

Watson, J. D., & Crick, F. H. (1953, January). The structure of DNA. In Cold Spring Harbor symposia on quantitative biology (Vol. 18, pp. 123-131). Cold Spring Harbor Laboratory Press.