Figure 1. Structure of DNA and basepairs. Note that bond lengths are not proportional and some have been exaggerated for clarity. Broken lines represent hydrogen bonds in each basepair. (a) Chemical structure of DNA. The nomenclature for each base and its corresponding nucleoside is indicated. Atoms are numbered for one sugar, one purine base and one pyrimidine base. A single phosphodiester linkage is shown between adjacent nucleosides on each strand. Arrows highlight the antiparallel orientation of each polynucleotide strand in a duplex. The major and minor groove edges of each base pair are indicated. (b) Hoogsteen and reverse Hoogsteen base pairs of dA·dT and dG·dC.

Figure 2. Three-dimensional space-filling models of B-form, A-form and Z-form helices. Major (M) and minor (m) grooves are indicated for each double helix.

Figure 3. Three-dimensional space-filling models and hydrogen-bonding patterns for triplexes and quadruplexes. Broken lines indicate hydrogen bonds and ‘R groups’ represent the continuation of polynucleotide structure through the phosphate and sugar backbone.