The secondary structure of DNA: B form, or Watson and Crick's model

Two complementary dinucleotides pair their bases to form a fragment of double-stranded B-DNA:

        5' pTpC  3'
        3'  ApGp 5'

The base pairs are positioned in parallel and internal, while pentoses and phosphates (backbone) are external.

In order to have all the bases paired, while keeping the phosphodiester bonds, each base pair shifts with respect to the former one by a rotation of about 36°. To see it, click on the following buttons in order so that base pairs are added or removed one at a time:  

You can appreciate that the tenth base pair after the first one has not yet completed a full turn, while the eleventh one surpasses the turn; this means that the number of base pairs per turn is between 10 and 11. In this Watson and Crick's model, or B form of DNA, each helix turn corresponds approximately to 10.5 base pairs.

The resulting whole molecule (polynucleotide) forms a dextrorse (right-handed) double helix:

Joining the phosphorus atoms with a fictitious line , to represent the backbone, allows a better view of the helicity.

of the molecule.

While , it is easier to appreciate that there is a slight bending of the axis of the double helix: the real structure of DNA is not exactly identical to Watson and Crick's model.

Spacefilling model, or , with bases and backbone: note the two grooves, major and minor.