DNA双螺旋

Deoxyribonucleic acid, or DNA, consists of two polynucleotide chains. Each one is made up of four types of nucleotide subunits, which consist of a single phosphate group and a nitrogen-containing base, adenine, A, cytosine, C, guanine, G, or thymine, T, attached to a sugar deoxyribose. This arrangement creates the sugar-phosphate backbone of the DNA structure.

On one strand, the sugar is oriented with the hydroxyl group exposed at the third carbon atom, which is ready to covalently link to another sugar molecule in the chain at the fifth carbon atom via phosphodiester bonds.

In the opposing antiparallel strand, each base is uniquely paired, C with G and A with T, thanks to differences in the number of hydrogen bonds that form between them. 

Such chemical polarity ensures efficient packing, winding the two strands around each other to form a 3-D double helix with ten base pairs every full turn.