Structure of DNA

DNA is an extremely long chain built from alternating sugar and phosphate units. The sugar is always deoxyribose, and it is always linked to the phosphate group in the same way, so the backbone repeats the identical sugar–phosphate linkage over and over, giving the chain a perfectly regular backbone. Each sugar in that backbone also carries a nitrogenous base — and it is the base, not the backbone, that varies from position to position along the chain.

The combination of a sugar, a phosphate and a base is called a nucleotide; the combination of just the base and the sugar (without the phosphate) is called a nucleoside. In a nucleoside, the base is attached to the sugar's first carbon (C-1); when a phosphate group is further attached at the sugar's fifth carbon (C-5), the result is a complete nucleotide. Four kinds of nucleoside — and so four kinds of nucleotide — occur in DNA, one for each of its four bases.

A single strand of DNA is built from many thousands of such nucleotides, linked one above the other: the phosphate group attached to the C-5 of one nucleotide's sugar forms a phosphodiester bond with the C-3 of the sugar of the nucleotide immediately above it, building up the sugar–phosphate backbone. …