What is the structure of DNA?

DNA is made up of two long chain backbones, bound together and forming a double helix shape. These backbones are made up of repeating units, each of which have the following structure:

A pentose sugar with a phosphate group bound on one side, and one of four bases bound on the other side. These bases are adenine, guanine, cystosine and thymine. These monomer units are called nucleotides, and are bound together by phosphodiester bonds between the phosphate group of one nucleotide and the OH group at the bottom of the other nucleotide. These form in a condensation reaction, so a molecule of water is produced.

The backbones join together by hydrogen bonds forming between the bases. Each base will only form hydrogen bonds and pair with one other base - guanine will pair with cytosine, adenine with thymine.