DNA is a molecule with a double helix structure. This means that it is shaped like a twisted ladder. The backbone of the molecule is made up of two long chains that spiral around each other. The struts of the ladder are base pairs, which are made up of two bases joined together.
There are four bases: adenine (A), cytosine (C), guanine (G) and thymine (T). These can only join together to form two possible base pairs: adenine–thymine or cytosine–guanine.