What is DNA?
Strands of DNA are made up of four different chemical building blocks, or bases - adenine, cytosine, thymine and guanine. These are the 'letters' of the 'digital' DNA code, usually labelled A, C, T and G. The bases are attached to a chemical backbone to make a strand of DNA. Two strands wind around each other to make a double helix, which looks like a spiral staircase. Each 'step' is a base pair: every A is paired with a T, and every C with a G.
