A-T Base Pair. Computer artwork of an A-T (adenine-thymine) base pair. These are nucleic acids which bond to each other in a DNA chain. The atoms shown as cylinders are colour coded: carbon (green), nitrogen (blue), hydrogen (white) and oxygen (red). Adenine is at lower right and thymine is at upper left. A-T is one of the two base pairs found in DNA (along with C-G (cytosine- guanine)). The bases in DNA are always present in these pairs only. The positioning of these base pairs along the DNA chain is what is known as the genetic code. It is this which gives organisms their individual characteristics.