DNA bonding. Molecular model of a molecule of DNA with an A-T base pair highlighted. A-T refers to the bases adenine and thymine. The atoms are colour-coded in the highlighted section: carbon (grey), hydrogen (white), oxygen (red), nitrogen (dark blue) and phosphorous (beige). DNA (deoxyribonucleic acid) is a molecule which consists of two strands coiled round each other, linked by the interactions between four bases. Adenine and thymine always bind to each other, as do cytosine and guanine. The sequence in which the bases appear along the strands makes up the genetic code, which determines the characteristics of all living things.