DNA molecule. Animation of a space-filled model of the helical structure of the molecule DNA (deoxyribonucleic acid). Atoms are represented as spheres. The backbone of the double helix is composed of two twisting sugar-phosphate strands. Nucleotide bases on the strands bond in complementary pairs to hold the double helix molecule together. It is the sequence of the four types of base along a strand that is the genetic code that controls the functioning and growth of every living organism. The text labels include the names of three of the four nucleotide bases: guanine (G), cytosine (C) and thymine (T). The fourth is adenine (A).