DNA double helix structure. Computer artwork of the atomic structure of a DNA (deoxyribonucleic acid) double helix. This model shows atoms (balls) and bonds (sticks). The backbone of the double helix is composed of two twisting sugar-phosphate strands (blue/green and pink/green). Nucleotide bases on the strands bond in complementary pairs (diagonal lines) 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 double nature of the helix structure allows the replication of DNA and hence the replication of cells and whole organisms.