DNA helix atomic 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. 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. DNA is found in the nuclei of cells, the microscopic structural units of living organisms.