Genetic research. Computer artwork of part of a DNA molecule in a conical flask. DNA (deoxyribonucleic acid) is a double helix molecule that carries the genetic code, which controls and transmits an organism's hereditary traits. DNA is made up of two strands of sugar-phosphate groups. The strands twist to form the double helix shape, and are cross-linked by complementary pairs of nucleotide bases (horizontal links). There are four types of nucleotide base: adenine, guanine, cytosine and thymine. The sequence of base pairs, arranged in discrete segments known as genes, determines the genetic code.