Abbreviation for the molecules that contain the biological instructions for rendering a species (a group with an ability to interbreed) of an organism uniquely different from other species. (A grouping of them make up the chromosome, a feature of the nucleus of a cell.) DNA stands for deoxyribonucleic acid and it is made of a double helix of chains of chemicals called nucleotides comprising phosphates and sugars in one of four nitrogen bases (labeled with the letters: A (adenine); T (thymine); G (guanine); and C (cystosine)).