DNA in space. Computer artwork of a DNA double helix in space above Earth. This may represent the possible uniqueness of Earth as a cradle of life in the universe, or the ubiquity of DNA and the related RNA in life on Earth. A molecule of DNA (deoxyribonucleic acid) consists of two helical chains coiled around the same axis, each chain consisting of alternate phosphate and sugar groups. The chains are held together in a double helix by the hydrogen bonds formed between nucleotide bases attached to the two chains. There are four bases; adenine (A), cytosine (C), guanine (G) and thymine (T). The sequence of the bases in the molecule forms the genetic code.