A nucleic acid is a genetic molecule. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids. They are polynucleotides, made up of a phosphate-sugar backbone and nitrogenous bases of which there are four types: adenine, thymine (uracil in RNA), cytosine and guanine. A single nucleotide is a single 'unit' such as this: a phosphate, sugar and base combination.