ADN, or deoxyribonucleic acid, is the molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms and many viruses. It is composed of two long strands that coil around each other to form a double helix. Each strand is made up of smaller units called nucleotides, which contain a sugar, a phosphate group, and a nitrogenous base. The sequence of these nitrogenous bases—adenine (A), thymine (T), cytosine (C), and guanine (G)—encodes the information necessary for building proteins, which perform various functions in the body. Genes are specific segments of ADN that determine traits and characteristics, making ADN essential for heredity and biological diversity.