DNA (Deoxyribonucleic Acid) is the molecule that carries genetic instructions for the growth, development, functioning, and reproduction of all known living organisms and many viruses. It is a long, double-helix structure made up of nucleotide units, which consist of a phosphate group, a sugar (deoxyribose), and a nitrogenous base. The four nitrogenous bases in DNA are Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). These bases pair specifically—A with T and C with G—forming the rungs of the DNA ladder.