DNA: deoxyribonucleic acid, the basic unit of heredity.  The sequence of nucleotides in DNA is the code in genes that control the characteristics and activity of cells. There are four kinds of nucleotides in DNA that differ in their nitrogen base. The nucleotides in DNA are bonded so the sugar of one is attached to the phosphate group of another. This creates a chain with the nitrogenous bases as side groups of the chain. The sequence in which the four nucleotides occur is essentially constant in DNA molecules of the same species, but differs slightly from species to species. The chains exist in pairs with the nitrogen bases and hydrogen bonds linking them.