Organic substance, found in all living cells, in which the hereditary information is stored and from which it can be transferred. Nucleic acid molecules are long chains that generally occur in combination with
proteins. The two chief types are
DNA (deoxyribonucleic acid), found mainly in cell nuclei, and
RNA (
ribonucleic acid), found mostly in
cytoplasm. Each nucleic acid chain is composed of subunits called nucleotides, each containing a sugar, a phosphate group, and one of four bases: adenine (symbolized A), guanine (G), cytosine (C) and thymine (T).
RNA contains the sugar ribose instead of deoxyribose and the base uracil (U) instead of thymine. The specific sequences of nucleotides constitute the cell's genetic information: Each three-nucleotide
DNA sequence specifies one particular amino acid. The long sequences of
DNA nucleotides thus correspond to the sequences of
amino acids in the cell's
proteins. In order to be expressed as protein, the genetic information is carried to the protein-synthesizing machinery of the cell, usually in the cell
cytoplasm. Forms of
RNA mediate this process.
DNA not only provides information, but also specifies its own exact replication. The cell replicates its
DNA by making a complementary copy of its exact nucleotide sequence: T for every A, C for every G, G for every C, A for every T. Although the triplet nucleotide code seems to be universal, the actual sequences of the nucleotides vary according to the species and individual. See also
Gene;
Genetic Engineering;
Mutation.