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Nucleic Acids, extremely complex molecules produced by living cells and viruses. Their name comes from their initial isolation from the nuclei of living cells. Certain nucleic acids, however, are found not in the cell nucleus but in cell cytoplasm. Nucleic acids have at least two functions: to pass on hereditary characteristics from one generation to the next, and to trigger the manufacture of specific proteins. How nucleic acids accomplish these functions is the object of some of the most intense and promising research currently under way. The nucleic acids are the fundamental substances of living things, believed by researchers to have first been formed about 3 billion years ago, when the most elementary forms of life began on earth. The origin of the so-called genetic code they carry has been accepted by researchers as being very close in time to the origin of life itself (see Evolution; Genetics). Biochemists have succeeded in deciphering the code, that is, determining how the sequence of nucleic acids dictates the structure of proteins.
The two classes of nucleic acids are the deoxyribonucleic acids (DNA) and the ribonucleic acids (RNA). The backbones of both DNA and RNA molecules are shaped like helical strands. Their molecular weights (see Molecule) are in the millions. To the backbones are connected a great number of smaller molecules (side groups) of four different types (see Amino Acids). The sequence of these molecules on the strand determines the code of the particular nucleic acid. This code, in turn, signals the cell how to reproduce either a duplicate of itself or the proteins it requires for survival.
All living cells contain the genetic material DNA. The cells of bacteria may have but one strand of DNA, but such a strand contains all the information needed by the cell in order to reproduce an identical offspring. The