A nucleic acid is a long chain, or polymer, of nucleotides. Nucleic acids – vital chemical constituents of living things – are a class of complex threadlike molecules comprising two main types: the deoxyribonucleic acids (DNA), which are double-stranded, and the ribonucleic acids (RNA), which is single-stranded. DNA is found exclusively in the nucleus of the living cell, where it forms the chief material of the chromosomes. It is the DNA molecule's ability to duplicate itself (replicate) that makes cell reproduction possible; and it is DNA, by directing protein translation, that controls heredity in all organisms other than certain viruses which contain only RNA. RNA performs several tasks connected with protein synthesis, and is found through the cell.
In both DNA and RNA the backbone of the molecule is a chain of alternate phosphate and sugar groups. To each sugar group is bonded one or other of the four nitrogenous side groups, which are either purines or pyrimidines. Each unit consists of a side group, a sugar and a phosphate is called a nucleotide. DNA differs chemically from RNA in that its sugar group has one less oxygen atom (hence the prefix "deoxy-") and one of its side group, thymine, is replaced in RNA by uracil. DNA molecules are usually very much longer than RNA and may contain a million or so phosphate-sugar links.
It is the sequence in which the side groups are arranged along the DNA molecule that constitutes stored genetic information and so makes the difference between one inherited characteristic and another. This information, in the form of coded instructions for the synthesis of particular protein molecules (see genetic code), is carried outside the cell nucleus by molecules of messenger RNA, each incorporating a side-group sequence determined by DNA. Floating freely outside the nucleus are amino acids, the building blocks of proteins, and molecules of another, smaller kind of RNA, transfer RNA. Each of these RNA molecules is able to capture an amino acid molecule of a particular type and locate it in its proper place in a sequence dictated by messenger RNA.