Any of a class of organic compounds, including structural subunits of nucleic acids. Each consists of a molecule of a five-carbon sugar (ribose in RNA, deoxyribose in DNA) and a nitrogen-containing base, either a purine or a pyrimidine. The base uracil occurs in RNA, thymine in DNA, and adenine, guanine, and cytosine in both, as part of the nucleosides uridine, deoxythymidine, adenosine or deoxyadenosine, guanosine or deoxyguanosine, and cytidine or deoxycytidine. Nucleosides usually have a phosphate group attached, forming nucleotides. Usually obtained by decomposition of nucleic acids, nucleosides are important in physiological and medical research. Those that are not part of nucleic acids include puromycin and certain other antibiotics produced by fungi.

This entry comes from Encyclopædia Britannica Concise.
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