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From The Collaborative International Dictionary of English v.0.48:
nucleoside \nucleoside\ n. (Biochem.) A type of molecule found in all living organisms, present mostly in chemically combined form as a component of nucleic acids, and also in smaller amounts in free form, consisting of a pentose sugar bound to a purine or pyrimidine base; two types of nucleoside, ribonucleoside and deoxyribonucleoside, are present. The most common bases present in nucleosides are adenine, cytosine, uracil, guanine, and thymine, and to a lesser extent hypoxanthine and other bases are found. The most commmon ribonucleosides composed from these bases are called adenosine, cytidine, uridine, and guanosine. The forms esterified with orthophosphoric at the 5-position of the pentose are called nucleotides. The nucleotides form the monomer units which are combined into DNA and RNA, which carry the genetic information required for reproduction in all known organisms. [PJC]