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.
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