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A nucleoside is a unit made up of a pentose sugar (ribose or deoxyribose) and a nitrogenous base (purine or pyrimidine), which are attatched by a ß N-glycosidic linkage (the base is above the plane of the sugar). The sugar will be either ribose (for RNA nucleosides) or deoxyribose (for DNA nucleosides). If the base is a purine, then it is attached to C-1' by N-9, and if the base is a pyrimidine then it is attached to C-1' by N-1.

There are four nucleoside units in RNA - adenosine, guanosine, cytidine and uridine. The four nucleoside units in DNA are called deoxyadenosine, deoxyguanosine, deoxycytidine and thymidine. A nucleoside can have a phosphate group attached to the C-5' through a condensation reaction, producing a nucleotide, a monomer of nucleic acid[1][2].


  1. Berg et al, Biochemistry, 6th Edition, New York: W.H. Freeman and Company, 2007
  2. Berg JM, Tymoczko JL and Stryer L, 2012, Biochemistry 7th edition, NY, W. H Freeman and Company, page 115
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