Nucleoside comprises a nucleobase and a five-carbon sugar (ribose or 2'-deoxyribose). In a nucleoside, the anomeric carbon is linked through a glycosidic bond to the N9 of a purine or the N1 of a pyrimidine.
Nucleotide is a compound consisting of a nucleoside linked to a phosphate group. Nucleotide is the molecular building block of nucleic acids, RNA and DNA, both of which are essential biomolecules within all life-forms on Earth. The four bases used in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T). In RNA, the base uracil (U) takes the place of thymine.
Phosphoramidite is a synthetic nucleoside derivative containing a phosphite ester group. Phosphoramidite is the building block used in RNA and DNA synthesis. These molecules permit the sequential addition of new bases to the chain in a highly efficient reaction. To avoid undesired side reactions, reactive hydroxy and exocyclic amino groups present in the nucleosides are appropriately protected.