Nucleotides are the phosphoric acid esters of nucleosides.

In ribose moiety of a ribonucleoside, phosphorylation is possible only at three position (C2′, C3′, and C5′) since C1′ and C4′ are involved in the furanose ring formation. In deoxyribosenucleoside, only two positions (C3′ and C5′) are available for phosphorylation.