Nucleoside

A nucleoside is a molecule formed when a nitrogenous base is covalently attached to a five-carbon sugar, either ribose or deoxyribose, and it contains no phosphate group. The bases can be purines (adenine and guanine) or pyrimidines (cytosine, thymine, and uracil). In ribonucleosides the sugar is ribose; in deoxyribonucleosides it is deoxyribose.

The linkage between the base and the sugar is an N-glycosidic bond. For purines, the bond is formed between the C1' of the sugar and the N9 atom of the base; for pyrimidines, it is formed with the N1 atom. This configuration yields a family of nucleosides such as adenosine, guanosine, cytidine, uridine, and thymidine (the latter being the deoxyribonucleoside corresponding to thymine).

Nucleosides differ from nucleotides, which are nucleosides that carry one to three phosphate groups on the 5' hydroxyl of the sugar. Nucleosides serve as precursors in nucleotide biosynthesis and can be phosphorylated enzymatically to form nucleotides. They are also involved in salvage pathways that recycle bases and ribose/deoxyribose units to regenerate nucleotides.

Biological roles vary by identity; for example, adenosine acts as a signaling molecule in the extracellular space, while many nucleoside analogs are used as antiviral or anticancer drugs. Common nucleosides include adenosine, guanosine, cytidine, uridine, and thymidine (deoxyribonucleoside). These compounds provide the foundational building blocks for RNA and DNA and participate in cellular metabolism and regulation.