Nucleoside- Definition, Types, Structure, Functions


A nucleoside is a type of biomolecule that consists of two components: a pentose sugar and a nitrogenous base. The pentose sugar can be either ribose or deoxyribose, depending on whether the nucleoside is part of RNA or DNA, respectively. The nitrogenous base can be one of the four main bases found in nucleic acids: adenine, guanine, cytosine, or thymine (in DNA) or uracil (in RNA). Alternatively, the nitrogenous base can be a modified or minor base that occurs in some nucleic acids.

The pentose sugar and the nitrogenous base are linked by a glycosidic bond, which is a covalent bond formed between the anomeric carbon atom of the sugar (C1`) and a nitrogen atom of the base (N9 for purines and N1 for pyrimidines). The glycosidic bond determines the orientation of the base relative to the sugar, which can be either syn or anti. In most nucleosides, the anti conformation is preferred, as it allows for more stable stacking interactions between adjacent bases in nucleic acids.

A nucleoside can be distinguished from a nucleotide by the absence of a phosphate group attached to the sugar. A nucleotide is formed when one or more phosphate groups are added to the hydroxyl group at the C5` position of the sugar. Nucleotides are the building blocks of nucleic acids, such as DNA and RNA, which store and transmit genetic information in living cells.

Nucleosides have various functions in biological systems. They serve as precursors for nucleotides, as well as signaling molecules that regulate cellular processes. Some nucleosides also have medical applications as antiviral or anticancer drugs, as they can interfere with the synthesis or function of nucleic acids in pathogens or tumor cells. Nucleosides can also be transported across membranes by specialized proteins called nucleoside transporters, which facilitate their uptake or efflux in different tissues.

In summary, a nucleoside is a molecule composed of a pentose sugar and a nitrogenous base, linked by a glycosidic bond. It differs from a nucleotide by lacking a phosphate group. Nucleosides are involved in various biological roles, such as precursors for nucleotides, signaling molecules, and therapeutic agents.