nucleosides

Nucleosides are basic molecular units of nucleic acids consisting of a nitrogenous base attached to a five-carbon sugar. The sugar is ribose in RNA and deoxyribose in DNA. Nucleosides lack phosphate groups, distinguishing them from nucleotides, which are nucleosides with one, two, or three phosphate groups.

The nitrogenous bases in nucleosides are divided into purines (adenine and guanine) and pyrimidines (cytosine, uracil in RNA, and thymine in DNA). Common nucleosides include adenosine, guanosine, cytidine, uridine, and thymidine. In chemical terms, the glycosidic bond linking the base to the sugar is formed at N9 for purines and N1 for pyrimidines.

Nucleosides are precursors for nucleotides and nucleic acids. They can be synthesized de novo or produced via salvage pathways that recycle free bases. Salvage reactions typically involve phosphoribosyltransferases, such as HGPRT, which attach a ribose phosphate to the base. Nucleosides can be phosphorylated by specific kinases to form monophosphates, diphosphates, and triphosphates, which enter metabolic pathways for nucleic acid synthesis.

In biology, nucleosides serve as components of RNA and DNA and occur as modified or special nucleosides in tRNA and rRNA, reflecting functional diversity. They also have signaling roles; for example, adenosine acts as a local signaling molecule in tissues. In medicine, many nucleoside analogs are used as antiviral and anticancer drugs; these compounds typically disrupt nucleic acid synthesis or function by interfering with polymerases or causing premature termination of chains.

Overall, nucleosides link the genetic information storage system to cellular metabolism and signaling, bridging genetics, biochemistry, and pharmacology.