nucleoside
Any of a class of organic compounds, including structural subunits of nucleic acids. Each consists of a molecule of a five-carbon sugar (ribose in RNA, deoxyribose in DNA) and a nitrogen-containing base, either a purine or a pyrimidine. The base uracil occurs in RNA, thymine in DNA, and adenine, guanine, and cytosine in both, as part of the nucleosides uridine, deoxythymidine, adenosine or deoxyadenosine, guanosine or deoxyguanosine, and cytidine or deoxycytidine. Nucleosides usually have a phosphate group attached, forming nucleotides. Usually obtained by decomposition of nucleic acids, nucleosides are important in physiological and medical research. Those that are not part of nucleic acids include puromycin and certain other antibiotics produced by fungi.
nucleoside [′nü·klē·ə‚sīd]
Nucleoside
a compound that consists of a nitrogenous base and a carbohydrate. A nucleoside whose carbohydrate portion is ribose is called a ribonucleoside, and a deoxyribose-containing nucleoside is called a deoxyribonucleoside. Nucleosides have the following structural formula:
In deoxyribonucleosides, X is replaced by a hydrogen atom, while in ribonucleosides, a hydroxyl group replaces X. R can be any purine or pyrimidine base.
A nucleoside can be regarded as the product of the dephos-phorylation of a nucleotide. In a nucleoside, the purine or pyrimidine base is connected to the 1′ carbon atom by a β-glycoside bond. Nucleosides are named according to their base; thus, adenosine contains adenine, guanosine—guanine, uridine—uracil, cytidine—cytosine, and thymidine—thymine. The nucleosides that contain hypoxanthine and xanthine are called inosine and xanthosine, respectively. A few other nucleosides exist, which are structurally similar to those mentioned but which differ in the particular base, carbohydrate component, or type of internal bonds.
Free nucleosides are present in small quantities in several biological substances. The significance of most nucleosides, however, is their occurrence as constituents of nucleotides and nucleic acids.
I. B. ZBARSKH
Want to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit the webmaster's page for free fun content.
| ?Page tools | ||
|---|---|---|
|