noun, plural: nucleosides
A nucleoside is a nitrogenous base (i.e. purine or pyrimidine) that is bound to a pentose sugar ribose or deoxyribose. It is a glycoside formed from the hydrolysis of nucleic acid. A phosphate group that is covalently attached to the pentose sugar of a nucleoside would make a nucleotide.
Depending on the pentose sugar component, a nucleoside may be a ribonucleoside or a deoxyribonucleoside. A ribonucleoside is a nucleoside with a ribose sugar component. It may be in the form of adenosine, guanosine, cytidine, and uridine. A deoxyribonucleoside is a nucleoside with a deoxyribose sugar. Deoxyribonucleosides include deoxyadenosine, deoxyguanosine, deoxycytidine, and deoxythymidine.
Nucleosides may be produced de novo in the liver. They may also be available in the diet containing nucleic acids. The nucleic acids are digested into nucleotides. The nucleotides are then broken down into nucleosides by the action of nucleotidases. The nucleosides, in turn, may be broken down in the lumen of the digestive system into their subcomponents (i.e. nitrogenous base and sugar) through the action of nucleosidases.
Word origin: nucleic- ("nucleus") + -ose + -ide