Nucleoside

Overview
Nucleosides are glycosylamines made by attaching a nucleobase (often referred to simply as base) to a ribose or deoxyribose ring. Examples of these include cytidine, uridine, adenosine, guanosine, thymidine and inosine. In short, a nucleoside is a base linked to sugar.

Nucleosides can be phosphorylated by specific kinases in the cell, producing nucleotides, which are the molecular building blocks of DNA and RNA.

Nucleosides are produced as the second step in nucleic acid digestion, whereby nucleotidases break down nucleotides (such as the thymine nucleotide) into nucleosides (such as thymidine) and phosphate. The nucleosides, in turn, are subsequently broken down Nucleosides can be produced by combining nucleobases with deoxyribose rings as well.
 * in the lumen of the digestive system by nucleosidases into nitrogenous bases and ribose (or deoxyribose), and
 * inside the cell by nucleoside phosphorylases into nitrogenous bases, and ribose-1-phosphate (or deoxyribose-1-phosphate).

Nucleosides differ from nucleotides by having a hydroxyl group attached to carbon number 5 (the one that isn't in the ring) of the ribose, rather than one or more phosphate groups.

In medicine several nucleoside analogues are used as antiviral or anticancer agents. The viral polymerase incorporates these compounds with non-canon bases. These compounds are activated in the cells by being converted into nucleotides, they are administered as nuclosides since charged nucleotides cannot easily cross cell membranes.

In molecular biology several analogues of the sugar back bone exist. Due to the low stability of RNA, which is prone to hydrolysis, several more stable alternative nucleoside/nucleotide analogues are used which correctly bind to RNA. This is achieved by using a different backbone sugar. These analogues includ LNA, morpholino, PNA.

In sequencing dideoxynucleotides are used. These nucleotides posses a non-canon sugar, dideoxyribose which lacks 3' hydroxyl group (which accepts the phosphate) and therefore cannot bond with the next base, terminating the chain as DNA polymerases mistake it for a regular deoxyribonucleotide.

