Nucleoside

Nucleosides are glycosylamines consisting of a nucleobase (often referred to simply base) bound to a ribose or deoxyribose sugar. Examples of these include cytidine, uridine, adenosine, guanosine, thymidine and inosine.

Nucleosides can be phosphorylated by specific kinases in the cell on the sugar's primary alcohol group (-CH2−OH), producing nucleotides, which are the molecular building blocks of DNA and RNA.

Nucleosides can be produced by de novo synthesis pathways, particularly in the liver, but they are more abundantly supplied via ingestion and digestion of nucleic acids in the diet, 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:

* in the lumen of the digestive system by nucleosidases into nucleobases and ribose or deoxyribose.
* inside the cell into nitrogenous bases, and ribose-1-phosphate or deoxyribose-1-phosphate.

In medicine several nucleoside analogues are used as antiviral or anticancer agents. The viral polymerase incorporates these compounds with non-canonical bases. These compounds are activated in the cells by being converted into nucleotides, they are administered as nucleosides 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 include LNA, morpholino, PNA.

In sequencing dideoxynucleotides are used. These nucleotides possess 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.