Nucleosides areglycosylamines that can be thought of asnucleotides without aphosphate group. A nucleoside consists simply of anucleobase (also termed a nitrogenous base) and a five-carbon sugar (ribose or 2'-deoxyribose) whereas a nucleotide is composed of a nucleobase, a five-carbon sugar, and one or more phosphate groups. In a nucleoside, theanomeric carbon is linked through a glycosidic bond to the N9 of apurine or the N1 of apyrimidine. Nucleotides are the molecular building blocks ofDNA andRNA.
This list does not includemodified nucleobases and the corresponding nucleosides
Each chemical has a short symbol, useful when the chemical family is clear from the context, and a longer symbol, if further disambiguation is needed. For example, long nucleobase sequences in genomes are usually described by CATG symbols, not Cyt-Ade-Thy-Gua (seeNucleic acid sequence § Notation).
| Nitrogenous base | Ribonucleoside | Deoxyribonucleoside |
|---|---|---|
 adenine symbolA orAde | adenosine symbolA orAdo | deoxyadenosine symboldA ordAdo |
 guanine symbolG orGua |  guanosine symbolG orGuo |  deoxyguanosine symboldG ordGuo |
 thymine (5-methyluracil) symbolT orThy |  5-methyluridine (ribothymidine) symbolm⁵U |  thymidine (deoxythymidine) symboldT ordThd (dated:T orThd) |
 uracil symbolU orUra |  uridine symbolU orUrd |  deoxyuridine symboldU ordUrd |
 cytosine symbolC orCyt |  cytidine symbolC orCyd |  deoxycytidine symboldC ordCyd |
Nucleosides can be produced from nucleotidesde novo, particularly in the liver, but they are more abundantly supplied via ingestion and digestion of nucleic acids in the diet, wherebynucleotidases break downnucleotides (such as thethymidine monophosphate) intonucleosides (such asthymidine) and phosphate. The nucleosides, in turn, are subsequently broken down in thelumen of the digestive system bynucleosidases into nucleobases and ribose or deoxyribose. In addition, nucleotides can be broken down inside the cell intonitrogenous bases, andribose-1-phosphate ordeoxyribose-1-phosphate.
In medicine severalnucleoside analogues are used as antiviral or anticancer agents.[1][2][3][4] 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, severalanalogues of the sugar backbone exist. Due to the low stability of RNA, which is prone to hydrolysis, several more stable alternative nucleoside/nucleotide analogues that correctly bind to RNA are used. This is achieved by using a different backbone sugar. These analogues includelocked nucleic acids (LNA),morpholinos andpeptide nucleic acids (PNA).
In sequencing,dideoxynucleotides are used. These nucleotides possess the non-canonical sugar dideoxyribose, which lacks 3' hydroxyl group (which accepts the phosphate). DNA polymerases cannot distinguish between these and regular deoxyribonucleotides, but when incorporated a dideoxynucleotide cannot bond with the next base and the chain is terminated.
In order to understand howlife arose, knowledge is required of the chemical pathways that permit formation of the key building blocks of life under plausibleprebiotic conditions. According to theRNA world hypothesis free-floating ribonucleosides and ribonucleotides were present in the primitive soup. Molecules as complex as RNA must have arisen from small molecules whose reactivity was governed by physico-chemical processes. RNA is composed ofpurine andpyrimidine nucleotides, both of which are necessary for reliable information transfer, and thus Darwinian natural selection andevolution. Nam et al.[5] demonstrated the direct condensation of nucleobases with ribose to give ribonucleosides in aqueous microdroplets, a key step leading to RNA formation. Also, a plausible prebiotic process for synthesizing pyrimidine and purine ribonucleosides and ribonucleotides using wet-dry cycles was presented by Becker et al.[6]
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