Introduction:

The association of nitrogenous bases and pentose sugars provides the compound known as nucleoside which is component of nucleotides (monomeric units of nucleic acids). Based on type of nitrogenous base and types of sugar it is liked to, different kinds of nucleotides are formed each containing its own characteristic and structure.

Definition of nucleosides:

Nucleosides are organic compounds having pentose sugars and nitrogenous base linked together by N-β-gylcosidic bond. Sugar can either be ribose sugar or deoxyribose sugar while nitrogenous bases are: Guanine, Adenine, Cytosine, Thymine or Uracil etc.

Components of nucleoside:

Components of nucleosides are essentially nitrogenous bases and pentose sugars.

Nitrogenous bases and their structures:

Nitrogenous bases present in nucleosides are either purines or pyrimidines and are given below. Purine bases contains purine ring (double ring system) while pyrimidine base contain pyrimidine ring (single ring structure).

The purine bases are:

Adenine denoted by A and has the structure:

Guanine denoted by G and has the following structure:

The pyrimidine bases are cytosine, thymine and uracil

Cytosine denoted by C and has the following structure

Thymine denoted by T and has the structure below

Uracil, denoted by U and has the structure below:

Additionally to purine bases A and G other unusual purine bases do exist and they comprise: hypoxathine, 1 methylguanine, 1 methylhypoxanthine etc. Unusual pyrimidine bases also exist and they are derived from cytosine, thymine and uracil and they comprise: 5-methylcytosine, Thiouracil etc.

Pentose sugars and their structures:

The pentose (5 carbon) sugars of nucleoides are basically ribose or (ribofuranose) 2' deoxyribose (2' deoxyribofuranose). The 2' deoxyribose sugar is found in deoxyribonucleic acid (DNA) while the ribose sugar is found in ribonucleic acid (RNA).

Linkages between pentose sugars and nitrogenous Bases in nucleosides:

In nucleosides, nitrogenous bases are joined to pentose sugar through the hemiacetal hydroxyl group on the C-1 (first carbon atom of the sugar). Generally, the purines are attached to the sugar through the N-9 nitrogen atom while pyrimidine is attached through the N-1 nitrogen atom.

Common purine and pyrimidine nucleosides and their Structures:

Purine Nucleosides:

Pyrimidine Nucleosides:

Nomenclature of nucleosides:

Nucleosides are named based on kind of ribose sugar attached to nitrogenous base and whether base present in nucleoside is the purine or pyrimidine.

All purine nucleoside names end with the suffix (-osine) added to name of purine base irrespective of whether it has ribose or deoxyribose sugar. Examples comprise: Adenosine, guanosine, deoxynadenosine and deoxyguanosine. If purine nucleoside has the deoxyribose sugar then prefix (-deoxy) is added to name of its ribonucleoide counterpart example deoxyadenosine and deoxyguanosine are deoxyribonucleosides of adenosine and guanosine respectively.

For naming of pyrimidine ribonucleoside the suffix (-idine) is added to name of base in nucleoside. Example ribonucleoside of uracil is named uridine, (-acil) of uracil is replaced by (-idine). Cytosine ribonucleoside is named Cytidine while that of Thymine is Thymidine. On other hand pyrimidine deoxyribonucleoside are named by adding the prefix 2' deoxy to name of its pyrimidine ribonucleosides counterpart like 2' deoxythymididine, 2' deoxycytidine, 2'deoxyadenosine, 2'deoxyguanosine and 2'deoxyuridrine.

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