Nucleoside

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See Nucleoside in Wikipedia, the free encyclopedia.

A nucleoside is a nucleic acid base (adenine, thymine, guanine, cytosine, or uracil) covalently bonded to either ribose or deoxyribose. Adding an inorganic phosphate group to the sugar component created a nucleotide.

Nomenclature

Nucleosides are named after the base involved:

• Adenine: Adenosine
• Guanine: Guanosine
• Cytosine: Cytidine
• Thymine: Thymidine
• Uracil: Uridine

Nucleotides take the name of the nucleoside and add the phosphate groups to the name, for example:

• Adenosine with one phosphate: Adenosine monophosphate (AMP)
• Adenosine with two phosphates: Adenosine diphosphate (ADP)
• Adenosine with three phosphates: Adenosine triphosphate (ATP)

Biological Roles

Nucleoside monophosphates join together by phosphodiester bonds (ie C-O-P-O-C) to form single-stranded DNA and RNA; these can then form either double stranded nucleic acids or DNA-RNA hybrids by hydrogen bonding between the bases.

ATP has another role. It is the primary energy currency of the cell; energy used to drive thermodynamically unfavourable biochemical reactions is obtained by cleaving the phosphoryl group from the end, creating ADP and orthophosphate.

ATP may also indirectly supply energy by donating a phosphoryl group to another molecule in order to change its charge and allow it to react. There are also a few specific biochemical reactions wihc use guanosine triphosphate (GTP) or uridine triphosphate (UTP) for similar purposes.

Cyclic AMP (cAMP) is generated when a pyrophosphate group is cleaved from ATP, and the remaining terminal oxygen atom performs a nucleophillic attack on a carbon atom in the sugar ring, forming a new ring in the molecule. cAMP is a common signalling molecule in cells.