Nucleotides are molecules that, when joined together, make up the structural units of DNA and RNA. In addition, nucleotides participate in cellular signaling (cGMP and cAMP), and are incorporated into important cofactors of enzymatic reactions (coenzyme A, FAD, FMN, and NADP+).
Nucleotide derivatives such as the nucleoside triphosphates play central roles in metabolism, in which capacity they serve as sources of chemical energy (ATP and GTP).

A nucleotide is composed of a nucleobase (nitrogenous base), a five-carbon sugar (either ribose or 2'-deoxyribose), and one phosphate group. Together, the nucleobase and sugar compose a nucleoside. The phosphate groups form bonds with either the 2, 3, or 5-carbon of the sugar, with the 5-carbon site most common. Cyclic nucleotides form when the phosphate group is bound to two of the sugar's hydroxyl groups. Ribonucleotides are nucleotides where the sugar is ribose, and deoxyribonucleotides contain the sugar deoxyribose. Nucleotides can contain either a purine or a pyrimidine base.

Nucleic acids are polymeric macromolecules made from nucleotide monomers. In DNA, the purine bases are adenine and guanine, while the pyrimidines are thymine and cytosine. RNA uses uracil in place of thymine. Adenine always pairs with thymine by 2 hydrogen bonds, while guanine pairs with cytosine through 3 hydrogen bonds, each due to their unique structures.

article source: wikipedia 

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