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Nucleotides are organic molecules that serve as the monomers, or subunits, of nucleic acids like DNA and RNA. The building blocks of nucleic acids, nucleotides are composed of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group. Thus a nucleoside plus a phosphate group yields a nucleotide. Nucleotides serve to carry packets of energy within the cell in the form of the nucleoside triphosphates (ATP, GTP, CTP and UTP), playing a central role in metabolism.〔Alberts B, Johnson A, Lewis J, Raff M, Roberts K & Wlater P (2002). ''Molecular Biology of the Cell'' (4th ed.). Garland Science. ISBN 0-8153-3218-1. pp. 120–121.〕 In addition, nucleotides participate in cell signaling (cGMP and cAMP), and are incorporated into important cofactors of enzymatic reactions (e.g. coenzyme A, FAD, FMN, NAD, and NADP+). In experimental biochemistry, nucleotides can be radiolabeled with radionuclides to yield radionucleotides. ==Structure== A nucleotide is made of a nucleobase (also termed a nitrogenous base), a five-carbon sugar (either ribose or 2-deoxyribose) depending on if it is DNA or RNA, and one or, depending on the definition, more than one phosphate groups. Authoritative chemistry sources such as the ACS Style Guide and IUPAC Gold Book clearly state that the term nucleotide refers only to a molecule containing one phosphate. However, common usage in molecular biology textbooks often extends this definition to include molecules with two or three phosphate groups.〔〔〕 Thus, the term "nucleotide" generally refers to a nucleoside monophosphate, but a nucleoside diphosphate or nucleoside triphosphate could be considered a nucleotide as well. Without the phosphate group, 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.〔 Nucleotides contain either a purine or a pyrimidine base. Ribonucleotides are nucleotides in which the sugar is ribose. Deoxyribonucleotides are nucleotides in which the sugar is deoxyribose. 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, in each case because of the unique structures of the bases. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nucleotide」の詳細全文を読む スポンサード リンク
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