page_title: Nucleotide
section_title: Structure
hierarchical_section_title: Nucleotide / Structure
caption_attribution_description: Version 8.25 from the Textbook OpenStax Anatomy and Physiology Published May 18, 2016
context_page_description: Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers deoxyribonucleic acid and ribonucleic acid, both of which are essential biomolecules within all life-forms on Earth.
  Nucleotides are composed of three subunit molecules: a nitrogenous base, a five-carbon sugar, and a phosphate group consisting of one to three phosphates. The four nitrogenous bases in DNA are guanine, adenine, cytosine and thymine; in RNA, uracil is used in place of thymine.
  Nucleotides also play a central role in metabolism at a fundamental, cellular level. They provide chemical energy—in the form of the nucleoside triphosphates, adenosine triphosphate, guanosine triphosphate, cytidine triphosphate and uridine triphosphate —throughout the cell for the many cellular functions that demand energy, including: amino acid, protein and cell membrane synthesis, moving the cell and cell parts, cell division, etc. In addition, nucleotides participate in cell signaling, and are incorporated into important cofactors of enzymatic reactions.
context_section_description: A nucleotide is composed of three distinctive chemical sub-units: a five-carbon sugar molecule, a nitrogenous base—which two together are called a nucleoside—and one phosphate group. With all three joined, a nucleotide is also termed a "nucleoside monophosphate", "nucleoside diphosphate" or "nucleoside triphosphate", depending on how many phosphates make up the phosphate group.
  In nucleic acids, nucleotides contain either a purine or a pyrimidine base—i.e., the nitrogenous base molecule, also known as a nucleobase—and are termed ribonucleotides if the sugar is ribose, or deoxyribonucleotides if the sugar is deoxyribose. Individual phosphate molecules repetitively connect the sugar-ring molecules in two adjacent nucleotide monomers, thereby connecting the nucleotide monomers of a nucleic acid end-to-end into a long chain. These chain-joins of sugar and phosphate molecules create a 'backbone' strand for a single- or double helix. In any one strand, the chemical orientation (directionality) of the chain-joins runs from the 5'-end to the 3'-end (read: 5 prime-end to 3 prime-end)—referring to the five carbon sites on sugar molecules in adjacent nucleotides. In a double helix, the two strands are oriented in opposite directions, which permits base pairing and complementarity between the base-pairs, all which is essential for replicating or transcribing the encoded information found in DNA.
  Nucleic acids then are polymeric macromolecules assembled from nucleotides, the monomer-units of nucleic acids. The purine bases adenine and guanine and pyrimidine base cytosine occur in both DNA and RNA, while the pyrimidine bases thymine (in DNA) and uracil (in RNA) occur in just one. Adenine forms a base pair with thymine with two hydrogen bonds, while guanine pairs with cytosine with three hydrogen bonds.
  In addition to being building blocks for construction of nucleic acid polymers, singular nucleotides play roles in cellular energy storage and provision, cellular signaling, as a source of phosphate groups used to modulate the activity of proteins and other signaling molecules, and as enzymatic cofactors, often carrying out redox reactions.  Signaling cyclic nucleotides are formed by binding the phosphate group twice to the same sugar molecule, bridging the 5'- and 3'- hydroxyl groups of the sugar.  Some signaling nucleotides differ from the standard single-phosphate group configuration, in having multiple phosphate groups attached to different positions on the sugar.  Nucleotide cofactors include a wider range of chemical groups attached to the sugar via the glycosidic bond, including nicotinamide and flavin, and in the latter case, the ribose sugar is linear rather than forming the ring seen in other nucleotides.
caption: Showing the arrangement of nucleotides within the structure of nucleic acids: At lower left, a monophosphate nucleotide; its nitrogenous base represents one side of a base-pair. At upper right, four nucleotides form two base-pairs: thymine and adenine (connected by double hydrogen bonds) and guanine and cytosine (connected by triple hydrogen bonds). The individual nucleotide monomers are chain-joined at their sugar and phosphate molecules, forming two 'backbones' (a double helix) of a nucleic acid, shown at upper left.
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