Editing Nucleotide (section)

== Structure ==
[[File:0322 DNA Nucleotides.jpg|thumb|370px|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 the 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 nucleic acid, shown at upper left.]]

A nucleo<u>tide</u> is composed of three distinctive chemical sub-units: a five-carbon sugar molecule, a [[nucleobase]] (the two of which together are called a [[nucleoside|nucleo<u>side</u>]]), and one [[phosphate group]]. With all three joined, a nucleotide is also termed a "nucleo<u>side</u> ''mono''phosphate", "nucleoside ''di''phosphate" or "nucleoside ''tri''phosphate", depending on how many phosphates make up the phosphate group.<ref>{{Cite book |last=Wiley |url=https://onlinelibrary.wiley.com/doi/book/10.1002/047001590X |title=Encyclopedia of Life Sciences |date=2005-09-09 |publisher=Wiley |isbn=978-0-470-01617-6 |edition=1 |language=en |doi=10.1002/9780470015902.a0001333.pub3}}</ref>

In [[nucleic acid]]s, nucleotides contain either a [[purine]] or a [[pyrimidine]] base—i.e., the nucleobase molecule, also known as a nitrogenous base—and are termed ''ribo''nucleotides if the sugar is ribose, or ''deoxyribo''nucleotides if the sugar is deoxyribose. Individual phosphate molecules repetitively connect the [[ribose|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 (molecular biology)|directionality]]) of the chain-joins runs from the [[Directionality (molecular biology)#5′-end|5'-end]] to the [[Directionality (molecular biology)#3'-end|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 (molecular biology)|complementarity]] between the base-pairs, all which is essential for [[DNA replication|replicating]] or [[transcription (genetics)|transcribing]] the encoded information found in DNA.{{cn|date=February 2024}}

Nucleic acids then are [[polymeric]] [[macromolecule]]s assembled from nucleotides, the [[monomer|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 the 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 [[Cofactor (biochemistry)|cofactors]], often carrying out [[redox]] reactions.  Signaling [[cyclic nucleotides]] are formed by binding the phosphate group twice to the same sugar [[Molecular geometry|molecule]], bridging the 5'- and 3'- [[hydroxyl group]]s of the sugar.<ref name="Alberts" />  Some signaling nucleotides differ from the standard single-phosphate group configuration, in having multiple phosphate groups attached to different positions on the sugar.<ref>{{cite book| veditors = Smith AD |title=Oxford Dictionary of Biochemistry and Molecular Biology | edition = Revised |year=2000|location=Oxford|publisher=Oxford University Press|page=460}}</ref>  Nucleotide cofactors include a wider range of chemical groups attached to the sugar via the [[glycosidic bond]], including [[nicotinamide]] and [[Flavin group|flavin]], and in the latter case, the ribose sugar is linear rather than forming the ring seen in other nucleotides.

[[File:Nucleotides 1.svg|class=skin-invert-image|thumb|center|660px|Structural elements of three nucleo<u>tides</u>—where one-, two- or three-phosphates are attached to the nucleo<u>side</u> (in yellow, blue, green) at center: 1st, the nucleotide termed as a ''nucleoside <u>mono</u>phosphate'' is formed by adding a phosphate (in red); 2nd, adding a second phosphate forms a ''nucleoside <u>di</u>phosphate''; 3rd, adding a third phosphate results in a ''nucleoside <u>tri</u>phosphate''. +&nbsp;The nitrogenous base ([[nucleobase]]) is indicated by [[nucleobase|"Base"]] and "[[glycosidic bond]]" (sugar bond). All five [[nucleobase|primary, or canonical, bases]]—the [[purines#Notable purines|purines]] and [[pyrimidine#Nucleotides|pyrimidines]]—are sketched at right (in blue).]]
<div class="skin-invert-image">
{{Gallery
|title=Examples of non-nucleic acid nucleotides
|align=center
|File:Cyclic-AMPchemdraw.png|[[Cyclic adenosine monophosphate|cAMP]], a cyclic nucleotide signaling molecule with a single phosphate linked to both 5- and 3-positions.
|File:PppGpp.svg|[[Guanosine pentaphosphate|pppGpp]], a nucleotide signaling molecule with both 5'- and 3'-phosphates.
|File:NADP+ phys.svg|[[Nicotinamide adenine dinucleotide phosphate|NADP]], a dinucleotide enzymatic [[Cofactor (biochemistry)|cofactor]].
|File:FAD.png|[[Flavin adenine dinucleotide|FAD]], a dinucleotide enzymatic cofactor in which one of the ribose sugars adopts a linear configuration rather than a ring.}}
</div>