Editing Nucleic acid (section)

==Molecular composition and size==
Nucleic acids are generally very large molecules. Indeed, DNA molecules are probably the largest individual molecules known. Well-studied biological nucleic acid molecules range in size from 21 nucleotides ([[small interfering RNA]]) to large chromosomes ([[chromosome 1|human chromosome 1]] is a single molecule that contains 247 million [[base pair]]s<ref>{{cite journal | vauthors = Gregory SG, Barlow KF, McLay KE, Kaul R, Swarbreck D, Dunham A, etal | title = The DNA sequence and biological annotation of human chromosome 1 | journal = Nature | volume = 441 | issue = 7091 | pages = 315–21 | date = May 2006 | pmid = 16710414 | doi = 10.1038/nature04727 | bibcode = 2006Natur.441..315G | doi-access = free }}</ref>).

In most cases, naturally occurring DNA molecules are [[double helix|double-stranded]] and RNA molecules are single-stranded.<ref>{{cite journal | vauthors = Todorov TI, Morris MD | title = Comparison of RNA, single-stranded DNA and double-stranded DNA behavior during capillary electrophoresis in semidilute polymer solutions | journal = Electrophoresis | volume = 23 | issue = 7–8 | pages = 1033–44 | date = April 2002 | pmid = 11981850 | doi = 10.1002/1522-2683(200204)23:7/8<1033::AID-ELPS1033>3.0.CO;2-7 | publisher = nih.gov | others = [[National Institutes of Health]] | s2cid = 33167686 }}</ref> There are numerous exceptions, however—some viruses have genomes made of [[Reoviridae|double-stranded RNA]] and other viruses have [[M13 bacteriophage|single-stranded DNA]] genomes,<ref>{{cite web |url=http://pathmicro.med.sc.edu/mhunt/rna-ho.htm |title= RN Virus Replication Strategies |author= Margaret Hunt |author2= University of South Carolina |publisher= sc.edu |year= 2010|author2-link= University of South Carolina }}</ref> and, in some circumstances, nucleic acid structures with [[triple-stranded DNA|three]] or [[G-quadruplex|four]] strands can form.<ref name="pmid10454599">{{cite journal | vauthors = McGlynn P, Lloyd RG | title = RecG helicase activity at three- and four-strand DNA structures | journal = Nucleic Acids Research | volume = 27 | issue = 15 | pages = 3049–56 | date = August 1999 | pmid = 10454599 | pmc = 148529 | doi = 10.1093/nar/27.15.3049}}</ref>

Nucleic acids are linear [[polymer]]s (chains) of nucleotides. Each nucleotide consists of three components: a [[purine]] or [[pyrimidine]] [[nucleobase]] (sometimes termed ''nitrogenous base'' or simply ''base''), a [[pentose]] [[sugar]], and a [[phosphate]] group which makes the molecule acidic. The substructure consisting of a nucleobase plus sugar is termed a [[nucleoside]]. Nucleic acid types differ in the structure of the sugar in their nucleotides–DNA contains 2'-[[deoxyribose]] while RNA contains [[ribose]] (where the only difference is the presence of a [[hydroxyl group]]). Also, the [[nucleobases]] found in the two nucleic acid types are different: [[adenine]], [[cytosine]], and [[guanine]] are found in both RNA and DNA, while [[thymine]] occurs in DNA and [[uracil]] occurs in RNA.{{cn|date=April 2023}}

The sugars and phosphates in nucleic acids are connected to each other in an alternating chain (sugar-phosphate backbone) through [[phosphodiester]] linkages.<ref name="Stryer">{{cite book |author1=Stryer, Lubert |author2=Berg, Jeremy Mark |author3=Tymoczko, John L. |title=Biochemistry |publisher=W.H. Freeman |location=San Francisco |year=2007 |isbn=978-0-7167-6766-4 |url-access=registration |url=https://archive.org/details/biochemistry0006berg }}</ref> In [[nucleic acid nomenclature|conventional nomenclature]], the carbons to which the phosphate groups attach are the 3'-end and the 5'-end carbons of the sugar. This gives nucleic acids [[directionality (molecular biology)|directionality]], and the ends of nucleic acid molecules are referred to as 5'-end and 3'-end. The nucleobases are joined to the sugars via an ''N''-glycosidic linkage involving a nucleobase ring nitrogen (''N''-1 for pyrimidines and ''N''-9 for purines) and the 1' carbon of the pentose sugar ring.

Non-standard nucleosides are also found in both RNA and DNA and usually arise from modification of the standard nucleosides within the DNA molecule or the primary (initial) RNA transcript. [[Transfer RNA]] (tRNA) molecules contain a particularly large number of modified nucleosides.<ref>{{cite journal | vauthors = Rich A, RajBhandary UL | title = Transfer RNA: molecular structure, sequence, and properties | journal = [[Annual Review of Biochemistry]] | volume = 45 | pages = 805–60 | year = 1976 | pmid = 60910 | doi = 10.1146/annurev.bi.45.070176.004105 }}</ref>