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((Short description|Synthetic nucleic acid analogues))
((Copy edit|for=Parts are like a school essay with detail about "all things _NA" (off-topic), and personal/informal writing.|date=October 2023))
((Use dmy dates|date=July 2018))
[[File:GNA-T vs. natural DNA-T.png|thumb|[[Glycol nucleic acid]] ''(left)'' is an example of a xeno nucleic acid because it has a different backbone than DNA ''(right)''.]]
'''Xeno nucleic acids''' ('''XNA''') are synthetic [[nucleic acid analogue]]s that have a different backbone than the [[ribose]] and [[deoxyribose]] found in the [[nucleic acids]] of naturally occurring [[RNA]] and [[DNA]].<ref name="Schmidt2012">((cite book |last=Schmidt|first=Markus | name-list-style = vanc |title=Synthetic Biology |url=https://books.google.com/books?id=lr0SgX1Hx2gC&pg=PA151|access-date=9 May 2013|year= 2012|publisher=John Wiley & Sons|isbn=978-3-527-65926-5|pages=151–))</ref> 

The same [[nucleobase]]s can be used to store [[genetic information]] and interact with DNA, RNA, or other XNA bases, but the change of backbone makes the structure have different stability and cannot be processed by naturally occurring cellular processes. For example, natural [[DNA polymerase]]s cannot read and duplicate this information, thus the genetic information stored in XNA is invisible to DNA-based organisms.<ref name="Schmidt1">((cite journal | vauthors = Schmidt M | title = Xenobiology: a new form of life as the ultimate biosafety tool | journal = BioEssays | volume = 32 | issue = 4 | pages = 322–331 | date = April 2010 | pmid = 20217844 | pmc = 2909387 | doi = 10.1002/bies.200900147 ))</ref>

((As of|2011)), at least six types of synthetic sugars have been shown to form nucleic acid backbones that can store and retrieve genetic information. Research is now being done to create synthetic polymerases to transform XNA. The study of its production and application has created a field known as [[xenobiology]].

==Background==
The structure of [[DNA]] was discovered in 1953. Around the early 2000s, researchers created a number of exotic DNA-like structures, XNA. XNA is a synthetic polymer that can carry the same information as DNA, but with different molecular constituents. The "X" in XNA stands for "xeno," meaning stranger or alien, indicating the difference in the molecular structure as compared to DNA or RNA.<ref name=":1" />

Not much was done with XNA until the development of special polymerase [[enzyme]], capable of copying XNA from a DNA template as well as copying XNA back into DNA.<ref name=":1">((Cite web |last=Gonzales |first=Robbie | name-list-style = vanc |title=XNA Is Synthetic DNA That's Stronger than the Real Thing |website=[[Io9]] |date=19 April 2012 |access-date=15 October 2015 |url=http://io9.com/5903221/meet-xna-the-first-synthetic-dna-that-evolves-like-the-real-thing))</ref> Pinheiro et al. (2012), for example, has demonstrated such an XNA-capable polymerase that works on sequences of ~100bp in length.<ref name="Synthetic genetic polymers capable">((cite journal | vauthors = Pinheiro VB, Taylor AI, Cozens C, Abramov M, Renders M, Zhang S, Chaput JC, Wengel J, Peak-Chew SY, McLaughlin SH, Herdewijn P, Holliger P | display-authors = 6 | title = Synthetic genetic polymers capable of heredity and evolution | journal = Science | volume = 336 | issue = 6079 | pages = 341–344 | date = April 2012 | pmid = 22517858 | pmc = 3362463 | doi = 10.1126/science.1217622 | bibcode = 2012Sci...336..341P ))</ref> More recently, synthetic biologists [[Philipp Holliger]] and Alexander Taylor managed to create XNAzymes, the XNA equivalent of a [[ribozyme]], enzymes made of RNA. This demonstrates that XNAs not only store hereditary information, but can also serve as enzymes, raising the possibility that life elsewhere could have begun with something other than RNA or DNA.<ref>((Cite web |website=[[Medical Research Council (United Kingdom)|Medical Research Council]] |title=World's first artificial enzymes created using synthetic biology |date=1 December 2014 |url=http://www.mrc.ac.uk/news/browse/world-s-first-artificial-enzymes-created-using-synthetic-biology/ |archive-url=https://webarchive.nationalarchives.gov.uk/ukgwa/20180103180513/https://www.mrc.ac.uk/news/browse/world-s-first-artificial-enzymes-created-using-synthetic-biology/ |archive-date=3 January 2018))</ref>

== Structure ==
Strands of DNA and RNA are formed by stringing together long chains of molecules called [[nucleotide]]s. A nucleotide is made up of three chemical components: a [[phosphate]], a five-carbon sugar group (this can be either a [[deoxyribose]] sugar—which gives us the "D" in DNA—or a [[ribose]] sugar—the "R" in RNA), and one of five standard bases ([[adenine]], [[guanine]], [[cytosine]], [[thymine]] or [[uracil]]).

The molecules that piece together to form the xeno nucleic acids are almost identical to those of DNA and RNA, with one exception: in XNA [[nucleotide]]s, the [[deoxyribose]] and [[ribose]] sugar groups of DNA and RNA have been replaced with other chemical structures. These substitutions make XNAs functionally and structurally analogous to DNA and RNA despite being unnatural and artificial.

XNA exhibits a variety of structural chemical changes relative to its natural counterparts. Types of synthetic XNA created so far include:<ref name="Schmidt1" />
* 1,5-Anhydrohexitol nucleic acid (HNA)
* Cyclohexene nucleic acid (CeNA)
* [[Threose nucleic acid]] (TNA)
* [[Glycol nucleic acid]] (GNA)
* [[Locked nucleic acid]]
* [[Peptide nucleic acid]] (PNA)
* Fluoroarabino nucleic acid (FANA)

HNA could be used to potentially act as a drug that can recognize and bind to specified sequences. Scientists have been able to isolate HNAs for the possible binding of sequences that target HIV.<ref>((Cite web |title = Polymers perform non-DNA evolution |url = http://www.rsc.org/chemistryworld/News/2012/April/xna-hna-primordial-life-dna-evolution-chemistry.asp |first=Andy |last=Extance | name-list-style = vanc |date=19 April 2012 |website = [[Royal Society of Chemistry]] |access-date = 15 October 2015))</ref> With cyclohexene nucleic acid, research has shown that CeNAs with stereochemistry similar to the D form can create stable duplexes with itself and RNA. It was shown that CeNAs are not as stable when they form duplexes with DNA.<ref>((cite journal | vauthors = Gu P, Schepers G, Rozenski J, Van Aerschot A, Herdewijn P | title = Base pairing properties of D- and L-cyclohexene nucleic acids (CeNA) | journal = Oligonucleotides | volume = 13 | issue = 6 | pages = 479–489 | year = 2003 | pmid = 15025914 | doi = 10.1089/154545703322860799 ))</ref>

==Implications==
The study of XNA is not intended to give scientists a better understanding of biological [[evolution]] as it has occurred historically, but rather to explore ways in which we can control and even reprogram the genetic makeup of biological organisms moving forward. XNA has shown significant potential in solving the current issue of [[genetic pollution]] in [[genetically modified organism]]s.<ref>((cite journal | vauthors = Herdewijn P, Marlière P | title = Toward safe genetically modified organisms through the chemical diversification of nucleic acids | journal = Chemistry & Biodiversity | volume = 6 | issue = 6 | pages = 791–808 | date = June 2009 | pmid = 19554563 | doi = 10.1002/cbdv.200900083 | s2cid = 8572188 ))</ref> While DNA is incredibly efficient in its ability to store genetic information and lend complex biological diversity, its four-letter genetic alphabet is relatively limited. Using a genetic code of six XNAs rather than the four naturally occurring DNA nucleotide bases yields endless opportunities for genetic modification and expansion of chemical functionality.<ref>((cite journal | vauthors = Pinheiro VB, Holliger P | title = The XNA world: progress towards replication and evolution of synthetic genetic polymers | journal = Current Opinion in Chemical Biology | volume = 16 | issue = 3–4 | pages = 245–252 | date = August 2012 | pmid = 22704981 | doi = 10.1016/j.cbpa.2012.05.198 ))</ref>

The development of various hypotheses and theories about XNAs have altered a key factor in our current understanding of nucleic acids: that [[heredity]] and evolution are not limited to DNA and RNA as once thought, but are simply processes that have developed from polymers capable of storing information.<ref name="Synthetic genetic polymers capable"/> Investigations into XNAs will allow for researchers to assess whether DNA and RNA are the most efficient and desirable building blocks of life, or if these two molecules were chosen randomly after evolving from a larger class of chemical ancestors.<ref>((cite journal | vauthors = Hunter P | title = XNA marks the spot. What can we learn about the origins of life and the treatment of disease through artificial nucleic acids? | journal = EMBO Reports | volume = 14 | issue = 5 | pages = 410–413 | date = May 2013 | pmid = 23579343 | pmc = 3642382 | doi = 10.1038/embor.2013.42 ))</ref>

== Applications ==
One theory of XNA utilization is its incorporation into medicine as a disease-fighting agent. Some enzymes and [[Antibody|antibodies]] that are currently administered for various disease treatments are broken down too quickly in the stomach or bloodstream. Because XNA is foreign and because it is believed that humans have not yet evolved the enzymes to break them down, XNAs may be able to serve as a more durable counterpart to the DNA and RNA-based treatment methodologies that are currently in use.<ref name=":0">((Cite web|title = XNA: Synthetic DNA That Can Evolve|url = http://www.popularmechanics.com/science/health/a7636/xna-synthetic-dna-that-can-evolve-8210483/|website = [[Popular Mechanics]] |date = 19 April 2012 |access-date = 17 November 2015))</ref>

Experiments with XNA have already allowed for the replacement and enlargement of this genetic alphabet, and XNAs have shown [[Complementarity (molecular biology)|complementarity]] with DNA and RNA nucleotides, suggesting potential for its transcription and recombination. One experiment conducted at the University of Florida led to the production of an XNA [[aptamer]] by the AEGIS-SELEX (artificially expanded genetic information system - systematic evolution of [[ligand]]s by exponential enrichment) method, followed by successful binding to a line of [[breast cancer]] cells.<ref>((cite journal | vauthors = Sefah K, Yang Z, Bradley KM, Hoshika S, Jiménez E, Zhang L, Zhu G, Shanker S, Yu F, Turek D, Tan W, Benner SA | display-authors = 6 | title = In vitro selection with artificial expanded genetic information systems | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 111 | issue = 4 | pages = 1449–1454 | date = January 2014 | pmid = 24379378 | pmc = 3910645 | doi = 10.1073/pnas.1311778111 | doi-access = free | bibcode = 2014PNAS..111.1449S ))</ref> Furthermore, experiments in the model bacterium ''[[Escherichia coli|E. coli]]'' have demonstrated the ability for XNA to serve as a biological template for DNA ''[[in vivo]].''<ref>((cite journal | vauthors = Pezo V, Liu FW, Abramov M, Froeyen M, Herdewijn P, Marlière P | title = Binary genetic cassettes for selecting XNA-templated DNA synthesis in vivo | journal = Angewandte Chemie | volume = 52 | issue = 31 | pages = 8139–8143 | date = July 2013 | pmid = 23804524 | doi = 10.1002/anie.201303288 | url = https://lirias.kuleuven.be/handle/123456789/413046 ))</ref>

In moving forward with genetic research on XNAs, various questions must come into consideration regarding [[biosafety]], [[biosecurity]], ethics, and governance/regulation.<ref name="Schmidt1" /> One of the key questions here is whether XNA in an ''in vivo'' setting would intermix with DNA and RNA in its natural environment, thereby rendering scientists unable to control or predict its implications in genetic [[mutation]].<ref name=":0" />

XNA also has potential applications to be used as [[Catalysis|catalysts]], much like RNA has the ability to be used as an [[enzyme]]. Researchers have shown XNA is able to cleave and [[Ligation (molecular biology)|ligate]] DNA, RNA and other XNA sequences, with the most activity being XNA catalyzed reactions on XNA molecules. This research may be used in determining whether DNA and RNA's role in life emerged through natural selection processes or if it was simply a coincidental occurrence.<ref>((cite journal | vauthors = Taylor AI, Pinheiro VB, Smola MJ, Morgunov AS, Peak-Chew S, Cozens C, Weeks KM, Herdewijn P, Holliger P | display-authors = 6 | title = Catalysts from synthetic genetic polymers | journal = Nature | volume = 518 | issue = 7539 | pages = 427–430 | date = February 2015 | pmid = 25470036 | pmc = 4336857 | doi = 10.1038/nature13982 | bibcode = 2015Natur.518..427T ))</ref>

XNA may be employed as molecular clamps in [[quantitative real-time polymerase chain reaction]]s (qPCR) by hybridizing with target DNA sequences.<ref name="A novel xenonucleic acid-mediated m">((cite journal | vauthors = Sun Q, Pastor L, Du J, Powell MJ, Zhang A, Bodmer W, Wu J, Zheng S, Sha MY | display-authors = 6 | title = A novel xenonucleic acid-mediated molecular clamping technology for early colorectal cancer screening | journal = PLOS ONE | volume = 16 | issue = 10 | pages = e0244332 | date = 2021-10-05 | pmid = 34610014 | pmc = 8491914 | doi = 10.1371/journal.pone.0244332 | bibcode = 2021PLoSO..1644332S | doi-access = free ))</ref> In a study published in [[PLOS One|PLOS ONE]], an XNA-mediated molecular clamping assay detected mutant cell-free DNA (cfDNA) from precancerous [[colorectal cancer]] (CRC) lesions and colorectal cancer.<ref name="A novel xenonucleic acid-mediated m"/> XNA may also act as highly specific molecular probes for detection of nucleic acid target sequence.<ref>((cite journal | vauthors = D'Agata R, Giuffrida MC, Spoto G | title = Peptide Nucleic Acid-Based Biosensors for Cancer Diagnosis | journal = Molecules | volume = 22 | issue = 11 | pages = 1951 | date = November 2017 | pmid = 29137122 | doi = 10.3390/molecules22111951 | pmc = 6150339 | doi-access = free ))</ref>

== References ==
((reflist))

((Nucleic acids))

[[Category:Helices]]
[[Category:Nucleic acids]]

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