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Xeno nucleic acid
Xeno nucleic acid
(XNA) is a synthetic alternative to the natural nucleic acids DNA
DNA
and RNA
RNA
as information-storing biopolymers that differs in the sugar backbone.[1] 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. Although the genetic information is still stored in the four canonical base pairs (unlike other nucleic acid analogues), natural DNA polymerases cannot read and duplicate this information. Thus the genetic information stored in XNA is “invisible” and therefore useless to natural DNA-based organisms.[2]

Contents

1 Background 2 Structure 3 Implications 4 Applications 5 See also 6 References

Background[edit] The structure of the DNA
DNA
was discovered in 1953 and many scientists assumed that our understandings for the chemical basis of life was perfect. However, around the early 2000s, researchers were able to create 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, named by scientists to indicate the difference in the molecular structure of XNA when compared to DNA
DNA
or RNA. Not much was done with XNA until the development of special polymerase Enzyme, capable of copying XNA from a DNA
DNA
template as well as copying XNA back into DNA.[3] More recently, synthetic biologists Philipp Holliger and Alexander Taylor, both from the University of Cambridge, managed to create XNAzymes, the XNA equivalent of a Ribozyme, enzymes made of DNA
DNA
or ribonucleic acid; 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
RNA
or DNA.[4] Structure[edit]

This image displays the differences in the sugar backbones used in XNAs compared to common and biologically used DNA
DNA
and RNA.

Strands of DNA
DNA
and RNA
RNA
are formed by stringing together long chains of molecules called nucleotides. 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
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 six xeno nucleic acids are almost identical to those of DNA
DNA
and RNA, with one exception: in XNA nucleotides, the deoxyribose and ribose sugar groups of DNA
DNA
and RNA
RNA
have been replaced. Some of these replacement molecules contain four carbons atoms instead of the standard five. Others cram in as many as seven carbons. FANA even contains a fluorine atom.[citation needed] These substitutions make XNAs functionally and structurally analogous to DNA
DNA
and RNA, but they also make them unnatural and artificial. XNA exhibits a variety of structural chemical changes relative to its natural counterparts. Types of synthetic 'XNA' created so far include 1,5-anhydrohexitol nucleic acid (HNA) and cyclohexene nucleic acid (CeNA). Threose nucleic acid (TNA), glycol nucleic acid (GNA), locked nucleic acid (LNA), and peptide nucleic acid (PNA) are also other currently known XNAs that have been made and have more background information currently available than the previous two mentioned.[2] 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.[5] 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.[6] Implications[edit] The study of XNA is intended not 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 organisms.[7] While DNA
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
DNA
nucleotide bases yields endless opportunities for genetic modification and expansion of chemical functionality.[8] 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
DNA
and RNA
RNA
as once thought, but are simply processes that have developed from polymers capable of storing information.[9] Investigations into XNAs will allow for researchers to assess whether DNA
DNA
and RNA
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.[10] Applications[edit] One theory of XNA utilization is its incorporation into medicine as a disease-fighting agent. Some enzymes and 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
DNA
and RNA-based treatment methodologies that are currently in use.[11] Experiments with XNA have already allowed for the replacement and enlargement of this genetic alphabet, and XNAs have shown complementarity with DNA
DNA
and RNA
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 ligands by exponential enrichment) method, followed by successful binding to a line of breast cancer cells.[12] Furthermore, experiments in the model bacterium E. coli have demonstrated the ability for XNA to serve as a biological template for DNA
DNA
in vivo.[13] In moving forward with genetic research on XNAs, various questions must come into consideration regarding biosafety, biosecurity, ethics, and governance/regulation.[2] One of the key questions here is whether XNA in an in vivo setting would intermix with DNA
DNA
and RNA
RNA
in its natural environment, thereby rendering scientists unable to control or predict its implications in genetic mutation.[11] XNA also has potential applications to be used as catalysts, much like RNA
RNA
has the ability to be used as an enzyme. Researchers have shown XNA is able to cleave and ligate DNA, RNA
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
DNA
and RNA's role in life emerged through natural selection processes or if it was simply a coincidental occurrence.[14] See also[edit]

Nucleic acid
Nucleic acid
analogue Xenobiology

References[edit]

^ Markus Schmidt (9 May 2012). Synthetic Biology. John Wiley & Sons. pp. 151–. ISBN 978-3-527-65926-5. Retrieved 9 May 2013.  ^ a b c Schmidt, Markus (April 2010). "Xenobiology: A new form of life as the ultimate biosafety tool". BioEssays. John Wiley & Sons. 32 (4): 322–331. doi:10.1002/bies.200900147. PMC 2909387 . PMID 20217844.  ^ Gonzales, Robbie. "XNA Is Synthetic DNA
DNA
That's Stronger than the Real Thing." Web log post. Io9, 09 Apr. 2012. Web. 15 Oct. 2015. <http://io9.com/5903221/meet-xna-the-first-synthetic-dna-that-evolves-like-the-real-thing>. ^ Medical Research Council "World's first artificial enzymes created using synthetic biology" 1st December 2014 <http://www.mrc.ac.uk/news/browse/world-s-first-artificial-enzymes-created-using-synthetic-biology/>. ^ "Polymers perform non- DNA
DNA
evolution". www.rsc.org. Retrieved 2015-10-15.  ^ Gu, Ping; Schepers, Guy; Rozenski, Jef; Aerschot, Arthur Van; Herdewijn, Piet. "Base Pairing Properties of D- and L-Cyclohexene Nucleic Acids (CeNA)". Oligonucleotides. 13 (6): 479–489. doi:10.1089/154545703322860799.  ^ Herdewijn, Piet; Marlière, Philippe. (2009). Toward Safe Genetically Modified Organisms through the Chemical Diversification of Nucleic Acids. Chemistry & Biodiversity, 6, pp. 791-808. Web. ^ Pinheiro, Vitor B.; Holliger, Philipp. (2012). XNA world: progress towards replication and evolution of synthetic genetic polymers. Current Opinion in Chemical Biology, 16(3-4), pp. 245-252. doi:10.1016/j.cbpa.2012.05.198 ^ Pinheiro, Vitor B.; Taylor, Alexander I.; Cozens, Christopher; Abramov, Mikhail; Renders, Marleen; Zhang, Su; Chaput, John C.; Wengel, Jesper; Peak-Chew, Sew-Yeu; McLaughlin, Stephen H.; Herdewijn, Piet; Holliger, Philipp. (2012). Synthetic Genetic Polymers Capable of Heredity
Heredity
and Evolution. Science, 336(6079), pp. 341-344. DOI: 10.1126/science.1217622 ^ Hunter, Philip (2013-05-01). "XNA marks the spot". EMBO Reports. 14 (5): 410–413. doi:10.1038/embor.2013.42. PMC 3642382 . PMID 23579343.  ^ a b "XNA: Synthetic DNA
DNA
That Can Evolve". Popular Mechanics. 2012-04-19. Retrieved 2015-11-17.  ^ Sefah, Kwame; Yang, Zunyi; Bradley, Kevin M.; Hoshika, Shuichi; Jiménez, Elizabeth; Zhang, Liqin; Zhu, Guizhi; Shanker, Savita; Yu, Fahong (2014-01-28). "In vitro selection with artificial expanded genetic information systems". Proceedings of the National Academy of Sciences. 111 (4): 1449–1454. doi:10.1073/pnas.1311778111. ISSN 0027-8424. PMC 3910645 . PMID 24379378.  ^ "http://onlinelibrary.wiley.com/store/10.1002/anie.201303288/asset/8139_ftp.pdf?v=1&t=ih3qexlz&s=75170ef8b7c4cf168dd55fe20f1439cf40c311b4" (PDF). doi:10.1002/anie.201303288/asset/8139_ftp.pdf.  External link in title= (help) ^ Taylor, Alexander I.; Pinheiro, Vitor B.; Smola, Matthew J.; Morgunov, Alexey S.; Peak-Chew, Sew; Cozens, Christopher; Weeks, Kevin M.; Herdewijn, Piet; Holliger, Philipp. "Catalysts from synthetic genetic polymers". Nature. 518 (7539): 427–430. doi:10.1038/nature13982. PMC 4336857 . PMID 25470036. 

v t e

Types of nucleic acids

Constituents

Nucleobases Nucleosides Nucleotides Deoxynucleotides

Ribonucleic acids (coding, non-coding)

Translational

Messenger

precursor, heterogenous nuclear

Transfer Ribosomal Transfer-messenger

Regulatory

Interferential

Micro Small interfering Piwi-interacting

Antisense Processual

Small nuclear Small nucleolar Small Cajal Body RNAs Y RNA

Enhancer RNAs

Others

Guide Ribozyme Small hairpin Small temporal Trans-acting small interfering Subgenomic messenger

Deoxyribonucleic acids

Complementary Chloroplast Deoxyribozyme Genomic Multicopy single-stranded Mitochondrial

Analogues

Xeno

Glycol Threose Hexose

Locked Peptide Morpholino

Cloning vectors

Phagemid Plasmid Lambda phage Cosmid Fosmid Artificial chromosomes

P1-derived Bac

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