Editing RNA world (section)

== Prebiotic RNA synthesis ==
[[File:Etls-2019-0024c.01.png|thumb|upright=2|The RNA world hypothesis proposes that spontaneous polymerization of ribonucleotides led to the emergence of [[ribozyme]]s and including an [[RNA-dependent RNA polymerase|RNA replicase]].]]

Nucleotides are the fundamental molecules that combine in series to form RNA. They consist of a nitrogenous base attached to a sugar-phosphate backbone. RNA is made of long stretches of specific nucleotides arranged so that their sequence of bases carries information. The RNA world hypothesis holds that in the [[primordial soup]] (or [[primordial sandwich|sandwich]]), there existed free-floating nucleotides. These nucleotides regularly formed bonds with one another, which often broke because the change in energy was so low. However, certain sequences of base pairs have catalytic properties that lower the energy of their chain being created, enabling them to stay together for longer periods of time. As each chain grew longer, it attracted more matching nucleotides faster, causing chains to now form faster than they were breaking down.

These chains have been proposed by some as the first, primitive forms of life. In an RNA world, different sets of RNA strands would have had different replication outputs, which would have increased or decreased their frequency in the population, i.e., [[natural selection]]. As the fittest sets of RNA molecules expanded their numbers, novel catalytic properties added by mutation, which benefitted their persistence and expansion, could accumulate in the population. Such an [[autocatalytic set]] of ribozymes, capable of self-replication in about an hour, has been identified. It was produced by molecular competition ([[Directed evolution|''in vitro'' evolution]]) of candidate enzyme mixtures.<ref>{{cite journal | vauthors = Lincoln TA, Joyce GF | title = Self-sustained replication of an RNA enzyme | journal = Science | volume = 323 | issue = 5918 | pages = 1229–1232 | date = February 2009 | pmid = 19131595 | pmc = 2652413 | doi = 10.1126/science.1167856 | bibcode = 2009Sci...323.1229L }}
* {{cite web |date=January 12, 2009 |title=First Examples Of RNA That Replicates Itself Indefinitely Developed By Scripps Scientists |website=Medical News Today |url=http://www.medicalnewstoday.com/articles/135031.php |archive-url=https://web.archive.org/web/20090731234701/http://www.medicalnewstoday.com/articles/135031.php |archive-date=2009-07-31}}</ref>

Competition between RNA may have favored the emergence of cooperation between different RNA chains, opening the way for the formation of the first [[protocell]]. Eventually, RNA chains developed with catalytic properties that help [[amino acid]]s bind together (a process called [[peptide bond|peptide-bonding]]). These amino acids could then assist with RNA synthesis, giving those RNA chains that could serve as ribozymes the selective advantage. The ability to catalyze one step in protein synthesis, [[Transfer RNA#Aminoacylation|aminoacylation]] of RNA, has been demonstrated in a short (five-nucleotide) segment of RNA.<ref>{{cite journal | vauthors = Turk RM, Chumachenko NV, Yarus M | title = Multiple translational products from a five-nucleotide ribozyme | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 10 | pages = 4585–4589 | date = March 2010 | pmid = 20176971 | pmc = 2826339 | doi = 10.1073/pnas.0912895107 | doi-access = free | bibcode = 2010PNAS..107.4585T }}
* {{cite press release |date=February 24, 2010 |title=Scientists create tiny RNA molecule with big implications for life's origins |website=ScienceDaily |url=https://www.sciencedaily.com/releases/2010/02/100222162009.htm}}</ref>

In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of [[life]], including uracil, cytosine, and thymine, have been formed in the laboratory under conditions found only in [[outer space]], using starting chemicals, like [[pyrimidine]], found in [[meteorite]]s. Pyrimidine, like [[polycyclic aromatic hydrocarbons]] (PAHs), may have been formed in [[red giant]] stars or in [[cosmic dust|interstellar dust]] and gas clouds, according to the scientists.<ref name="NASA-20150303">{{cite web |last=Marlaire |first=Ruth |name-list-style=vanc |title=NASA Ames Reproduces the Building Blocks of Life in Laboratory |url=https://www.nasa.gov/content/nasa-ames-reproduces-the-building-blocks-of-life-in-laboratory |date=3 March 2015 |work=[[NASA]] |access-date=5 March 2015 |url-status=live |archive-url=https://web.archive.org/web/20150305083306/http://www.nasa.gov/content/nasa-ames-reproduces-the-building-blocks-of-life-in-laboratory/ |archive-date=5 March 2015 }}</ref>

In 2018, researchers at [[Georgia Institute of Technology]] identified three molecular candidates for the bases that might have formed an earliest version of proto-RNA: [[barbituric acid]], [[melamine]], and [[pyrimidine|2,4,6-triaminopyrimidine]] (TAP). These three molecules are simpler versions of the four bases in current RNA, which could have been present in larger amounts and could still be [[forward compatibility|forward-compatible]] with them but may have been discarded by evolution in exchange for more optimal base pairs.<ref>{{Cite web | url=https://www.astrobio.net/news-exclusive/new-study-identifies-possible-ancestors-of-rna/ | title=New Study Identifies Possible Ancestors of RNA| date=2018-09-14 |archive-url=https://web.archive.org/web/20201109042024/https://www.astrobio.net/news-exclusive/new-study-identifies-possible-ancestors-of-rna/ |archive-date=2020-11-09 |url-status=usurped}}</ref> Specifically, TAP can form nucleotides with a large range of sugars.<ref>{{cite journal | vauthors = Fialho DM, Clarke KC, Moore MK, Schuster GB, Krishnamurthy R, Hud NV | title = Glycosylation of a model proto-RNA nucleobase with non-ribose sugars: implications for the prebiotic synthesis of nucleosides | journal = Organic & Biomolecular Chemistry | volume = 16 | issue = 8 | pages = 1263–1271 | date = February 2018 | pmid = 29308815 | doi = 10.1039/c7ob03017g | bibcode = 2018OrgBC..16.1263F }}</ref> Both TAP and melamine base pair with barbituric acid. All three spontaneously form nucleotides with ribose.<ref>{{cite journal | vauthors = Cafferty BJ, Fialho DM, Khanam J, Krishnamurthy R, Hud NV | title = Spontaneous formation and base pairing of plausible prebiotic nucleotides in water | journal = Nature Communications | volume = 7 | issue = 1 | pages = 11328 | date = April 2016 | pmid = 27108699 | pmc = 4848480 | doi = 10.1038/ncomms11328 | bibcode = 2016NatCo...711328C }}</ref>