Editing RNA world (section)

=== RNA in information storage ===
RNA is a very similar molecule to DNA, with only two significant chemical differences (the backbone of RNA uses ribose instead of deoxyribose and its nucleobases include [[uracil]] instead of [[thymine]]). The overall structure of RNA and DNA are immensely similar—one strand of DNA and one of RNA can bind to form a double helical structure. This makes the storage of information in RNA possible in a very similar way to the storage of information in DNA. However, RNA is less stable, being more prone to hydrolysis due to the presence of a hydroxyl group at the ribose 2' position.

[[File:Ribonucleic acid chemical structure.svg|thumb|The major difference between RNA and DNA is the presence of a [[hydroxyl]] group at the 2'-position.]]

==== Comparison of DNA and RNA structure ====
{{Main|RNA|DNA}}

The major difference between RNA and DNA is the presence of a [[hydroxyl]] group at the 2'-position of the [[ribose]] sugar in RNA (illustration, right).<ref name="Atk06" /> This group makes the molecule less stable because, when not constrained in a double helix, the 2' hydroxyl can chemically attack the adjacent [[phosphodiester bond]] to cleave the phosphodiester backbone. The hydroxyl group also forces the ribose into the C3'-''endo'' sugar conformation unlike the C2'-''endo'' conformation of the [[deoxyribose]] sugar in DNA. This forces an RNA double helix to change from a [[B-DNA]] structure to one more closely resembling [[A-DNA]].

RNA also uses a different set of bases than DNA—[[adenine]], [[guanine]], [[cytosine]] and [[uracil]], instead of adenine, guanine, cytosine and [[thymine]]. Chemically, uracil is similar to thymine, differing only by a [[methyl group]], and its production requires less energy.<ref>{{cite web |title=Uracil |url=http://www.humpath.com/spip.php?article7304 |access-date=2020-07-24 |url-status=live |archive-url=https://web.archive.org/web/20150908055138/http://www.humpath.com/spip.php?article7304 |archive-date=2015-09-08}}</ref> In terms of base pairing, this has no effect. Adenine readily binds uracil or thymine. Uracil is, however, one product of [[Deamination#Cytosine|damage to cytosine]] that makes RNA particularly susceptible to mutations that can replace a '''GC''' base pair with a '''GU''' ([[wobble base pair|wobble]]) or '''AU''' [[base pair]].

RNA is thought to have preceded DNA, because of their ordering in the biosynthetic pathways.<ref name="Robertson2012" /> The deoxyribonucleotides used to make DNA are made from ribonucleotides, the building blocks of RNA, by removing the 2'-hydroxyl group. As a consequence, a cell must have the ability to make RNA before it can make DNA.

==== Limitations of information storage in RNA ====
The chemical properties of RNA make large RNA [[molecule]]s inherently fragile, and they can easily be broken down into their constituent nucleotides through [[hydrolysis]].<ref>{{cite journal | vauthors = Lindahl T | title = Instability and decay of the primary structure of DNA | journal = Nature | volume = 362 | issue = 6422 | pages = 709–715 | date = April 1993 | pmid = 8469282 | doi = 10.1038/362709a0 | s2cid = 4283694 | bibcode = 1993Natur.362..709L }}</ref><ref>{{cite journal | vauthors = Pääbo S | title = Ancient DNA | journal = Scientific American | volume = 269 | issue = 5 | pages = 86–92 | date = November 1993 | pmid = 8235556 | doi = 10.1038/scientificamerican1193-86 | s2cid = 5288515 | bibcode = 1993SciAm.269e..86P }}</ref> These limitations do not make use of RNA as an [[information storage]] system impossible, simply energy intensive (to repair or replace damaged RNA molecules) and prone to mutation. While this makes it unsuitable for current 'DNA optimised' life, it may have been acceptable for more primitive life.