Editing RNA (section)

===Basic chemical composition===
[[File:Piwi-siRNA-basepairing.png|thumb|upright=1.15|Watson-Crick base pairs in a [[siRNA]]. Hydrogen atoms are not shown.]]
Each [[nucleotide]] in RNA contains a [[ribose]] sugar, with carbons numbered 1' through 5'. A base is attached to the 1' position, in general, [[adenine]] (A), [[cytosine]] (C), [[guanine]] (G), or [[uracil]] (U). Adenine and guanine are [[purine]]s, and cytosine and uracil are [[pyrimidine]]s. A [[phosphate]] group is attached to the 3' position of one ribose and the 5' position of the next. The phosphate groups have a negative charge each, making RNA a charged molecule (polyanion). The bases form [[hydrogen bond]]s between cytosine and guanine, between adenine and uracil and between guanine and uracil.<ref name="pmid15561141"/> However, other interactions are possible, such as a group of adenine bases binding to each other in a bulge,<ref>{{cite book|title=RNA biochemistry and biotechnology| vauthors= Barciszewski J, Frederic B, Clark C | date = 1999 | pages = 73–87 | publisher = Springer | isbn = 978-0-7923-5862-6 | oclc = 52403776 }}</ref>
or the GNRA [[tetraloop]] that has a guanine–adenine base-pair.<ref name="pmid15561141">{{cite journal |vauthors=Lee JC, Gutell RR | title = Diversity of base-pair conformations and their occurrence in rRNA structure and RNA structural motifs | journal = Journal of Molecular Biology | volume = 344 | issue = 5 | pages = 1225–49 | date = December 2004 | pmid = 15561141 | doi = 10.1016/j.jmb.2004.09.072 }}</ref>