Question: Where can you find RNA nucleotides?
About the article "RNA", section "In RNA processing".

Candidate passages:
1986–2000.RNA sequences can be edited within cells.
Messenger RNA precursors from a wide range of organisms can be edited before being translated into protein.
In this process, non-encoded nucleotides may be inserted into specific sites in the RNA, and encoded nucleotides may be removed or replaced.
RNA editing was first discovered within the mitochondria of kinetoplastid protozoans, where it has been shown to be extensive.
For example, some protein-coding genes encode fewer than 50% of the nucleotides found within the mature, translated mRNA.
Other RNA editing events are found in mammals, plants, bacteria and viruses.
These latter editing events involve fewer nucleotide modifications, insertions and deletions than the events within kinetoplast DNA, but still have high biological significance for gene expression and its regulation. 
The ryfA RNA gene is a non-coding RNA present in "E. coli", "Shigella flexneri" and "Salmonella" species where it is found between the "ydaN" and "dbpA" genes.
These RNA genes are about 300 nucleotides in length.
The function of this RNA is unknown. 
The RNA world hypothesis is supported by RNA's ability to store, transmit, and duplicate genetic information, as DNA does.
RNA can act as a ribozyme, a special type of enzyme.
Because it can perform the tasks of both DNA and enzymes, RNA is believed to have once been capable of supporting independent life forms.
Some viruses use RNA as their genetic material, rather than DNA.
Further, while nucleotides were not found in Miller-Urey's origins of life experiments, their formation in prebiotically plausible conditions has now been reported, as noted above; the purine base known as adenine is merely a pentamer of hydrogen cyanide.
Experiments with basic ribozymes, like Bacteriophage Qβ RNA, have shown that simple self-replicating RNA structures can withstand even strong selective pressures (e.g., opposite-chirality chain terminators).
Since there were no known chemical pathways for the abiogenic synthesis of nucleotides from pyrimidine nucleobases cytosine and uracil under prebiotic conditions, it is thought by some that nucleic acids did not contain these nucleobases seen in life's nucleic acids.
The nucleoside cytosine has a half-life in isolation of 19 days at and 17,000 years in freezing water, which some argue is too short on the geologic time scale for accumulation.
Others have questioned whether ribose and other backbone sugars could be stable enough to find in the original genetic material, and have raised the issue that all ribose molecules would have had to be the same enantiomer, as any nucleotide of the wrong chirality acts as a chain terminator.
Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised by a sequence of reactions that by-pass free sugars and assemble in a stepwise fashion by going against the dogma that nitrogenous and oxygenous chemistries should be avoided.
In a series of publications, The "Sutherland Group" at the School of Chemistry, University of Manchester have demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene.
One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater, of possible interest towards biological homochirality.
This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallised out from a mixture of the other pentose aminooxazolines.
Aminooxazolines can react with cyanoacetylene in a mild and highly efficient manner, controlled by inorganic phosphate, to give the cytidine ribonucleotides.
Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry, one problem with this chemistry is the selective phosphorylation of alpha-cytidine at the 2' position.
However, in 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerise into RNA.
This was hailed as strong evidence for the RNA world.
The paper also highlighted the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates.
A potential weakness of these routes is the generation of enantioenriched glyceraldehyde, or its 3-phosphate derivative (glyceraldehyde prefers to exist as its keto tautomer dihydroxyacetone).
On August 8, 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks of RNA (adenine, guanine and related organic molecules) may have been formed extraterrestrially in outer space.
On August 29, 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system.
The molecule was found around the protostellar binary "IRAS 16293-2422", which is located 400 light years from Earth.
Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA.
This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation. 
Many RNAs are involved in modifying other RNAs.
Introns are spliced out of pre-mRNA by spliceosomes, which contain several small nuclear RNAs (snRNA), or the introns can be ribozymes that are spliced by themselves.RNA can also be altered by having its nucleotides modified to other nucleotides than A, C, G and U.
In eukaryotes, modifications of RNA nucleotides are in general directed by small nucleolar RNAs (snoRNA; 60-300 nt), found in the nucleolus and cajal bodies.
snoRNAs associate with enzymes and guide them to a spot on an RNA by basepairing to that RNA.
These enzymes then perform the nucleotide modification.
rRNAs and tRNAs are extensively modified, but snRNAs and mRNAs can also be the target of base modification.
RNA can also be methylated.
One strand of the DNA, the "template strand" (or noncoding strand), is used as a template for RNA synthesis.
As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy.
Although RNA polymerase traverses the template strand from 3' → 5', the coding (non-template) strand and newly formed RNA can also be used as reference points, so transcription can be described as occurring 5' → 3'.
This produces an RNA molecule from 5' → 3', an exact copy of the coding strand (except that thymines are replaced with uracils, and the nucleotides are composed of a ribose (5-carbon) sugar where DNA has deoxyribose (one fewer oxygen atom) in its sugar-phosphate backbone).mRNA transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription (amplification of particular mRNA), so many mRNA molecules can be rapidly produced from a single copy of a gene.
Elongation also involves a proofreading mechanism that can replace incorrectly incorporated bases.
In eukaryotes, this may correspond with short pauses during transcription that allow appropriate RNA editing factors to bind.
These pauses may be intrinsic to the RNA polymerase or due to chromatin structure.