Question: What is the most common modified base in RNA?
About the article "Nucleic acid sequence", section "Notation".

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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 purines, cytosine and uracil are pyrimidines.
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 at physiological pH, making RNA a charged molecule (polyanion).
The bases form hydrogen bonds between cytosine and guanine, between adenine and uracil and between guanine and uracil.
However, other interactions are possible, such as a group of adenine bases binding to each other in a bulge,or the GNRA tetraloop that has a guanine–adenine base-pair.
An important structural feature of RNA that distinguishes it from DNA is the presence of a hydroxyl group at the 2' position of the ribose sugar.
The presence of this functional group causes the helix to adopt the A-form geometry rather than the B-form most commonly observed in DNA.
This results in a very deep and narrow major groove and a shallow and wide minor groove.
A second consequence of the presence of the 2'-hydroxyl group is that in conformationally flexible regions of an RNA molecule (that is, not involved in formation of a double helix), it can chemically attack the adjacent phosphodiester bond to cleave the backbone.RNA is transcribed with only four bases (adenine, cytosine, guanine and uracil), but these bases and attached sugars can be modified in numerous ways as the RNAs mature.
Pseudouridine (Ψ), in which the linkage between uracil and ribose is changed from a C–N bond to a C–C bond, and ribothymidine (T) are found in various places (the most notable ones being in the TΨC loop of tRNA).
Another notable modified base is hypoxanthine, a deaminated adenine base whose nucleoside is called inosine (I).
Inosine plays a key role in the wobble hypothesis of the genetic code.
There are more than 100 other naturally occurring modified nucleosides, The greatest structural diversity of modifications can be found in tRNA, while pseudouridine and nucleosides with 2'-O-methylribose often present in rRNA are the most common.
The specific roles of many of these modifications in RNA are not fully understood.
However, it is notable that, in ribosomal RNA, many of the post-transcriptional modifications occur in highly functional regions, such as the peptidyl transferase center and the subunit interface, implying that they are important for normal function.
The functional form of single-stranded RNA molecules, just like proteins, frequently requires a specific tertiary structure.
The scaffold for this structure is provided by secondary structural elements that are hydrogen bonds within the molecule.
This leads to several recognizable "domains" of secondary structure like hairpin loops, bulges, and internal loops.
Since RNA is charged, metal ions such as Mg2+ are needed to stabilise many secondary and tertiary structures. 
Nucleobases are nitrogen-containing biological compounds (nitrogenous bases) found within nucleotides—the basic building blocks of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Nucleobases are also found in nucleosides (a nucleobase linked to a sugar).
Often simply called "bases" in genetics, their ability to form base-pairs and to stack upon one another lead directly to the helical structure of DNA and RNA.
Use of the word "base" is historical, in reference to the chemical properties of nucleobases in acid-base reactions within the test tube, and is not especially relevant or important for understanding most of their biological functions.
The primary nucleobases are cytosine (DNA and RNA), guanine (DNA and RNA), adenine (DNA and RNA), thymine (DNA) and uracil (RNA), abbreviated as C, G, A, T, and U, respectively.
Because A, G, C, and T appear in the DNA, these molecules are called DNA-bases; A, G, C, and U are called RNA-bases.
Uracil replaces thymine in RNA.
These two bases are identical except that uracil lacks the 5' methyl group.
Adenine and guanine belong to the double-ringed class of molecules called purines (abbreviated as R).
Cytosine, thymine, and uracil are all pyrimidines (abbreviated as Y).
In normal spiral DNA the bases form pairs between the two strands: A with T and C with G. Purines pair with pyrimidines mainly for dimensional reasons - only this combination fits the constant width geometry of the DNA spiral.
The A-T and C-G pairings are required to match the hydrogen bonds between the amine and carbonyl groups on the complementary bases.
The compound formed when a nucleobase forms a glycosidic bond with the 1' anomeric carbon of a ribose or deoxyribose is called a "nucleoside", and a nucleoside with one or more phosphate groups attached at the 5' carbon is called a "nucleotide".
Apart from adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U), DNA and RNA also contain bases that have been modified after the nucleic acid chain has been formed.
In DNA, the most common modified base is 5-methylcytosine (m5C).
In RNA, there are many modified bases, including those contained in the nucleosides pseudouridine (Ψ), dihydrouridine (D), inosine (I), and 7-methylguanosine (m7G).
Hypoxanthine and xanthine are two of the many bases created through mutagen presence, both of them through deamination (replacement of the amine-group with a carbonyl-group).
Hypoxanthine is produced from adenine, xanthine from guanine.
In similar manner, deamination of cytosine results in uracil.
In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting nucleobases (such as adenine, guanine, xanthine, hypoxanthine, purine, 2,6-diaminopurine, and 6,8-diaminopurine) may have been formed in outer space. 
RNA silencing (associated with the concept of post-transcriptional gene silencing (PTGS) or RNA interference) refers to a family of gene silencing effects by which the expression of one or more genes is downregulated or entirely suppressed by small non-coding RNAs.
It may also refer to the introduction of a synthetic antisense RNA molecule used in scientific experiments on gene expression.
RNA silencing may also be defined as sequence-specific regulation of gene expression triggered by double-stranded RNA (dsRNA).
RNA silencing mechanisms are highly conserved in most eukaryotes.
The most common and well-studied example is RNA interference (RNAi), in which endogenously expressed microRNA (miRNA) or exogenously derived small interfering RNA (siRNA) induces the degradation of complementary messenger RNA.
Other classes of small RNA have been identified, including piwi-interacting RNA (piRNA) and its subspecies repeat associated small interfering RNA (rasiRNA). 
RNA extraction is the purification of RNA from biological samples.
This procedure is complicated by the ubiquitous presence of ribonuclease enzymes in cells and tissues, which can rapidly degrade RNA.
Several methods are used in molecular biology to isolate RNA from samples, the most common of these is Guanidinium thiocyanate-phenol-chloroform extraction.RNA extraction in liquid nitrogen, commonly using a mortar and pestle, or specialised motorized grinding device cell lysis device is also useful in preventing ribonuclease activity. 
In ribonucleotides, the sugar component is ribose while in deoxyribonucleotides, the sugar component is deoxyribose.
Instead of a hydroxyl group at the second carbon in the ribose ring, it is replaced by a hydrogen atom.
Both types of pentoses in DNA and RNA are in their β-furanose (closed five-membered ring) form and they define the identity of a nucleic acid.
DNA is defined by containing 2'-deoxy-D-ribose nucleic acid while RNA is defined by containing D-ribose nucleic acid.
In some occasions, DNA and RNA may contain some minor bases.
Methylated forms of the major bases are most common in DNA.
In viral DNA, some bases may be hydroxymethylated or glucosylated.
In RNA, minor or modified bases occur more frequently.
Some examples include hypoxanthine, dihydrouracil, methylated forms of uracil, cytosine, and guanine, as well as modified nucleoside pseudouridine.
Nucleotides with phosphate groups in positions other than on the 5' carbon have also been observed.
Examples include ribonucleoside 2',3'-cyclic monophosphates which are isolatable intermediates, and ribonucleoside 3'-monophosphates which are end products of the hydrolysis of RNA by certain ribonucleases.
Other variations include adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP).