For other uses see RNA (disambiguation). A hairpin loop from a pre-mRNA. Highlighted are the nucleobases (green) and the ribose-phosphate backbone (blue).

Ribonucleic acid (RNA) is one of the three major macromolecules (along with DNA and proteins) that are essential for all known forms of life.

Like DNA RNA is made up of a long chain of components called nucleotides. Each nucleotide consists of a nucleobase (sometimes called a nitrogenous base) a ribose sugar and a phosphate group. The sequence of nucleotides allows RNA to encode genetic information. For example some viruses use RNA instead of DNA as their genetic material and all organisms use messenger RNA (mRNA) to carry the genetic information that directs the synthesis of proteins.

Like proteins some RNA molecules play an active role in cells by catalyzing biological reactions controlling gene expression or sensing and communicating responses to cellular signals. One of these active processes is protein synthesis a universal function whereby mRNA molecules direct the assembly of proteins on ribosomes. This process uses transfer RNA (tRNA) molecules to deliver amino acids to the ribosome where ribosomal RNA (rRNA) links amino acids together to form proteins.

The chemical structure of RNA is very similar to that of DNA with two differences (a) RNA contains the sugar ribose while DNA contains the slightly different sugar deoxyribose (a type of ribose that lacks one oxygen atom) and (b) RNA has the nucleobase uracil while DNA contains thymine (uracil and thymine have similar base-pairing properties).

Unlike DNA most RNA molecules are single-stranded. Single-stranded RNA molecules adopt very complex three-dimensional structures since they are not restricted to the repetitive double-helical form of double-stranded DNA. RNA is made within living cells by RNA polymerases enzymes that act to copy a DNA or RNA template into a new RNA strand through processes known as transcription or RNA replication respectively. Contents 1 Comparison with DNA 2 Structure 3 Synthesis 4 Types of RNA 4.1 Overview 4.2 In translation 4.3 Regulatory RNAs 4.4 In RNA processing 4.5 RNA genomes 4.6 In reverse transcription 4.7 Double-stranded RNA 5 Key discoveries in RNA biology 6 See also 7 References 8 External links Comparison with DNA Three-dimensional representation of the 50S ribosomal subunit. RNA is in ochre protein in blue. The active site is in the middle (red).

RNA and DNA are both nucleic acids but differ in three main ways. First unlike DNA which is in general double-stranded RNA is a single-stranded molecule in many of its biological roles and has a much shorter chain of nucleotides. Second while DNA contains deoxyribose RNA contains ribose (in deoxyribose there is no hydroxyl group attached to the pentose ring in the 2' position). These hydroxyl groups make RNA less stable than DNA because it is more prone to hydrolysis. Third the complementary base to adenine is not thymine as it is in DNA but rather uracil which is an unmethylated form of thymine.1

Like DNA most biologically active RNAs including mRNA tRNA rRNA snRNAs and other non-coding RNAs contain self-complementary sequences that allow parts of the RNA to fold and pair with itself to form double helices. Structural analysis of these RNAs has revealed that they are highly structured. Unlike DNA their structures do not consist of long double helices but rather collections of short helices packed together into structures akin to proteins. In this fashion RNAs can achieve chemical catalysis like enzymes.2 For instance determination of the structure of the ribosomean enzyme that catalyzes peptide bond formationrevealed that its active site is composed entirely of RNA.3 Structure 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 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 may form hydrogen bonds between cytosine and guanine between adenine and uracil and between guanine and uracil.4 However other interactions are possible such as a group of adenine bases binding to each other in a bulge5 or the GNRA tetraloop that has a guanineadenine base-pair.4 Chemical structure of RNA

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.6 This results in a very deep and narrow major groove and a shallow and wide minor groove.7 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.8 Secondary structure of a telomerase RNA. RNA is transcribed with only four bases (adenine cytosine guanine and uracil)9 but these bases and attached sugars can be modified in numerous ways as the RNAs mature. Pseudouridine () in which the linkage between uracil and ribose is changed from a CN bond to a CC bond and ribothymidine (T) are found in various places (the most notable ones being in the TC loop of tRNA).10 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.11 There are nearly 100 other naturally occurring modified nucleosides12 of which pseudouridine and nucleosides with 2'-O-methylribose are the most common.13 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.14 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.15 Since RNA is charged metal ions such as Mg2+ are needed to stabilise many secondary and tertiary structures.16 Synthesis Synthesis of RNA is usually catalyzed by an enzymeRNA polymeraseusing DNA as a template a process known as transcription. Initiation of transcription begins with the binding of the enzyme to a promoter sequence in the DNA (usually found "upstream" of a gene). The DNA double helix is unwound by the helicase activity of the enzyme. The enzyme then progresses along the template strand in the 3 to 5 direction synthesizing a complementary RNA molecule with elongation occurring in the 5 to 3 direction. The DNA sequence also dictates where termination of RNA synthesis will occur.17 RNAs are often modified by enzymes after transcription. For example a poly(A) tail and a 5' cap are added to eukaryotic pre-mRNA and introns are removed by the spliceosome. There are also a number of RNA-dependent RNA polymerases that use RNA as their template for synthesis of a new strand of RNA. For instance a number of RNA viruses (such as poliovirus) use this type of enzyme to replicate their genetic material.18 Also RNA-dependent RNA polymerase is part of the RNA interference pathway in many organisms.19 Types of RNA See also: List of RNAs Overview Structure of a hammerhead ribozyme a ribozyme that cuts RNA Messenger RNA (mRNA) is the RNA that carries information from DNA to the ribosome the sites of protein synthesis (translation) in the cell. The coding sequence of the mRNA determines the amino acid sequence in the protein that is produced.20 Many RNAs do not code for protein however (about 97% of the transcriptional output is non-protein-coding in eukaryotes 21222324). These so-called non-coding RNAs ("ncRNA") can be encoded by their own genes (RNA genes) but can also derive from mRNA introns.25 The most prominent examples of non-coding RNAs are transfer RNA (tRNA) and ribosomal RNA (rRNA) both of which are involved in the process of translation.1 There are also non-coding RNAs involved in gene regulation RNA processing and other roles. Certain RNAs are able to catalyse chemical reactions such as cutting and ligating other RNA molecules26 and the catalysis of peptide bond formation in the ribosome;3 these are known as ribozymes. In translation Messenger RNA (mRNA) carries information about a protein sequence to the ribosomes the protein synthesis factories in the cell. It is coded so that every three nucleotides (a codon) correspond to one amino acid. In eukaryotic cells once precursor mRNA (pre-mRNA) has been transcribed from DNA it is processed to mature mRNA. This removes its intronsnon-coding sections of the pre-mRNA. The mRNA is then exported from the nucleus to the cytoplasm where it is bound to ribosomes and translated into its corresponding protein form with the help of tRNA. In prokaryotic cells which do not have nucleus and cytoplasm compartments mRNA can bind to ribosomes while it is being transcribed from DNA. After a certain amount of time the message degrades into its component nucleotides with the assistance of ribonucleases.20 Transfer RNA (tRNA) is a small RNA chain of about 80 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has sites for amino acid attachment and an anticodon region for codon recognition that binds to a specific sequence on the messenger RNA chain through hydrogen bonding.25 Ribosomal RNA (rRNA) is the catalytic component of the ribosomes. Eukaryotic ribosomes contain four different rRNA molecules: 18S 5.8S 28S and 5S rRNA. Three of the rRNA molecules are synthesized in the nucleolus and one is synthesized elsewhere. In the cytoplasm ribosomal RNA and protein combine to form a nucleoprotein called a ribosome. The ribosome binds mRNA and carries out protein synthesis. Several ribosomes may be attached to a single mRNA at any time.20 rRNA is extremely abundant and makes up 80% of the 10 mg/ml RNA found in a typical eukaryotic cytoplasm.27 Transfer-messenger RNA (tmRNA) is found in many bacteria and plastids. It tags proteins encoded by mRNAs that lack stop codons for degradation and prevents the ribosome from stalling.28 Regulatory RNAs Several types of RNA can downregulate gene expression by being complementary to a part of an mRNA or a gene's DN. (Downregulation or DN is the process by which a cell decreases the quantity of a cellular component such as RNA or protein in response to an external variable. An increase of a cellular component is called upregulation). MicroRNAs (miRNA; 21-22 nt) are found in eukaryotes and act through RNA interference (RNAi) where an effector complex of miRNA and enzymes can break down mRNA to which the miRNA is complementary block the mRNA from being translated or accelerate its degradation.2930 While small interfering RNAs (siRNA; 20-25 nt) are often produced by breakdown of viral RNA there are also endogenous sources of siRNAs.3132 siRNAs act through RNA interference in a fashion similar to miRNAs. Some miRNAs and siRNAs can cause genes they target to be methylated thereby decreasing or increasing transcription of those genes.333435 Animals have Piwi-interacting RNAs (piRNA; 29-30 nt) which are active in germline cells and are thought to be a defense against transposons and play a role in gametogenesis.3637 Many prokaryotes have CRISPR RNAs a regulatory system similar to RNA interference.38 Antisense RNAs are widespread; most downregulate a gene but a few are activators of transcription.39 One way antisense RNA can act is by binding to an mRNA forming double-stranded RNA that is enzymatically degraded.40 There are many long noncoding RNAs that regulate genes in eukaryotes41 one such RNA is Xist which coats one X chromosome in female mammals and inactivates it.42 An mRNA may contain regulatory elements itself such as riboswitches in the 5' untranslated region or 3' untranslated region; these cis-regulatory elements regulate the activity of that mRNA.43 The untranslated regions can also contain elements that regulate other genes.44 In RNA processing Uridine to pseudouridine is a common RNA modification. Many RNAs are involved in modifying other RNAs. Introns are spliced out of pre-mRNA by spliceosomes which contain several small nuclear RNAs (snRNA)1 or the introns can be ribozymes that are spliced by themselves.45 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 generally directed by small nucleolar RNAs (snoRNA; 60-300 nt)25 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.4647 RNA genomes Like DNA RNA can carry genetic information. RNA viruses have genomes composed of RNA and a variety of proteins encoded by that genome. The viral genome is replicated by some of those proteins while other proteins protect the genome as the virus particle moves to a new host cell. Viroids are another group of pathogens but they consist only of RNA do not encode any protein and are replicated by a host plant cell's polymerase.48 In reverse transcription Reverse transcribing viruses replicate their genomes by reverse transcribing DNA copies from their RNA; these DNA copies are then transcribed to new RNA. Retrotransposons also spread by copying DNA and RNA from one another49 and telomerase contains an RNA that is used as template for building the ends of eukaryotic chromosomes.50 Double-stranded RNA Double-stranded RNA (dsRNA) is RNA with two complementary strands similar to the DNA found in all cells. dsRNA forms the genetic material of some viruses (double-stranded RNA viruses). Double-stranded RNA such as viral RNA or siRNA can trigger RNA interference in eukaryotes as well as interferon response in vertebrates.515253 Key discoveries in RNA biology Further information: History of RNA biology Research on RNA has led to many important biological discoveries and numerous Nobel Prizes. Nucleic acids were discovered in 1868 by Friedrich Miescher who called the material 'nuclein' since it was found in the nucleus.54 It was later discovered that prokaryotic cells which do not have a nucleus also contain nucleic acids. The role of RNA in protein synthesis was suspected already in 1939.55 Severo Ochoa won the 1959 Nobel Prize in Medicine (shared with Arthur Kornberg) after he discovered an enzyme that can synthesize RNA in the laboratory.56 Ironically the enzyme discovered by Ochoa (polynucleotide phosphorylase) was later shown to be responsible for RNA degradation not RNA synthesis. The sequence of the 77 nucleotides of a yeast tRNA was found by Robert W. Holley in 196557 winning Holley the 1968 Nobel Prize in Medicine (shared with Har Gobind Khorana and Marshall Nirenberg). In 1967 Carl Woese hypothesized that RNA might be catalytic and suggested that the earliest forms of life (self-replicating molecules) could have relied on RNA both to carry genetic information and to catalyze biochemical reactionsan RNA world.5859 During the early 1970s retroviruses and reverse transcriptase were discovered showing for the first time that enzymes could copy RNA into DNA (the opposite of the usual route for transmission of genetic information). For this work David Baltimore Renato Dulbecco and Howard Temin were awarded a Nobel Prize in 1975. In 1976 Walter Fiers and his team determined the first complete nucleotide sequence of an RNA virus genome that of bacteriophage MS2.60 In 1977 introns and RNA splicing were discovered in both mammalian viruses and in cellular genes resulting in a 1993 Nobel to Philip Sharp and Richard Roberts. Catalytic RNA molecules (ribozymes) were discovered in the early 1980s leading to a 1989 Nobel award to Thomas Cech and Sidney Altman. In 1990 it was found in petunia that introduced genes can silence similar genes of the plant's own now known to be a result of RNA interference.6162 At about the same time 22 nt long RNAs now called microRNAs were found to have a role in the development of C. elegans.63 Studies on RNA interference gleaned a Nobel Prize for Andrew Fire and Craig Mello in 2006 and another Nobel was awarded for studies on transcription of RNA to Roger Kornberg in the same year. 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"The potential of oligonucleotides for therapeutic applications". Trends in Biotechnology 24 (12): 56370. doi:10.1016/j.tibtech.2006.10.003. PMID 17045686.  External links RNA World website Link collection (structures sequences tools journals) Nucleic Acid Database Images of DNA RNA and complexes. v d eGenetics Introduction  History  Related topics  List of organizations Key components Chromosome  DNA  Nucleotide  RNA  Genome Fields of genetics Classical genetics  Conservation genetics  Ecological genetics  Immunogenetics  Molecular genetics  Population genetics  Quantitative genetics Archaeogenetics of... Americas  British Isles  Europe  Italy  Near East  South Asia Related topics Geneticist  Genomics  Genetic code  Medical genetics  Molecular evolution  Reverse genetics  Genetic engineering  Genetic diversity  Heredity  Genetic monitoring v d eGene expression Introduction to genetics General flow: DNA > RNA > Protein special transfers (RNA > RNA RNA > DNA Protein > Protein) Genetic code Transcription (Transcription factors RNA Polymerasepromoter) Prokaryotic / Archaeal / Eukaryotic post-transcriptional modification (hnRNA5' cappingSplicingPolyadenylation) Translation (RibosometRNA) Prokaryotic / Archaeal / Eukaryotic post-translational modification (functional groups peptides structural changes) Gene regulation epigenetic regulation (Genomic imprinting) transcriptional regulation post-transcriptional regulation (sequestration alternative splicing miRNA) translational regulation post-translational regulation (reversible irreversible) v d eTypes of RNA Protein synthesis Messenger RNA  Ribosomal RNA  Signal recognition particle RNA  Transfer RNA  Transfer-messenger RNA RNA processing Small nuclear RNA  Small nucleolar RNA  Guide RNA  RNase P  RNase MRP  Y RNA Gene regulation Antisense RNA  Cis-natural antisense transcript  CRISPR RNA  Long noncoding RNA  MicroRNA  Piwi-interacting RNA  Repeat-associated siRNA  Small interfering RNA  Small temporal RNA  Trans-acting siRNA Cis-regulatory elements Riboswitch  SECIS element Parasites Retrotransposon  Reverse transcribing virus  RNA virus  Viroid Other Telomerase RNA  Vault RNA v d eTypes of nucleic acids Constituents Nucleobases  Nucleosides  Nucleotides  Deoxynucleotides Ribonucleic acids (coding and non-coding) translation: mRNA (pre-mRNA/hnRNA)  tRNA  rRNA  tmRNA regulatory: miRNA  siRNA  piRNA  aRNA   RNAi   RNA processing: snRNA  snoRNA other/ungrouped: gRNA  shRNA  stRNA  ta-siRNA Deoxyribonucleic acids cDNA  cpDNA  gDNA  msDNA  mtDNA Nucleic acid analogues GNA  LNA  BNA  PNA  TNA  morpholino Cloning vectors phagemid  plasmid  lambda phage  cosmid  fosmid  PAC  BAC  YAC  HAC biochemical families: prot  nucl  carb (glpr alco glys)  lipd (fata/i phld strd gllp eico)  amac/i  ncbs/i  ttpy/i