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American Heritage Dictionary:

RNA

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(är'ĕn-ā') pronunciation
n.
A polymeric constituent of all living cells and many viruses, consisting of a long, usually single-stranded chain of alternating phosphate and ribose units with the bases adenine, guanine, cytosine, and uracil bonded to the ribose. RNA molecules are involved in protein synthesis and sometimes in the transmission of genetic information. Also called ribonucleic acid.

[R(IBO)N(UCLEIC) A(CID).]


Britannica Concise Encyclopedia:

RNA

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One of the two main types of nucleic acid (the other being DNA), which functions in cellular protein synthesis in all living cells and replaces DNA as the carrier of genetic information in some viruses. Like DNA, it consists of strands of repeating nucleotides joined in chainlike fashion, but the strands are single (except in certain viruses), and it has the nucleotide uracil (U) where DNA has thymine (T). Messenger RNA (mRNA), a single strand copied from a DNA strand that acts as its template, carries the message of the genetic code from DNA (in chromosomes) to the site of protein synthesis (on ribosomes). Ribosomal RNA (rRNA), part of the building blocks of ribosomes, participates in protein synthesis. Transfer RNA (tRNA), the smallest type, has fewer than 100 nucleotide units (mRNA and rRNA contain thousands). Each nucleotide triplet on mRNA specifies which amino acid comes next on the protein being synthesized, and a tRNA molecule with that triplet's complement on its protruding end brings the specified amino acid to the site of synthesis to be linked into the protein. Various minor types of RNA also exist; at least some act as catalysts (ribozymes), a function long ascribed only to proteins.

For more information on RNA, visit Britannica.com.

Gale Genetics Encyclopedia:

RNA

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Ribonucleic acid (RNA) molecules, which are linear chains (or polymers) of ribonucleotides, perform a number of critical functions. Many of these functions are related to protein synthesis. Some RNA molecules bring genetic information from a cell's chromosomes to its ribosomes, where proteins are assembled. Others help ribosomes translate genetic information to assemble specific sequences of amino acids.

Molecular Structure

Ribonucleotides, the building blocks of RNA, are molecules that consist of a nitrogen-containing base, a phosphate group, and ribose, a five-carbon sugar. The nitrogen-containing base may be adenine, cytosine, guanine, or uracil. These four bases are abbreviated as A, C, G, and U.

RNA is similar to deoxyribonucleic acid (DNA), another class of nucleic acid. However, DNA nucleotides contain deoxyribose, not ribose, and they use the nitrogen-containing base thymine (T), not uracil, along with ade-nine, cytosine, and guanine.

The nucleotides in DNA and RNA molecules are linked together to form chains. The link between two nucleotides is between a phosphate group attached to the fifth (5′ or "five prime") carbon of the sugar on one nucleotide and a hydroxyl group on the third (3′ or "three prime") carbon of the sugar on the other. The link is called a 5′-3′ phosphodiester bond.

RNA, therefore, can be described as a chain of ribose sugars linked together by phosphodiester bonds, with a base protruding from each sugar, as shown in the figure below. The 5′-3′ linkage gives RNA directionality, or polarity, and results in its having two ends with different chemical structures. The 5′ end usually has one or three free phosphate groups, and the 3′ end usually has a free hydroxyl group.

Whereas DNA is usually double-stranded, with the bases on one strand pairing up with those on the other, RNA usually exists as single chains of nucleotides. The bases in RNA do, however, follow Watson-Crick base-pair rules: A and U can pair with each other, as can G and C. There is usually extensive pairing of bases within a single strand of RNA.

RNA strands fold, with the bases in one part of the strand pairing with the bases in another. Folding can create both "secondary" and "tertiary" structures. Secondary structures are those that can be described in two dimensions and that can be thought of as simple loops or helices. Tertiary structures are complex, three-dimensional shapes.

The most common secondary structures, "hairpins," "loops," and "pseudo-knots," are shown in the figure below. Such secondary structures are formed when hydrogen bonds form between bases in the nucleotides and by the stacking of bases to form helical structures.

Tertiary structures usually involve interactions between nucleotides that are distant from each other along an RNA strand. Such interactions may arise from hydrogen bonding between bases, as in regular Watson-Crick base pairing, or from interactions among other chemical groups in the nucleotides. Some RNA molecules, such as ribosomal RNA (rRNA) and transfer RNA (tRNA), have structures that are very complex. In structure they resemble proteins more than they do DNA.

To understand the function of a given RNA molecule, scientists often need to know its structure. There are three general strategies for analyzing RNA structure. First, using the relatively simple base-pairing rules for RNA and the basic principles of thermodynamics, computers can be used to predict secondary RNA structure, although not always with complete success.

Second, researchers can analyze RNA molecules from various organisms and compare those molecules that have the same function. Even when the nucleotide sequences vary between species, important structures are usually preserved.

Third, the structure of an RNA molecule can be determined experimentally, using enzymes to cut it or chemicals to modify it. Some enzymes and chemicals cut or modify only nonpaired, single-stranded portions of the RNA molecule, allowing researchers to identify double-stranded regions by examining which ones remain uncut and unmodified.

Despite the usefulness of each of these methods, none can provide a complete and accurate three-dimensional structure. A more complete determination of structure can be achieved by the biophysical methods of X-ray crystallography and nuclear magnetic resonance.

Synthesis

RNA molecules are synthesized by enzymes known as RNA polymerases in a process called transcription. Usually, one strand of a double-stranded DNA molecule is used as a template for the RNA. The order of ribonucleotides that are assembled to form the RNA molecule is determined by the order of the deoxyribonucleotides in the DNA strand. The genetic information in the DNA sequence is thus reproduced in the RNA molecule. Sometimes, but rarely, an RNA molecule is synthesized using another RNA molecule as the template.

Often, when RNA molecules are synthesized, they are in a form that prevents them from carrying out their function. To become functional, they must undergo processing, which can involve removing segments of the strands or modifying specific nucleotides. The link between a base and a ribose may be altered, or extra chemical groups may be added to the bases or ribose molecules. Many RNA molecules are associated with proteins during or after their synthesis. Together, the RNA and protein are referred to as RNA-protein particles (RNPs).

In eukaryotes, RNA that is encoded by nuclear chromosomes is synthesized in the nucleus. The processing and assembly of many small RNA molecules in higher eukaryotes is accomplished in Cajal bodies, which are coiled structures in the nucleus that were identified more than 100 years ago but that have begun to be investigated in detail only recently. The synthesis of those RNA molecules that are components of ribosomes occurs in the nucleolus, a part of the nucleus. RNA synthesis and processing also occurs in the mitochondria and chloroplasts, when the RNA will be used in those organelles.

After being processed and assembled, RNPs either remain in the nucleus or are exported to the cytoplasm through the nuclear pores. Some are also exported and modified in the cytoplasm and then imported back into the nucleus. In prokaryotes, where there is no nucleus, the synthesis and processing of RNA, as well as the assembly of RNPs, occurs in the cytoplasm.

Function

Almost all types of RNA play a role in translation, which is the process of protein synthesis. Translation requires three types of RNA: messenger RNA (mRNA), which ranges in length from a few hundred to many thousands of nucleotides; tRNA, which is 75 to 85 nucleotides long; and rRNA, which is 1,500 to 4,000 nucleotides long.

Molecules of mRNA, each of which contains a copy of at least one gene, are the intermediates between DNA and protein. These mRNA molecules bring the genetic code from the DNA, which is in the nucleus, to ribosomes, which are in the cytoplasm. They attach to the ribosomes and determine the order in which amino acids are assembled to synthesize a protein. Of the three types of RNA required for translation, mRNA molecules have the simplest structure.

Next, tRNA molecules function as adapters that help translate the nucleotide sequences in mRNA into amino acid sequences, so specific proteins can be constructed. There are many different types of tRNA, each of which is capable of binding to one of the twenty amino acids that are the building blocks of proteins.

Finally, rRNA molecules, which account for most of a ribosome's mass, are, according to recent experiments, the part of the ribosome responsible for linking amino acids into a growing protein chain. Ribosomes, the organelles that assemble a particular sequence of amino acids to form proteins, contain three or four different molecules of rRNA, along with at least fifty different proteins.

Both rRNA and tRNA are stable forms of RNA that last through several cell divisions. In contrast, mRNA is normally unstable, with a lifetime that can be as short as a few minutes. This instability has probably evolved because it lets cells quickly stop synthesizing proteins that are no longer needed. In some cases, enzymes called ribonucleases (RNases) actively degrade certain mRNA molecules. For example, mRNA that encodes a particular protein regulating the cell cycle is degraded when the protein has carried out its function.

In certain cells, mRNA can exist in a stable form for decades. When egg cells are formed, for example, some of the mRNA in the cells is associated with "storage proteins" and lasts until after the eggs are fertilized. During embryonic development, this maternal mRNA becomes activated for translation and associates with translating ribosomes. It usually decays after it has been used to produce a certain amount of protein.

Less Common Types of Rna

Several types of less abundant, small RNA molecules perform essential functions in both the nucleus and the cytoplasm. All organisms contain cytoplasmic RNPs that are involved in exporting proteins from cells. During the synthesis of proteins that are destined to be exported, the ribosome and mRNA associate with an "export-RNP," which helps them dock at an export pore in the cell membrane. As it formed, the protein is threaded through the membrane to the outside of the cell. In eukaryotes, this same strategy is used to transport proteins into the endoplasmic reticulum, where some newly synthesized proteins are sorted and modified.

RNase P is another RNP found in all forms of life. This RNA-containing enzyme helps turn precursor tRNA into mature tRNA molecules. It does so by cleaving a section off the 5′ end of the precursor molecules.

Small nucleolar RNAs, which are known as snoRNAs and which are found in the nucleoli of eukaryotes and in Archaea, are required for the processing of precursor rRNA. During the assembly of new ribosomes, snoRNAs help remove regions of the precursor molecules and modify specific nucleotides.

Often, mRNA molecules in eukaryotes and in Archaea contain sequences that do not code for amino acids. These sequences, called introns, must be spliced out before translation begins. In eukaryotes, small nuclear RNAs (snRNAs) in the nucleus remove these introns. Once the introns are removed, the mature mRNA molecules are exported, through nuclear pores, into the cytoplasm, where they associate with ribosomes for translation.

Some viral genomes consist of single-stranded or double-stranded RNA, not DNA. Examples are found among both prokaryotic and eukaryotic viruses and include HIV, as well as viruses causing some forms of cancer.

Bibliography

Lodish, Harvey, et al. Molecular Cell Biology, 4th ed. New York: W. H. Freeman, 2000.

Meili, M., B. Albert-Fournier, and M. C. Maurel. "Recent Findings in the Modern RNA World." International Microbiology 4 (2001): 5-11.

Robinson, Richard. Biology. Farmington Hills, MI: Macmillan Reference USA, 2002.

Storz, G. "An Expanding Universe of Non-coding RNAs." Science 296 (2002): 1260-1263.

—Lasse Lindahl

Dictionary of Cultural Literacy: Science:

RNA

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One of a group of molecules similar in structure to a single strand of DNA. The function of RNA is to carry the information from DNA in the cell's nucleus into the body of the cell, to use the genetic code to assemble proteins, and to comprise part of the ribosomes that serve as the platform on which protein synthesis takes place.

Oxford Dictionary of Biochemistry:

RNA

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abbr. and common name for ribonucleate (def. 1) or ribonucleic acid; one of the two main types of nucleic acid, consisting of a long, unbranched macromolecule formed from ribonucleotides, the 3′-phosphate group of each constituent ribonucleotide (except the last) being joined in 3′,5′-phosphodiester linkage to the 5′-hydroxyl group on each ribose moiety of the next one. The presence of a free 2′-hydroxyl group on each ribose moiety renders these phosphodiester bonds susceptible to hydrolytic attack by alkali, in contrast to those of DNA. The RNA chain has polarity, with one 5′ end and one 3′end. Two purines, adenine and guanine, and two pyrimidines, cytosine and uracil, are the major bases usually present. In addition, minor bases may occur; transfer RNA, however, contains unusual bases in relatively large amounts. The sequence of bases carries information, whereas the sugar and phosphate groups play a structural role. RNA is fundamental to protein biosynthesis in all living cells. Messenger RNA (mRNA) is responsible for carrying the coded genetic 'message', transcribed from DNA, to sites of protein assembly at the ribosomes. The latter are composed of ribosomal RNA (rRNA) plus proteins. A third species, transfer RNA (tRNA), is instrumental in importing amino acids to the assembly site, according to instructions carried by mRNA. In some viruses, RNA is the genetic material instead of DNA. Specific forms, functions, molecules, or preparations of RNA may be designated by prefixes or suffixes, thus: A-RNA or RNA-A, A-form of RNA; cRNA, complementary RNA; dsRNA, double-strand(ed) RNA; hnRNA or H-RNA or HnRNA, heterogeneous nuclear RNA; mRNA, messenger RNA; micRNA, messenger-RNA-interfering complementary RNA; mtRNA, mitochondrial RNA; nRNA, nuclear RNA; rRNA, ribosomal RNA; sRNA or S-RNA, soluble RNA; scRNA, small cytoplasmic RNA; snRNA, small nuclear RNA; snoRNA, small nucleolar RNA; ssRNA, single-strand(ed) RNA; tRNA, transfer RNA; tcRNA, translational-control RNA; Z-RNA or RNA-Z, Z-form of RNA.

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Saunders Veterinary Dictionary:

RNA

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Ribonucleic acid.

  • RNA interference (RNAi) — the functional inactivation of specific genes by experimental introduction of a corresponding double stranded RNA, which induces degradation of the complementary single-stranded mRNA encoded by the gene but not that of mRNAs with different sequences. See microRNA and gene silencing.
  • microRNA (miRNA) — small RNAs containing 21 to 33 nucleotides that associated with multiple proteins in a RNA-induced silencing complex (RISC) that repress transcription of specific target mRNA by hybridizing to its 3’ untranslated region.
  • RNA primer — a sequence of about 10 nucleotides long copied from DNA by RNA primase and required for the priming of the synthesis of each Okazaki fragment during DNA replication.
  • secondary structure RNA — folding of single-stranded RNA molecules which arises from intramolecular base pairing.
  • small cytoplasmic RNA (scRNA) — small (7S; 129 nucleotides) RNA molecules found in the cytosol and rough endoplasmic reticulum associated with proteins that are involved in specific selection and transport of other proteins.
  • small nuclear RNA (snRNA) — a general term for many diferent kinds of small RNA molecules found in the nucleus of a cell that include as examples species involved in splicing of introns from mRNA and in RNA interference.
  • RNA viruses — viruses distinguished by having a ribonucleic acid genome, usually as a single strand which may be positive or negative sense, a single molecule or a segmented; in at least two families the genome is a double-strand segmented form.
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Wikipedia on Answers.com:

RNA

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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 /raɪbɵ.njuːˌkleɪ.ɨk ˈæsɪd/, or RNA, is part of a group of molecules known as the nucleic acids, which are one of the four major macromolecules (along with lipids, carbohydrates and proteins) 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, a ribose sugar, and a phosphate group. The sequence of nucleotides allows RNA to encode genetic information. All cellular organisms use messenger RNA (mRNA) to carry the genetic information that directs the synthesis of proteins. In addition, many viruses use RNA instead of DNA as their genetic material.

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. Unlike DNA, most RNA molecules are single-stranded and can adopt very complex three-dimensional structures.

Contents

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:

  • Unlike double-stranded DNA, RNA is a single-stranded molecule in many of its biological roles and has a much shorter chain of nucleotides.
  • 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.
  • 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[2] and pair with itself to form double helices. 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.[3] For instance, determination of the structure of the ribosome—an enzyme that catalyzes peptide bond formation—revealed that its active site is composed entirely of RNA.[4]

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.[5] However, other interactions are possible, such as a group of adenine bases binding to each other in a bulge,[6] or the GNRA tetraloop that has a guanine–adenine base-pair.[5]

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.[7] This results in a very deep and narrow major groove and a shallow and wide minor groove.[8] 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.[9]

Secondary structure of a telomerase RNA.

RNA is transcribed with only four bases (adenine, cytosine, guanine and uracil),[10] 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).[11] 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.[12]

There are nearly 100 other naturally occurring modified nucleosides,[13] of which pseudouridine and nucleosides with 2'-O-methylribose are the most common.[14] 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.[15]

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.[16] Since RNA is charged, metal ions such as Mg2+ are needed to stabilise many secondary and tertiary structures.[17]

Synthesis

Synthesis of RNA is usually catalyzed by an enzyme—RNA polymerase—using 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.[18]

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.[19] Also, RNA-dependent RNA polymerase is part of the RNA interference pathway in many organisms.[20]

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.[21] Many RNAs do not code for protein however (about 97% of the transcriptional output is non-protein-coding in eukaryotes [22][23][24][25]).

These so-called non-coding RNAs ("ncRNA") can be encoded by their own genes (RNA genes), but can also derive from mRNA introns.[26] 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 molecules,[27] and the catalysis of peptide bond formation in the ribosome;[4] 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 introns—non-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.[21]

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.[26]

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.[21] Nearly all the RNA found in a typical eukaryotic cell is rRNA.

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 DNA. MicroRNAs (miRNA; 21-22 nt) are found in eukaryotes and act through RNA interference (RNAi), where an effector complex of miRNA and enzymes can cleave complementary mRNA, block the mRNA from being translated, or accelerate its degradation.[29][30] While small interfering RNAs (siRNA; 20-25 nt) are often produced by breakdown of viral RNA, there are also endogenous sources of siRNAs.[31][32]

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.[33][34][35] Animals have Piwi-interacting RNAs (piRNA; 29-30 nt) that are active in germline cells and are thought to be a defense against transposons and play a role in gametogenesis.[36][37]

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 eukaryotes,[41] 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 in general directed by small nucleolar RNAs (snoRNA; 60-300 nt),[26] 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.[46][47] RNA can also be methylated.[48][49]

RNA genomes

Like DNA, RNA can carry genetic information. RNA viruses have genomes composed of RNA that encodes a number of proteins. 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.[50]

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 another,[51] and telomerase contains an RNA that is used as template for building the ends of eukaryotic chromosomes.[52]

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.[53][54][55]

Key discoveries in RNA biology

Further information: History of RNA biology
Robert W. Holley, left, poses with his research team.

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.[56] 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.[57] 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.[58] However, 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 1965,[59] 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 reactions—an RNA world.[60][61]

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.[62]

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.[63][64]

At about the same time, 22 nt long RNAs, now called microRNAs, were found to have a role in the development of C. elegans.[65] 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. The discovery of gene regulatory RNAs has led to attempts to develop drugs made of RNA, such as siRNA, to silence genes.[66]

See also

References

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External links
Key components
Fields of genetics
Archaeogenetics of...
Related topics
Introduction to genetics
Transcription
Translation

(Ribosome,tRNA) Prokaryotic / Archaeal / Eukaryotic

post-translational modification (functional groups, peptides, structural changes)
Gene regulation
Types of RNA
Protein synthesis
RNA processing
Gene regulation
Cis-regulatory elements
Parasites
Other
Types of nucleic acids
Constituents
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 · SgRNASutherland RNA
Deoxyribonucleic acids
cDNA · cpDNA · gDNA · msDNA · mtDNA
Nucleic acid analogues
GNA · LNA · PNA · TNA · morpholino
Cloning vectors
phagemid · plasmid · lambda phage · cosmid · fosmid · PAC · BAC · YAC · HAC


This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Translations:

Rna

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Dansk (Danish)
n. - RNA (ribonukleinsyre)

Nederlands (Dutch)
RNA, ribonucleïnezuur

Français (French)
n. - (Biol) acide ribonucléique

Deutsch (German)
n. - RNS

Ελληνική (Greek)
abbr. - ριβο(ζο)νουκλεϊνικό οξύ

Italiano (Italian)
acido ribonucleico

Português (Portuguese)
abbr. - ácido ribonucléico

Русский (Russian)
РНК-рибонуклеиновая кислота

Español (Spanish)
n. - ARN - ácido ribonucleico

Svenska (Swedish)
abbr. - (kem) ribunokleinsyra

中文（简体）(Chinese (Simplified))
核糖核酸

中文（繁體）(Chinese (Traditional))
n. - 核糖核酸

한국어 (Korean)
n. - 리보헥산

日本語 (Japanese)
abbr. - リボ核酸

العربيه (Arabic)
‏(اختصار) حامض يعيش في كروموسومات الأشياء الحيه‏

עברית (Hebrew)
n. - ‮רנ"א (חומצה ריבונוקלאית)‬


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