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RIBONUCLEIC ACID (RNA) is a polymeric molecule essential in various biological roles in coding , decoding , regulation , and expression of genes . RNA
RNA
and DNA
DNA
are nucleic acids , and, along with lipids , proteins and carbohydrates , constitute the four major macromolecules essential for all known forms of life . Like DNA, RNA
RNA
is assembled as a chain of nucleotides , but unlike DNA
DNA
it is more often found in nature as a single-strand folded onto itself, rather than a paired double-strand. Cellular organisms use messenger RNA
RNA
(_MRNA_) to convey genetic information (using the letters G, U, A, and C to denote the nitrogenous bases guanine , uracil , adenine , and cytosine ) that directs synthesis of specific proteins. Many viruses encode their genetic information using an RNA
RNA
genome .

Some RNA
RNA
molecules play an active role within 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 where RNA molecules direct the assembly of proteins on ribosomes . This process uses transfer RNA
RNA
(_TRNA_) molecules to deliver amino acids to the ribosome, where ribosomal RNA
RNA
(_RRNA_) then links amino acids together to form proteins.

CONTENTS

* 1 Comparison with DNA
DNA
* 2 Structure * 3 Synthesis

* 4 Types of RNA
RNA

* 4.1 Overview * 4.2 In length * 4.3 In translation * 4.4 Regulatory RNAs * 4.5 In RNA processing * 4.6 RNA
RNA
genomes * 4.7 In reverse transcription * 4.8 Double-stranded RNA
RNA
* 4.9 Circular RNA
Circular RNA

* 5 Key discoveries in RNA
RNA
biology * 6 Evolution * 7 See also * 8 References * 9 External links

COMPARISON WITH DNA

Bases in an RNA
RNA
molecule. Three-dimensional representation of the 50S
50S
ribosomal subunit. Ribosomal RNAis in ochre, proteins in blue. The active site is a small segment of rRNA, indicated in red.

The chemical structure of RNA
RNA
is very similar to that of DNA
DNA
, but differs in three main ways:

* Unlike double-stranded DNA, RNA
RNA
is a single-stranded molecule in many of its biological roles and has a much shorter chain of nucleotides. However, RNA
RNA
can, by complementary base pairing, form intrastrand (i.e., single-strand) double helixes, as in tRNA. * While DNA
DNA
contains _deoxyribose _, RNA
RNA
contains _ribose _ (in deoxyribose there is no hydroxyl group attached to the pentose ring in the 2\' position). These hydroxyl groups make RNA
RNA
less stable than DNA because it is more prone to hydrolysis . * The complementary base to adenine in DNA
DNA
is thymine , whereas in RNA, it is uracil , which is an unmethylated form of thymine.

Like DNA, most biologically active RNAs, including m RNA
RNA
, t RNA
RNA
, rRNA , snRNAs , and other non-coding RNAs , contain self-complementary sequences that allow parts of the RNA
RNA
to fold 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). 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.

STRUCTURE

Main article: Nucleic acid
Nucleic acid
structure Watson-Crick base pairs in a si RNA
RNA
(hydrogen atoms are not shown)

Each nucleotide in RNA
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
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, making RNA
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. Chemical structure of RNA
RNA

An important structural feature of RNA
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 mostly adopt the A-form geometry , although in single strand dinucleotide contexts, RNA
RNA
can rarely also adopt the B-form most commonly observed in DNA. The A-form geometry 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
RNA
molecule (that is, not involved in formation of a double helix), it can chemically attack the adjacent phosphodiester bond to cleave the backbone. Secondary structure of a telomerase RNA
RNA
.

RNA
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 t RNA
RNA
). Another notable modified base is hypoxanthine, a deaminated adenine base whose nucleoside is called inosine (I). Inosine
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 t RNA
RNA
, while pseudouridine and nucleosides with 2\'-O-methylribose often present in r RNA
RNA
are the most common. The specific roles of many of these modifications in RNA
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
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
RNA
is charged, metal ions such as Mg2+ are needed to stabilise many secondary and tertiary structures .

The naturally occurring enantiomer of RNA
RNA
is D- RNA
RNA
composed of D-ribonucleotides. All chirality centers are located in the D-ribose. By the use of L-ribose or rather L-ribonucleotides, L- RNA
RNA
can be synthesized. L- RNA
RNA
is much more stable against degradation by RNase .

Like other structured biopolymers such as proteins, one can define topology of a folded RNA
RNA
molecule. This is often done based on arrangement of intra-chain contacts within a folded RNA, termed as circuit topology .

SYNTHESIS

Synthesis of RNA
RNA
is usually catalyzed by an enzyme— RNA
RNA
polymerase —using DNA
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
DNA
(usually found "upstream" of a gene). The DNA
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
RNA
molecule with elongation occurring in the 5’ to 3’ direction. The DNA
DNA
sequence also dictates where termination of RNA
RNA
synthesis will occur.

Primary transcriptRNAs are often modified by enzymes after transcription. For example, a poly(A) tail and a 5\' cap are added to eukaryotic pre-m RNA
RNA
and introns are removed by the spliceosome .

There are also a number of RNA-dependent RNA
RNA
polymerases that use RNA as their template for synthesis of a new strand of RNA. For instance, a number of RNA
RNA
viruses (such as poliovirus) use this type of enzyme to replicate their genetic material. Also, RNA-dependent RNA polymerase is part of the RNA interference
RNA interference
pathway in many organisms.

TYPES OF RNA

See also: List of RNAs

OVERVIEW

Structure of a hammerhead ribozyme , a ribozyme that cuts RNA
RNA

Messenger RNA
Messenger RNA
(mRNA) is the RNA
RNA
that carries information from DNA
DNA
to the ribosome , the sites of protein synthesis (translation ) in the cell. The coding sequence of the m RNA
RNA
determines the amino acid sequence in the protein that is produced. However, many RNAs do not code for protein (about 97% of the transcriptional output is non-protein-coding in eukaryotes ).

These so-called non-coding RNAs ("ncRNA") can be encoded by their own genes ( RNA
RNA
genes), but can also derive from m RNA
RNA
introns . The most prominent examples of non-coding RNAs are transfer RNA
RNA
(tRNA) and ribosomal RNA
RNA
(rRNA), both of which are involved in the process of translation. There are also non-coding RNAs involved in gene regulation, RNA processingand other roles. Certain RNAs are able to catalyse chemical reactions such as cutting and ligating other RNA molecules, and the catalysis of peptide bond formation in the ribosome ; these are known as ribozymes .

IN LENGTH

According to the length of RNA
RNA
chain, RNA
RNA
includes small RNA
RNA
and long RNA. Usually, small RNAs are shorter than 200 nt in length, and long RNAs are greater than 200 nt long. Long RNAs, also called large RNAs, mainly include long non-coding RNA
RNA
(lncRNA) and m RNA
RNA
. Small RNAs mainly include 5.8S ribosomal RNA
RNA
(rRNA), 5S rRNA, transfer RNA (tRNA), micro RNA
RNA
(miRNA), small interfering RNA
RNA
(siRNA), small nucleolar RNA
RNA
(snoRNAs), Piwi-interacting RNA(piRNA), tRNA-derived small RNA
RNA
(tsRNA) and small rDNA-derived RNA
RNA
(srRNA).

IN TRANSLATION

Messenger RNA
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 ) corresponds to one amino acid. In eukaryotic cells, once precursor m RNA
RNA
(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 t RNA
RNA
. In prokaryotic cells, which do not have nucleus and cytoplasm compartments, m RNA
RNA
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 .

Transfer RNA(tRNA) is a small RNA
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.

Ribosomal RNA(rRNA) is the catalytic component of the ribosomes. Eukaryotic
Eukaryotic
ribosomes contain four different r RNA
RNA
molecules: 18S, 5.8S, 28S and 5S rRNA. Three of the r RNA
RNA
molecules are synthesized in the nucleolus , and one is synthesized elsewhere. In the cytoplasm, ribosomal RNA
RNA
and protein combine to form a nucleoprotein called a ribosome. The ribosome binds m RNA
RNA
and carries out protein synthesis. Several ribosomes may be attached to a single m RNA
RNA
at any time. Nearly all the RNA
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.

REGULATORY RNAS

Several types of RNA
RNA
can downregulate gene expression by being complementary to a part of an m RNA
RNA
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 mi RNA
RNA
and enzymes can cleave complementary mRNA, block the m RNA
RNA
from being translated, or accelerate its degradation.

While small interfering RNAs (siRNA; 20-25 nt) are often produced by breakdown of viral RNA, there are also endogenous sources of siRNAs. siRNAs act through RNA interference
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. 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 .

Many prokaryotes have CRISPR
CRISPR
RNAs, a regulatory system similar to RNA interference. Antisense RNAs are widespread; most downregulate a gene, but a few are activators of transcription. One way antisense RNA
RNA
can act is by binding to an mRNA, forming double-stranded RNA
RNA
that is enzymatically degraded. There are many long noncoding RNAs that regulate genes in eukaryotes, one such RNA
RNA
is Xist , which coats one X chromosome in female mammals and inactivates it.

An m RNA
RNA
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. The untranslated regions can also contain elements that regulate other genes.

IN RNA
RNA
PROCESSING

Uridine
Uridine
to pseudouridine is a common RNA
RNA
modification.

Many RNAs are involved in modifying other RNAs. Introns are spliced out of pre-m RNA
RNA
by spliceosomes , which contain several small nuclear RNAs (snRNA), or the introns can be ribozymes that are spliced by themselves. RNA
RNA
can also be altered by having its nucleotides modified to nucleotides other than A , C , G and U . In eukaryotes, modifications of RNA
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
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.

RNA
RNA
GENOMES

Like DNA, RNA
RNA
can carry genetic information. RNA
RNA
viruses have genomes composed of RNA
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.

IN REVERSE TRANSCRIPTION

Reverse transcribing viruses replicate their genomes by reverse transcribing DNA
DNA
copies from their RNA; these DNA
DNA
copies are then transcribed to new RNA. Retrotransposons also spread by copying DNA and RNA
RNA
from one another, and telomerase contains an RNA
RNA
that is used as template for building the ends of eukaryotic chromosomes.

DOUBLE-STRANDED RNA

Double-stranded RNA
RNA
(dsRNA) is RNA
RNA
with two complementary strands, similar to the DNA
DNA
found in all cells. ds RNA
RNA
forms the genetic material of some viruses (double-stranded RNA
RNA
viruses ). Double-stranded RNA
RNA
such as viral RNA
RNA
or si RNA
RNA
can trigger RNA interference in eukaryotes , as well as interferon response in vertebrates .

CIRCULAR RNA

Recently, it was shown that there is a single stranded covalently closed, _i.e._ circular form of RNA
RNA
expressed throughout the animal and plant kingdom (see circ RNA
RNA
). circRNAs are thought to arise via a "back-splice" reaction where the spliceosome joins a downstream donor to an upstream acceptor splice site. So far the function of circRNAs is largely unknown, although for few examples a micro RNA
RNA
sponging activity has been demonstrated.

KEY DISCOVERIES IN RNA
RNA
BIOLOGY

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

Research on RNA
RNA
has led to many important biological discoveries and numerous Nobel Prizes. Nucleic acids were discovered in 1868 by Friedrich Miescher
Friedrich Miescher
, who called the material 'nuclein' since it was found in the nucleus . It was later discovered that prokaryotic cells, which do not have a nucleus, also contain nucleic acids. The role of RNA
RNA
in protein synthesis was suspected already in 1939. Severo Ochoa
Severo Ochoa
won the 1959 Nobel Prize in Medicine(shared with Arthur Kornberg ) after he discovered an enzyme that can synthesize RNA
RNA
in the laboratory. However, the enzyme discovered by Ochoa (polynucleotide phosphorylase ) was later shown to be responsible for RNA
RNA
degradation, not RNA
RNA
synthesis. In 1956 Alex Rich and David Davies hybridized two separate strands of RNA
RNA
to form the first crystal of RNA
RNA
whose structure could be determined by X-ray crystallography.

The sequence of the 77 nucleotides of a yeast t RNA
RNA
was found by Robert W. Holley
Robert W. Holley
in 1965, winning Holley the 1968 Nobel Prize in Medicine (shared with Har Gobind Khorana
Har Gobind Khorana
and Marshall Nirenberg). In 1967, Carl Woese
Carl Woese
hypothesized that RNA
RNA
might be catalytic and suggested that the earliest forms of life (self-replicating molecules) could have relied on RNA
RNA
both to carry genetic information and to catalyze biochemical reactions—an RNA
RNA
world .

During the early 1970s, retroviruses and reverse transcriptase were discovered, showing for the first time that enzymes could copy RNA into DNA
DNA
(the opposite of the usual route for transmission of genetic information). For this work, David Baltimore
David Baltimore
, Renato Dulbecco
Renato Dulbecco
and Howard Teminwere awarded a Nobel Prize in 1975. In 1976, Walter Fiers and his team determined the first complete nucleotide sequence of an RNA virus
RNA virus
genome, that of bacteriophage MS2 .

In 1977, introns and RNA splicing
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
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
Petunia
_ that introduced genes can silence similar genes of the plant's own, now known to be a result of RNA interference
RNA interference
.

At about the same time, 22 nt long RNAs, now called microRNAs , were found to have a role in the development of _C. elegans _. Studies on RNA interference
RNA interference
gleaned a Nobel Prize for Andrew Fire and Craig Mello in 2006, and another Nobel was awarded for studies on the transcription of RNA
RNA
to Roger Kornbergin the same year. The discovery of gene regulatory RNAs has led to attempts to develop drugs made of RNA, such as si RNA
RNA
, to silence genes.

EVOLUTION

In March 2015, complex DNA
DNA
and RNA
RNA
nucleotides , including uracil , cytosine and thymine , were reportedly formed in the laboratory under outer space conditions, using starter chemicals, such as pyrimidine , an organic compound commonly found in meteorites . Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), is one of the most carbon-rich compounds found in the Universe
Universe
and may have been formed in red giants or in interstellar dust and gas clouds.

SEE ALSO

* Biomolecular structure * Macromolecule
Macromolecule
* DNA
DNA
* History of RNA
RNA
Biology * List of RNA
RNA
Biologists * Transcriptome
Transcriptome

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