Ribonucleic acid (
RNA) is one of the three major that are essential for all known forms of life.
Like DNA, RNA is made up of a long chain of components called . Each nucleotide consists of a (sometimes called a nitrogenous base), a
sugar, and a group. The sequence of nucleotides allows RNA to encode genetic information. For example, some use RNA instead of DNA as their genetic material, and all organisms use
(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 or sensing and communicating responses to cellular signals. One of these active processes is a universal function whereby mRNA molecules direct the assembly of proteins on This process uses (tRNA) molecules to deliver amino acids to the ribosome, where links amino acids together to form proteins.
The chemical structure of RNA is very similar to that ofth two differences--(a) RNA contains the sugar ribose while DNA contains the slightly different sugar e of ribose that lacks one oxygen atom), and (b) RNA has the nucleobase le DNA containsuracil 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 which act to copy a DNA or RNA template into a new RNA strand through processes known asr espectively.
RNA and are both
, but differ in three main ways. First, unlike DNA which is generally 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
RNA contains (there is no hydroxyl group attached to the pentose ring in theosition in DNA). These hydroxyl groups make RNA less stable than DNA because it is more prone to . Third, the complementary base to
is not as it is in DNA, but rather
which is an
form of thymine.
Like DNA, most biologically active RNAs, including 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 ike enzymer 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
Watson-Crick base pairs in a(hydrogen atoms are not shown)
Eachn RNA contains a
sugar, with carbons numbered 1' through 5'. A base is attached to the 1' position, generally
(U). Adenine and guanine are cytosine and uracil are 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 formbetween cytosine and guanine, between adenine and uracil and between guanine and uraciowever other interactions are possible, such as a group of adenine bases binding to each other in a bulgeor the GNRA hat has a guanine–adenine base-pair
Chemical structure of RNA
An important structural feature of RNA that distinguishes it from DNA is the presence of agroup at the 2' position of the ribose sugar. The presence of this functional group causes the helix to adopt the
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 maturen 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 (most notably in the TΨC loop of Another notable modified base is hypoxanthine, a deaminated adenine base whose
There are nearly 100 other naturally occurring modified nucleosidesof which pseudouridine and nucleosides with are the most commonThe 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 funct
The functional form of single stranded RNA molecules, just like proteins, frequently requires a specific
he scaffold for this structure is provided by elements which are hydrogen bonds within the molecule. This leads to several recognizable "domains" of secondary structu bulges and internal lce RNA is charged, metal ions such are needed to stabilise many secondary and
Synthesis
Synthesis of RNA is usually catalyzed by an enzymeusing DNA as a template, a process known as Initiation of transcription begins with the binding of the enzyme to a sequence in the DNA (usually found "upstream" of a gene). The DNA double helix is unwound by 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 occ
RNAs are ofre removed by the
There are also a number o 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 materia
] Also, RNA-dependent RNA polymerase is part of the pathway in many organisms
Types of RNA
[Overview
Structure of ribozyme that cuts RNA
Messenger RNA (mRNA) is the RNA that carries information from DNA the sites of protein synthesis in the cell. The coding sequence of the mRNA determines the sequeat is prodany RNAs do not code for protein however (about 97% of the transcriptional output is non-protein-coding in eukaryot
These so-called ("ncRNA") can be encoded by their own genes (RNA genes), but can also derive from mRNA The most prominent examples of non-coding RNA(tRN (rRNA), both of which are involved in the process of transhere are also non-coding RNAs involved in gene reguland other roles. Certain RNAs are ablechemical reactions such as cuttinother RNA m the catalthese are known
In translation
(mRNA) carries information about a protein sequencee protein synthesis factories in the cell. Itso te precursor mRNA (pre-mRNA) has been transcribed from DNA, it is processed to mature mRNA. This coding sections of the pre-mRNA. The mRNA is then exported from the nucleus to the cytoplasm, where it is bound to ribosomes ainto its corresponding protein form withbind 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
(tRNA) is a small RNA chain of aat transfers a specific amino acid to a gain at the ribosomal site of protein synthesis during translation. It has sites for amino acid attachment aegrecognition that binds to a specific sequence on the messenger RNA chain through hydrogen b
(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 synthesize 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 timrRNA is extremely abundant and makes up 80% of the 10 mg/ml RNA found in a typical eukaryoti
(tmRNA) is found in t tags proteins encoded by mRNAs that lack stop codons for degradation and prevents the ribosome from st
Regulatory RNAs
Several types of RNA can downregulate gene expression by being complementary to a part of an mRNA or a gene's DNA.
(miRNA; 21-22 are found in eukaryotes and act through (RNAi), where an effector complex of miRNA and enzymes can break down mRNA which the miRNA is complementary to, block the mRNA from being translated, or accelerate its degradWhile(siRNA; 20-25 nt) are often produced by breakdown of viral RNA, there are also endogenous sources of siRNAs
siRNAs act through RNA interference in a fashion similar to miRNAs. Some miRNAs and siRNAs can cause genes they target t transcription of those geneAnimals hav(piRNA; 29-30 nt) which are actcells and are thought to be a defense agains and play a role i
Many prokaryotes havRNAs, a regulatory system similar to RNA inaisense RNA can act is by binding to an mRNA, forming double-stranded RNA that is enzymatically degraThere are many regulate the activity of that mR untranslated regions can also contain elements that regulate other genes.
[44]
[edit] In RNA processing
Uridine to pseudouridine is a common RNA modification.
Many RNAs are involved in modifying other RNAs. which contain several (snRNA)e introns can be ribozymes that are spliced by themselveNA can also be altered by having its nucleotides modified to other nucleotides thankaryotes, modifications of RNA nucleotides are generally directed by
(snoRNA; 60-300 nt),d in thenoRNAs 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 genomes
Like DNA, RNA can carry genetic information. 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 celle 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
replicate their genomes by
DNA copies from their RNA; these DNA copies are then transcribed to new RNA.lso spread by copying DNA and RNA from one anothercontains an RNA that is used as template for building the ends of eukaryotic chromosomes.
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 Double-stranded RNA such as viral RNA ocan trigger response
Key discoveries in RNA biology
For further information, see article entitled
ies and numerous Nobel Prizes.t 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.
won the 1959(shared with
after he discovered an enzyme that can synthesize RNA in the laboratorynically, the enzyme discovered by Ochoa (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 n 1965winning Holley the 1968 Nobel Prize in Medicine (shared with 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
During the early 1970s 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, were awarded a Nobel Prize in 1975. In 1976,
nd his team determined the first complete nucleotide sequence of an RNA virus genome, that of
In 1977,
were discovered in both mammalian viruses and in cellular genes, resulting in a 1993 Nobel to
and
. Catalytic RNA molecules (discovered in the early 1980s, leading to a 1989 Nobel award toand
. In 1990 it was found in that introduced genes can silence similar genes of the plant's own, now known to be a result of
