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'''Ribonucleic acid''' (RNA) is a molecule found in all cells, comprised of many individual units of nucleic acid. It differs from Deoxyribonucleic acid ([[DNA]]) in that it only contains a single, not double strand, and substitutes [[uracil]] for [[thymine]]. The primary role of RNA is to transfer a copy of the information coded in DNA to a [[ribosome]] to be expressed as a protein. It is also a component of the ribosome itself.
'''Ribonucleic acid''' (RNA) is a family of biomolecules which perform several essential functions within all cells. As a type of nucleic acid, RNA is structurally and chemically very similar to [[DNA]]; the two largest differences being that the RNA backbone contains ribose instead of 2'-deoxyribose and uses [[uracil]] instead of [[thymine]] as one of its four bases.<ref>Nelson & Cox. (2008). ''Principles of biochemistry''.</ref>
The polio virus is an example of an organism which contains only RNA to carry its genetic information.
Unlike DNA, which is primarily used as a template for [[transcription]], RNA molecules perform a diverse set of functions within the cell. Subsets of RNA are generally classified by their function, which can range from acting as the template for [[Translation (biology)|protein synthesis]] ("messenger RNA" or "[[mRNA]]") to performing [[ribozyme|enzymatic functions]]. Additionally, many viruses use RNA instead of DNA as their genetic material (e.g. [[retrovirus]]es).
As with DNA, RNAs can exist in single-stranded (annotated "ssRNA") or double-stranded ("dsRNA") forms; however, the vast majority of cellular RNAs are single-stranded. dsRNAs generally function in post-transcriptional [[gene regulation]] or as viral genetic material. Targeted degradation of dsRNAs is a major intracellular defence mechanism against viruses.<ref>Alberts et al. (2008). ''Molecular biology of the cell''.</ref><ref>Weaver, Robert F. ''Molecular Biology''. 4th ed. Boston: McGraw-Hill, 2008.</ref>
==Structure==
Every nucleotide of RNA consists of a ribose molecule with a nitrogenous base attached to its 1' carbon and a phosphate group attached to its 5' carbon. Individual RNA nucleotides are linked by a [[phosphodiester bond]] between the 3' carbon of one nucleotide's ribose and the 5' carbon on the next.
The four nitrogenous bases used in RNA are [[adenine]], [[guanine]], [[cytosine]], and [[uracil]]. Adenine and guanine are the ''purine'' bases, cytosine and uracil are the ''pyrimidine'' bases. Like in DNA, these bases can hybridize through [[hydrogen bond]]s; adenine hybridizes with uracil and guanine hybridizes cytosine.
Hybridization of nucleotides within RNA molecules allows for the formation of ''secondary structures'' such as RNA hairpins, these secondary structures give RNA molecules an overall ''tertiary structure'' which is often essential for the RNA to perform its function. Additionally, RNAs can hybridize through base pairing with other RNAs or with complementary DNA sequences.
==Function==
RNA performs a wide variety of functions within the cell. Types of RNA are specified by the functions that they perform. More than 21 functional classes of RNA have been identified, the most common types are described below.
===mRNA===
''Messenger RNAs'' ([[mRNA]]) are the template molecules which are ''translated'' into [[protein]]s by [[ribosome]]s. They are generally synthesized by [[transcription]] of a DNA template (a [[gene]]) by [[RNA polymerase]], although some viral mRNAs are directly transcribed from an RNA template.
In [[eukaryote]]s, nascent RNA transcripts must generally be processed from ''pre-mRNA'' into mature mRNA, via ''RNA processing'', before they are exported from the nucleus to be translated into proteins.
===rRNA===
''Ribosomal RNAs'' ([[rRNA]]) are the RNA molecules that form a major component of the [[ribosome]]s. There are three rRNAs in [[prokaryote]]s and four rRNAs in [[eukaryote]]s and [[archaea]]ns.
The genes encoding rRNAs are among the most highly conserved (low level of sequence variance between individuals and species) genes in any genome. As such, rRNA sequences are often used to generate very precise phylogenetic trees.
The 28S rRNA (23S rRNA in prokaryotes) is a [[ribozyme]], and is responsible for the aminoacyltransferase (polypeptide-lengthening) activity of the ribosome.
===tRNA===
''Transfer RNAs'' ([[tRNA]]) are the ''adapter molecules'' that recognize ''codons'' during translation and bring specific [[amino acids]] to the [[ribosome]] to be added to the growing [[protein|polypeptide]] chain.
The "cloverleaf" tertiary structure of tRNAs is a common textbook example of the link between RNA structure and function.
===microRNA===
''Micro RNAs'' ([[microRNA]])are short RNA molecules involved in post-transcriptional [[gene regulation]]. microRNAs work by binding to complementary sequences on target mRNAs; thus targeting the mRNA for degradation via the [[RISC|RNA-induced silencing complex]] or, less commonly, blocking translation of the mRNA by [[steric hindrance|steric]] (mechanical) hindrance.
In their mature, functional form, microRNA molecules are 20 to 24 nucleotides long. This length allows for microRNAs to have a very high degree of target specificity (some microRNAs may only have a single target mRNA).
Because the mechanics of [[RNA polymerase]] require a minimum transcript length of about 50-100 nucleotides, precursor microRNAs are at least 80 nucleotides long (most are longer). As such, several different microRNAs are usually derived from a single precursor transcript.
Together with siRNAs, microRNAs are a major component of ''gene regulatory networks'', which allow the cell to rapidly and efficiently respond to environmental stimuli.
===siRNA===
''Short interfering RNAs'' ([[siRNA]]) are short (~21 base pairs), double-stranded RNA molecules which function in a manner similar to microRNAs (described above).
siRNAs are commonly used in research for targeted gene knockdown (blocking the expression of a specific gene) and they are also used in several gene therapies.
===snRNA===
''Small nuclear RNAs'' ([[snRNA]]) are involved in several different processes that occur within the nucleus, notably [[RNA splicing]] and the regulation of certain [[transcription factor]]s. They generally function in complexes with specific proteins; these snRNA and protein complexes are called ''small nuclear ribonucleoproteins'' ("snRNPs", colloquially pronounced "snurps").
==References==
<references/>
[[Category:Genetics]]
[[Category:Biochemistry]]
[[Category:Biology]]