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 RNA contains ribose instead of 2'-deoxyribose and uses uracil instead of thymine as one of its four bases.
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 protein synthesis ("messenger RNA" or "mRNA") to performing enzymatic functions. Additionally, many viruses use RNA instead of DNA as their genetic material (e.g. retroviruses).
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.
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 bonds; 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.
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.
Messenger RNAs (mRNA) are the template molecules which are translated into proteins by ribosomes. 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 eukaryotes, 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.
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.
The "cloverleaf" tertiary structure of tRNAs is a common textbook example of the link between RNA structure and function.
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 RNA-induced silencing complex or, less commonly, blocking translation of the mRNA by 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.
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.