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Small RNA is a term used to describe several classes of non-coding ribonucleic acid (ncRNA). Although the term small RNA is often used to describe the short chains of ncRNA found in bacterial cells, it also applies as an umbrella term for other types of ncRNA that have been studied in animals, plants, and fungi. Small RNA is heavily involved in gene regulation and may have applications in gene therapy.
ncRNA differs in function from other RNA in that it does not directly code for a protein. Instead, ncRNA is involved in other cellular functions, including gene expression and modification. Small RNA molecules are usually made up of about 20 to 30 nucleotides. Many types of small RNA exist, including microRNAs (miRNAs), short interfering RNAs (siRNAs), and small nucleolar RNAs (snoRNAs).
Certain types of small RNA, such as microRNA and siRNA, are important in gene silencing. Gene silencing is a process of genetic regulation in which a gene that would normally be expressed is “turned off” by an intracellular element, in this case, the small RNA. The protein that would normally be formed by this genetic information is not formed due to interference, and the information coded in the gene is blocked from expression.
Gene silencing has an important role in naturally protecting organisms from viral attack. Research has shown that siRNA may also have therapeutic uses, because it can be used to artificially induce the knockout of a target gene. The complexity of genetic regulation makes it difficult to develop the therapeutic potential of siRNA, especially since it behaves differently in different cell types.
snoRNAs, another class of small RNA, act as mediators in the chemical modifications of other RNA molecules. For example, snoRNAs help guide the modification of pre-ribosomal RNA (pre-rRNA) and facilitate its development into mature ribosomal RNA (rRNA), which is one of the main components of ribosomes. Some snoRNAs appear not to target a specific RNA molecule and may have other functions, such as regulating alternative splicing, or reconnection, of RNA sequences.
The actions of small RNA have been studied in several model organisms. Many of these are plants, such as the flowering plant Arabidopsis thaliana, maize, and rice. Other models include the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, mice, and humans. Using these and other organisms, the potential of small RNA as a tool for genetic manipulation is being researched by scientists in the fields of applied biology and medicine.