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The most prominent examples of RNA genes are '''transfer RNA''' ('''tRNA''') and '''ribosomal RNA''' ('''rRNA'''), both of which are involved in the process of translation. However, since the late 1990s, many new RNA genes have been found, and thus RNA genes may play a much more significant role than previously thought. Even so, they are probably not as significant or numerous as the protein-coding genes.
Transfer RNA plays a critical role in the process of translation. tRNA is the "adaptor" molecule hypothesized by [[Francis Crick]], which mediates recognition of the [[codon]] sequence in mRNA and allows its translation into the appropriate amino acid.
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See [[translation (biology)|translation]] for more on the role of tRNA.
Ribosomal RNA (rRNA) is the primary constituent of [[ribosome]]s. Ribosomes are the protein-manufacturing [[organelle]]s of cells and exist in the [[cytoplasm]]. rRNA is transcribed from DNA, like all RNA, and in eukaryotes it is processed in the [[nucleolus]] before being transported through the [[nuclear membrane]]. This type of RNA makes up the vast majority of RNA found in a typical cell (~95%). While proteins are also present in the ribosomes, the [[Svedberg|23s]] rRNA forms the active site for peptide bond formation, making that molecule a [[ribozyme]].
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Small nuclear RNA (snRNA) is a class of small RNA molecules that are found within the nucleus of eukaryotic cells. They are involved in a variety of important processes such as RNA splicing (removal of [[intron]]s from hnRNA) and maintaining the [[telomere]]s. They are always associated with specific proteins, and the complexes are referred to as [[small nuclear ribonucleoprotein]]s (snRNP) or sometimes as snurps.
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Small nucleolar RNA (snoRNA) is a class of small RNA molecules that are involved in chemical modifications of ribosomal RNAs (rRNAs) and other RNA genes, for example by [[methylation]].
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The snoRNA guides the snoRNP complex to the modification site of the target RNA gene via sequences in the snoRNA that hybridize to the target site. The proteins then catalyze modification of the RNA gene.
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microRNA (also miRNA) are RNA genes that are the reverse complement of another gene's mRNA transcript and inhibit the expression of the target gene.
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See [[miRNA]].
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gRNAs (for guide RNA) are RNA genes that function in RNA editing. Thus far, RNA editing has been found only in the mitochondria of [[kinetoplastid]]s, in which mRNAs are edited by inserting or deleting stretches of [[uracil|uridylates]] (Us). The gRNA forms part of the '''editosome''' and contains sequences that hybridize to matching sequences in the mRNA, to guide the mRNA modifications.
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The term "guide RNA" is also sometimes used generically to mean any RNA gene that guides an RNA/protein complex via hybridization of matching sequences.
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The [[signal recognition particle]] (SRP) is an RNA-protein complex present in the cytoplasm of cells that binds to the mRNA of proteins that are destined for secretion from the cell. The RNA component of the SRP in eukaryotes is called 4.5S RNA.
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At least one species of DNA-containing [[phage]]s, phi-29, uses a complex of six identical short RNA sequences as mechanical components (utilizing ATP for energy) of its DNA packaging machinery. How common this phenomenon is has yet to be determined.
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