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Noncoding RNA-protein complexes (ncRNPs) are ubiquitous in eukaryotic cells and inhabit specific cellular compartments. The most famous noncoding nuclear RNPs (snRNPs) participate in pre-mRNA splicing by recognizing important intron signals and assembling to form an active splicing complex called a spliceosome. There are many other kinds, including those where the RNA is made by an infecting virus.

RNA Machines of the Mammalian Cell Nucleus

Three distinct kinds of RNA molecules orchestrate gene expression in all cells. Messenger RNAs (mRNAs) bring the information from the genome to the protein synthesis factory, the ribosome, which itself is composed mostly of RNA (rRNA). Transfer RNAs (tRNAs) serve as the adapter molecules that align the correct amino acids on the mRNA template during the elongation of protein chains.

A fourth class of RNA molecules, called noncoding RNAs, is also crucial for gene expression in high cells. Small RNAs (having chain lengths of 60-300 nucleotides), which were the first to be characterized, are highly conserved and particularly abundant in the nuclei of mammalian cells. Small nuclear RNAs (snRNAs) exist tightly bound to one or more proteins in particles called small nuclear ribonucleoproteins (snRNPs: pronounced "snurps"). Some snRNPs inhabit the nucleoplasm, which contains the DNA and is devoted to the production of mRNAs for export to the cytoplasm. Others occupy the nucleolus, where ribosomes are assembled before being shunted to the cytoplasm to function in protein synthesis. More recently, a novel family of tiny regulatory RNAs, dubbed microRNAs because they are only about 22 nucleotides long, has been discovered. They too bind proteins (creating microRNPs), but control gene expression as the level of translation and mRNA stability in the cytoplasm. MicroRNAs play important roles in cancer and infectious disease, as well as in normal development.