Spliceosome

A spliceosome is a large and complex molecular machine found primarily within the splicing speckles of the cell nucleus of eukaryotic cells. The spliceosome is assembled from snRNAs and protein complexes. The spliceosome removes introns from a transcribed pre-mRNA, a type of primary transcript. This process is generally referred to as splicing. Only eukaryotes have spliceosomes and some organisms have a second spliceosome, the minor spliceosome. An analogy is a film editor, who selectively cuts out irrelevant or incorrect material (equivalent to the introns) from the dailies and sends the cleaned-up version to be screened for the producer.

Each spliceosome is composed of five small nuclear RNAs (snRNA), and a range of associated protein factors. When these small RNAs are combined with the protein factors, they make an RNA-protein complex called snRNP (small nuclear ribonucleo proteins). The snRNAs that make up the major spliceosome are named U1, U2, U4, U5, and U6, and participate in several RNA-RNA and RNA-protein interactions. The RNA component of the small nuclear ribonucleic protein or snRNP (pronounced "snurp") is rich in uridine (the nucleoside analog of the uracil nucleotide).

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