RNA splicing

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In molecular biology, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation. For many eukaryotic introns, splicing is done in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs), but there are also self-splicing introns.

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Splicing pathways

Several methods of RNA splicing occur in nature: the type of splicing depends on the structure of the spliced intron and the catalysts required for splicing to occur.

Spliceosomal introns

Spliceosomal introns often reside in eukaryotic protein-coding genes. Within the intron, a 3' splice site, 5' splice site, and branch site are required for splicing. The 5' splice site or splice donor site includes an almost invariant sequence GU at the 5' end of the intron, within a larger, less highly conserved consensus region. The 3' splice site or splice acceptor site terminates the intron with an almost invariant AG sequence. Upstream (5'-ward) from the AG there is a region high in pyrimidines (C and U), or polypyrimidine tract. Upstream from the polypyrimidine tract is the branch point, which includes an adenine nucleotide.[1] Point mutations in the underlying DNA or errors during transcription can activate a "cryptic splice site" in part of the transcript that usually is not spliced. This results in a mature messenger RNA with a missing section of an exon. In this way a point mutation, which usually only affects a single amino acid, can manifest as a deletion in the final protein.

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