thumb|simple illustration of exons and introns in pre-mrna and the formation of mature mrna by splicing. in molecular biology and genetics, splicing is a modification of the nascent pre-messenger rna (pre-mrna) transcript in which introns are removed and exons are joined. for nuclear encoded genes, splicing takes place within the nucleus after or concurrently with transcription. splicing is needed for the typical eukaryotic messenger rna (mrna) 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. ==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==== the word intron is derived from the term intragenic region, that is, a region inside a gene. the term intron refers to both the dna sequence within a gene and the corresponding sequence in the unprocessed rna transcript. as part of the rna processing pathway, introns are removed by rna splicing either shortly after or concurrent with transcription. introns are found in the genes of most organisms and many viruses. they can be located in a wide range of genes, including those that generate proteins, ribosomal rna (rrna), and transfer rna (trna). spliceosomal introns often reside within the sequence of eukaryotic protein-coding genes. within the intron, a donor site (5' end of the intron), a branch site (near the 3' end of the intron) and an acceptor site (3' end of the intron) are required for splicing. the splice donor site includes an almost invariant sequence gu at the 5' end of the intron, within a larger, less highly conserved region. the spl
Gene_expression RNA_splicing Spliceosome