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Splicing (Genetics)In genetics, splicing is a modification of genetic information prior to translation. In eukaryotes, a gene often contains altering sequences known as exons (expressed codons) and introns (interrupting codons). In contrast to prokaryotes, which do not usually have introns, eukaryotes initially create a primary mRNA transcript called pre-mRNA that is composed of both introns and exons (see transcription). Pre-mRNA has to be spliced before it can be translated into protein. During splicing, the introns are removed from the mRNA using one of three methods: - Protein-mediated: spliceosomes, huge molecular structures composed of small nuclear ribonuclear proteins (snRPS (pronounced "snurps")), splice the 3' end of the first exon, form the intron into a lariat, splice the 5' end of the next exon, and fuse the exon-exon junction;
- Self-splicing: specialized introns serves as ribozymes, performing the functions of the spliceosome intramolecularly;
- Group 1: do not form lariats but instead bind introns to free-floating guanine;
- Group 2: perform the same two-step chemical reaction as the protein-mediated reaction (these are thought to be the evolutionary precursor to the much more common spliceosome).
Regardless of which process is used, the excised lariats are discarded. The final mRNA product must have each of the following: - A 5' cap, a guanine triphosphate nucleotide, thus named because it binds to the 5' end of the mRNA;
- The methylated cap prevents the 5' end from being degraded by exonucleases
- A leader that follows the 5' cap but precedes the exons;
- A trailer that follows the exons;
- A poly-A tail that follows the trailer;
- The poly-A tail serves as a stalling mechanism; when exonucleases begin to degrade the 3' end, the long trail of extra As takes extra time to degrade, elongating the mRNA's lifespan.
Mutations in the introns can prevent splicing and thus prevent protein biosynthesis. In many cases, the splicing process can create many unique proteins from a large collection of exons. This phenomenon is called alternative splicing. XXXXEEEEIIIEEEEEEEEEEIIIIEEEEEEEEEEXXXX DNA with exons and introns ↓transcription↓ EEEEIIIEEEEEEEEEEIIIIEEEEEEEEEE mRNA (primary transcript) with exons and introns ↓ splicing ↓ CLLLEEEEEEEEEEEEEEEEEEEEEEEETTTTAAAA mRNA (spliced) with exons, 5' cap, leader, trailer and poly-A tail ↓ translation ↓ polypeptide - See also : genetics -- cDNA
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