Splicing

In genetics, splicing is a modification of genetic information prior to translation.

In eukaryotes, a gene often contains altering sequences known as exons and introns. In contrast to prokaryotes, which do not have introns, the primary mRNA transcript called pre-mRNA (see transcription) from the DNA has to be spliced, that means, the introns are removed from the mRNA in an intramolecular reaction, where the mRNA acts as a ribozyme, with the assistance of spliceosomes. The spliceosome also attaches new noncoding units:

• A 5' cap, a guanine triphosphate nucleotide, thus named because it binds to the 5' end of the mRNA;
• 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.

Mutations in the introns can prevent splicing and thus protein biosynthesis. In many cases, the splicing process leads to several mRNA's. 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