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.
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