Group I introns occur in nuclear, mitochondrial and chloroplast rRNA genes, group II in mitochondrial and chloroplast mRNA genes. Many of the group I and group II introns are self-splicing in that no additional protein factors are necessary for the intron to be efficiently and accurately excised and the strands reattached.
Group I introns require an external guanosine nucleotide as a cofactor. The 3'-OH of the guanosine nucleotide acts as a nucleophile to attack the 5'-phosphate of the intron's 5' nucleotide. The 3' end of the 5' exon is termed the splice donor site. The 3'-OH at the 3' splice donor end of the 5' exon next attacks the splice acceptor site at the 5' nucleotide of the 3' exon, releasing the intron and covalently attaching the two exons together.
Pre-mRNA processing takes place in the nucleus of eukaryotes, whereas lack of a nuclear membrane in prokaryotes permits initiation of translation while transcription is not yet complete.Pre-mRNA processing events include capping of the 5’ end on the pre-mRNA, pre-mRNA splicing to remove intronic sequences, and polyadenylation of the 3’ end of the pre-mRNA.
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