Final answer:
Spliceosomes consist of proteins and snRNAs, recognizing intronic signal sequences for splicing mRNA by binding snRNPs to the exon-intron borders. They target specific sites for two transesterification reactions, resulting in the release of introns and ligation of exons to form mature mRNA ready for translation.
Step-by-step explanation:
Composition and Function of Spliceosomes
Spliceosomes are ribonucleoprotein complexes essential for the pre-mRNA splicing process within eukaryotic cells. These complexes are composed of proteins and small nuclear RNAs (snRNAs), which together form the subunits known as snRNPs (small nuclear ribonucleoproteins).
Recognition of Signal Sequences
Spliceosomes recognize intronic signal sequences that delineate the borders of introns and exons. These signals are typically found at the 5' and 3' ends of introns. Recognition involves binding of snRNPs to specific sequences at the intron-exon boundaries to carry out splicing.
Targeted Sites for Transesterification Reactions
The sites in mRNA that are targeted for the two transesterification reactions are the 5' splice site at the beginning of the intron, the branch point adenine within the intron, and the 3' splice site at the end of the intron.
Mechanism of Splicing
- The spliceosome assembles at the 5' splice site and branch point, aligning the pre-mRNA in a precise configuration for splicing.
- The first transesterification reaction occurs, cutting the RNA at the 5' splice site and joining the 5' end of the intron to the branch point adenine to form a lariat structure.
- The second transesterification reaction then cuts the RNA at the 3' splice site and joins the two exons together, resulting in the ligation of adjacent exons and the release of the intron lariat.
The final outcome is a continuous mRNA with its codons intact, ready for translation, and the removal of the lariat structure which is then degraded.