The spliceosome is a large ribonucleoprotein (RNP) complex that catalyzes pre-mRNA splicing, a critical step in gene expression. It consists of five small nuclear ribonucleoproteins (snRNPs) and numerous proteins. The spliceosome's dynamic structure allows it to accurately and flexibly perform its function, with its dynamics conserved between yeast and metazoans. Structural studies using electron microscopy, X-ray crystallography, and NMR have provided insights into the spliceosome's organization and the interactions between RNA and proteins. These studies suggest that the spliceosome is an RNP enzyme, but the precise nature of its active site remains unclear. The spliceosome assembly involves a complex network of RNA-RNA and RNP interactions that align the reactive groups of pre-mRNA for catalysis. The U2-dependent spliceosome, which is more common in eukaryotes, catalyzes the removal of U2-type introns, while the less common U12-dependent spliceosome catalyzes the removal of U12-type introns. The spliceosome undergoes dynamic conformational changes during splicing, with the U2 snRNP playing a key role in the first catalytic step. The U6 snRNA forms an intramolecular stem-loop structure that is crucial for splicing catalysis. The spliceosome's protein composition is highly dynamic, with proteins being exchanged during different stages of splicing. This dynamic nature allows the spliceosome to adapt to different splicing requirements. The spliceosome's active site is thought to be composed of both RNA and protein, with recent structural studies suggesting that the U6 snRNA and group II introns share common RNA motifs that form the basis of the active site. The spliceosome's catalytic center is modeled as a conformational two-state system, with the spliceosome existing in two distinct conformations during splicing. The spliceosome's dynamic nature is also influenced by posttranslational modifications, such as phosphorylation and ubiquitination, which regulate splicing events. These modifications help in the proper positioning of splice sites and the stability of the spliceosome. The spliceosome's structure and function are essential for the accurate processing of pre-mRNA, and understanding these processes is crucial for elucidating the mechanisms of gene expression and the development of therapeutic strategies for splicing-related diseases.The spliceosome is a large ribonucleoprotein (RNP) complex that catalyzes pre-mRNA splicing, a critical step in gene expression. It consists of five small nuclear ribonucleoproteins (snRNPs) and numerous proteins. The spliceosome's dynamic structure allows it to accurately and flexibly perform its function, with its dynamics conserved between yeast and metazoans. Structural studies using electron microscopy, X-ray crystallography, and NMR have provided insights into the spliceosome's organization and the interactions between RNA and proteins. These studies suggest that the spliceosome is an RNP enzyme, but the precise nature of its active site remains unclear. The spliceosome assembly involves a complex network of RNA-RNA and RNP interactions that align the reactive groups of pre-mRNA for catalysis. The U2-dependent spliceosome, which is more common in eukaryotes, catalyzes the removal of U2-type introns, while the less common U12-dependent spliceosome catalyzes the removal of U12-type introns. The spliceosome undergoes dynamic conformational changes during splicing, with the U2 snRNP playing a key role in the first catalytic step. The U6 snRNA forms an intramolecular stem-loop structure that is crucial for splicing catalysis. The spliceosome's protein composition is highly dynamic, with proteins being exchanged during different stages of splicing. This dynamic nature allows the spliceosome to adapt to different splicing requirements. The spliceosome's active site is thought to be composed of both RNA and protein, with recent structural studies suggesting that the U6 snRNA and group II introns share common RNA motifs that form the basis of the active site. The spliceosome's catalytic center is modeled as a conformational two-state system, with the spliceosome existing in two distinct conformations during splicing. The spliceosome's dynamic nature is also influenced by posttranslational modifications, such as phosphorylation and ubiquitination, which regulate splicing events. These modifications help in the proper positioning of splice sites and the stability of the spliceosome. The spliceosome's structure and function are essential for the accurate processing of pre-mRNA, and understanding these processes is crucial for elucidating the mechanisms of gene expression and the development of therapeutic strategies for splicing-related diseases.