spliceosomer

The spliceosome is a large and dynamic molecular machine found within the nucleus of eukaryotic cells. It is responsible for the critical process of pre-mRNA splicing. Pre-mRNA, or pre-messenger RNA, is the initial RNA transcript produced from a gene during transcription. This pre-mRNA contains both coding regions, called exons, and non-coding regions, called introns. Introns must be removed and the exons precisely joined together to create mature messenger RNA (mRNA), which can then be translated into protein.

The spliceosome is composed of several small nuclear ribonucleoproteins (snRNPs, pronounced "snurps"), which are complexes of small nuclear RNAs (snRNAs) and proteins. The major spliceosome consists of five snRNPs: U1, U2, U4, U5, and U6, along with numerous associated proteins. These components assemble onto the pre-mRNA in a step-wise manner, forming a catalytic core that recognizes and binds to specific sequences at the boundaries of introns.

Through a series of conformational changes and RNA-RNA and RNA-protein interactions, the spliceosome catalyzes two transesterification reactions. The first reaction removes the 5' end of the intron and joins it to a branch point within the intron. The second reaction cleaves the 3' end of the intron and ligates the two adjacent exons together. This precise removal of introns and joining of exons ensures the fidelity of the genetic code that will be translated into functional proteins. Alternative splicing, a process where different combinations of exons from the same pre-mRNA can be joined, significantly expands the protein diversity encoded by a genome and is also mediated by the spliceosome in conjunction with regulatory proteins.