Spliceosomal introns are introns spliced by the spliceosome and a series of snRNAs (small nuclear RNAs).
A spliceosome is a complex of specialized RNA and protein subunits that removes introns from a transcribed pre-mRNA (hnRNA) segment. This process is generally referred to as splicing.
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Nuclear or spliceosomal introns are spliced by the spliceosome and a series of snRNAs (small nuclear RNAs). Certain splice signals (or consensus sequences) abet the splicing (or identification) of these introns by the spliceosome.
Spliceosomal introns often reside in eukaryotic protein-coding genes. Within the intron, a 3' splice site, 5' splice site, and branch site are required for splicing. The 5' splice site or splice donor site includes an almost invariant sequence GU at the 5' end of the intron, within a larger, less highly conserved consensus region. The 3' splice site or splice acceptor site terminates the intron with an almost invariant AG sequence. Upstream (5'-ward) from the AG there is a region high in pyrimidines (C and U), or polypyrimidine tract. Upstream from the polypyrimidine tract is the branch point, which includes an adenine nucleotide. Point mutations in the underlying DNA or errors during transcription can activate a "cryptic splice site" in part of the transcript that usually is not spliced. This results in a mature messenger RNA with a missing section of an exon. In this way a point mutation, which usually only affects a single amino acid, can manifest as a deletion in the final protein.
Some introns, such as the Group I and Group II introns, after transcription possess ribozyme activity, enabling them to catalyze their own splicing out of a primary RNA transcript. These introns are thus self splicing introns and are relatively rare compared to spliceosomal introns. This self-splicing activity was discovered by Thomas Cech, who shared the 1989 Nobel Prize in Chemistry with Sidney Altman for the discovery of the catalytic properties of RNA.
Intron evolution: There are two competing theories that offer alternative scenarios for the origin and early evolution of spliceosomal introns. Other classes of introns such as self-splicing and tRNA introns are not subject to much debate, but see for the former. These are popularly referred to as the Introns-Early (IE) and the Introns-Late (IL) views.
The IE model, championed by Walter Gilbert, proposes that introns are extremely old and numerously present in the earliest ancestors of prokaryotes and eukaryotes (the progenote). In this model, introns were subsequently lost from prokaryotic organisms, allowing them to attain growth efficiency. A central prediction of this theory is that the early introns were mediators that facilitated the recombination of exons that represented the protein domains. This model cannot account for some observed positional variation of introns shared among related genes.
The IL model proposes that introns were recently inserted into originally intron-less contiguous genes after the divergence of eukaryotes and prokaryotes. In this model, introns probably originated from transposable elements. This model is based on the observation that the spliceosomal introns are restricted to eukaryotes alone. However, there is considerable debate over the presence of introns in the early prokaryote-eukaryote ancestors and the subsequent intron loss-gain during eukaryotic evolution. The evolution of introns and of the intron-exon structure may be largely independent of the evolution of coding-sequences.
The minor spliceosome is a ribonucleoprotein complex that catalyses the removal (splicing) of an atypical class of spliceosomal introns (U12-type) from eukaryotic messenger RNAs in plant, insects, vertebrates and some fungi (Rhizopus oryzae). This process is called noncanonical splicing, as opposed to U2-dependent canonical splicing. U12-type introns represent less than 1% of all introns in human cells. However they are found in genes performing essential cellular functions.
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