Simple illustration of exons and introns in pre-mRNA. The mature mRNA is formed by splicing.
Splicing is a modification of genetic information after transcription, in which introns are removed and exons are joined. Splicing prepares precursor messenger RNA in eukaryotes to produce mature messenger RNA. This mature messenger RNA is then prepared to undergo translation as part of protein synthesis to produce proteins. Splicing occurs by a series of biochemical reactions between RNA nucleotides, which are catalyzed by proteins, RNA, or both.
Several methods of RNA splicing occur in nature. The type of splicing depends on the structure of the spliced intron and the catalysts required for splicing to occur. Regardless of which pathway is used, the excised introns are discarded.
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. Splicing is catalyzed by the spliceosome which is a large RNA-protein complex composed of five small nuclear ribonucleoproteins (snRNPs, pronounced "snurps"). The RNA components of snRNPs interact with the intron and may be involved in catalysis. Two types of spliceosomes have been identified (the major and minor) which contain different snRNPs.- Major
U1- binds 5' splice site U2- binds the branch U4- inhibits U6, lost to activate spliceosome U5 - binds U1 and U2 to create lariat U6 - When, activated, displaces U1 and binds U2. U2-U6 forms active catalytic complex- Minor
Self-splicing occurs for rare introns that form a ribozyme, performing the functions of the spliceosome by RNA alone. There are three kinds of self-splicing introns, Group I, II, and III. Group II and III introns perform splicing similar to the spliceosome without requiring any protein. This similarity suggests that Group II and III introns may be evolutionarily related to the spliceosome. Self-splicing may also be very ancient, and may have existed in an RNA world that was present before protein.
tRNA (also tRNA-like) splicing is another rare form of splicing that usually occurs in tRNA. The splicing reaction involves a different biochemistry than the spliceomsomal and self-splicing pathways. Ribonucleases cleave the RNA and ligases join the exons together. This form of splicing does also not require any RNA components for catalysis.
Splicing occurs in all the kingdoms or domains of life, however, the extent and types of splicing can be very different between the major divisions. Eukaryotes splice many protein-coding messenger RNAs and some non-coding RNAs. Prokaryotes, on the other hand, splice rarely, but mostly non-coding RNAs. Another important difference between these two groups of organisms is that prokaryotes completely lack the spliceosomal pathway.
Because spliceosomal introns are not conserved in all species, there is debate concerning when spliceosomal splicing evolved. Two models have been proposed: the intron late and intron early models.
Spliceosomal splicing and self-splicing involves a two-step biochemical process. Both steps involve transesterification reactions that occur between RNA nucleotides. tRNA splicing, however, is an exception and does not occur by transesterification.
Spliceosomal and self-splicing transesterification reactions occur in a specific order. First, a specific branch-point nucleotide within the intron reacts with the first nucleotide of the intron, forming an intron lariat. Second, the last nucleotide of the first exon reacts with the first nucleotide of the second exon, joining the exons and releasing the intron lariat.
In many cases, the splicing process can create many unique proteins by variations in the splicing of the same messenger RNA. This phenomenon is called alternative splicing.
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