![]() U4 is released, U6/U2 catalyzes transesterification, making the 5'-end of the intron ligate to the A on intron and form a lariat, U5 binds exon at 3' splice site, and the 5' site is cleaved, resulting in the formation of the lariat.The U1 snRNP is released, U5 shifts from exon to intron, and the U6 binds at the 5' splice site.The U5/U4/U6 snRNP trimer binds, and the U5 snRNP binds exons at the 5' site, with U6 binding to U2.The U2 snRNP displaces SF1 and binds to the branch point sequence and ATP is hydrolyzed.U2AF2 binds to the polypyrimidine tract.U2AF1 binds at the 3' splice site of the intron.Splicing factor 1 binds to the intron branch point sequence.The U1 snRNP binds to the GU sequence at the 5' splice site of an intron.The spliceosome forms different complexes during the splicing process: In addition, a number of proteins including U2 small nuclear RNA auxiliary factor 1 (U2AF35), U2AF2 (U2AF65) and SF1 are required for the assembly of the spliceosome. It is composed of the U1, U2, U4, U5, and U6 snRNPs and is active in the nucleus. The major spliceosome splices introns containing GU at the 5' splice site and AG at the 3' splice site.Two types of spliceosomes have been identified (major and minor) which contain different snRNPs. The RNA components of snRNPs interact with the intron and are involved in catalysis. Assembly and activity of the spliceosome occurs during transcription of the pre-mRNA. Splicing is catalyzed by the spliceosome, a large RNA-protein complex composed of five small nuclear ribonucleoproteins ( snRNPs). Intron Exon Boundary in pre-mRNA 1 - 3' Splice site 2 - Poly pyrimidine Tract 3 - Branch site 4 - 5' splice site Formation and activity ![]() In this way, a point mutation, which might otherwise affect only a single amino acid, can manifest as a deletion or truncation in the final protein. This results in a mature messenger RNA with a missing section of an exon. Also, 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. However, it is noted that the specific sequence of intronic splicing elements and the number of nucleotides between the branchpoint and the nearest 3’ acceptor site affect splice site selection. Y-U-R-A-C (branch sequence 20-50 nucleotides upstream of acceptor site). The consensus sequence for an intron (in IUPAC nucleic acid notation) is: G-G-G-U-R-A-G-U (donor site). Further upstream from the polypyrimidine tract is the branchpoint, which includes an adenine nucleotide involved in lariat formation. Upstream (5'-ward) from the AG there is a region high in pyrimidines (C and U), or polypyrimidine tract. The splice acceptor site at the 3' end of the intron terminates the intron with an almost invariant AG sequence. The splice donor site includes an almost invariant sequence GU at the 5' end of the intron, within a larger, less highly conserved region. Within introns, a donor site (5' end of the intron), a branch site (near the 3' end of the intron) and an acceptor site (3' end of the intron) are required for splicing. They can be located in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA). Introns are found in the genes of most organisms and many viruses. ![]() As part of the RNA processing pathway, introns are removed by RNA splicing either shortly after or concurrent with transcription. The term intron refers to both the DNA sequence within a gene and the corresponding sequence in the unprocessed RNA transcript. The word intron is derived from the terms intragenic region, and intracistron, that is, a segment of DNA that is located between two exons of a gene. 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. Process of RNA splicing Splicing pathways The process of transcription, splicing and translation is called gene expression, the central dogma of molecular biology. There exist self-splicing introns, that is, ribozymes that can catalyze their own excision from their parent RNA molecule. For many eukaryotic introns, splicing occurs in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins ( snRNPs). For those eukaryotic genes that contain introns, splicing is usually needed to create an mRNA molecule that can be translated into protein. For nuclear-encoded genes, splicing occurs in the nucleus either during or immediately after transcription. It works by removing all the introns (non-coding regions of RNA) and splicing back together exons (coding regions). RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre- mRNA) transcript is transformed into a mature messenger RNA ( mRNA). ![]()
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