RNA processing RNA species in cells RNA processing Types of introns and their removal Types of introns Complex organisms have intron-rich genes Examples of introns in human protein-coding genes Removal of introns from mRNA in the nucleus of eukaryotes Consensus elements of human GU-AG introns Chemistry of mRNA splicing for GU-AG introns Three way junction at the internal (A) branch site U1 snRNP Spliceosome-catalyzed splicing of GU-AG introns Different snRNPs and proteins bind and dissociate during splicing RNA processing Similarities in spliceosome-catalyzed removal of introns and self-splicing group I and group II introns Splicing of AU-AC introns by “minor” spliceosomes proceeds as for GU-AG introns Trans-splicing joins exons from different RNAs Possible errors in splice-site selection SR proteins are essential for correct splice-site selection SR proteins = serine, arginine-rich proteins ESE = exonic splicing enhancers Alternative splicing results in alternative mRNAs that are translated into different proteins Alternative splicing results in alternative mRNAs that are translated into different proteins Alternative splicing of the human slo gene (codes for a membrane protein that regulates the flow of potassium ions across the membrane) Optional exons participating in alternative splicing events. About 500 mRNA variants are produced in cells of the inner ear. Figure 12.34 Genomes 3 (© Garland Science 2007) Alternative splicing is regulated Assembly of major and minor spliceosomes Nonsense-mediated transcript decay Example: alternative splicing in Drosophila Dscam transcripts Example: mutual exclusive alternative splicing of exon 6 in Drosophila Dscam transcripts Splicing activators and repressors Splicing activators and repressors Exons may code for protein domains Exon shuffling may lead to protein diversity Exon shuffling may be responsible for protein diversity (chromatin-modifying enzymes of yeast, worms, flies, and humans) Types of introns Removal of introns from tRNAs Figure 12.40 Genomes 3 (© Garland Science 2007) RNA processing Processing of ribosomal RNA by cutting and trimming Processing of a tRNA by cutting and trimming Figure 12.17 Genomes 3 (© Garland Science 2007) RNA processing Chemical modifications in tRNAs Figure 12.18 Genomes 3 (© Garland Science 2007) Methylation of ribosomal RNAs Figure 12.41a Genomes 3 (© Garland Science 2007) Figure 12.41b Genomes 3 (© Garland Science 2007) Table 11.3 Genomes 3 (© Garland Science 2007) RNA processing RNA editing in apolipoprotein B mRNA Deamination of bases leads to codon changes RNA-guided insertion of Us into mRNAs Processing of RNAs is required for transport into the cytoplasm
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