made from the death cap mushroom and is a known potent inhibitor of RNA

Made from the death cap mushroom and is a known

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made from the death cap mushroom and is a known potent inhibitor of RNA polymerase II One single mushroom could very easily lead to a fast death in 10 days The mechanism of action is that alpha amanitin inhibits RNA polymerase II at both the initiation and elongation states of transcription CHAPTER 14: RNA MOLECULES AND RNA PROCESSING RNA Molecules and RNA Processing In prokaryotes, the coding region of a gene isn’t interrupted: the sequence of the gene is co-linear with the amino acid sequence of the protein Therefore, the prokaryotic messenger RNA sequence corresponds to the gene from which it was transcribed
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The Shine-Dalgarno sequence, 5’ UAAGGAGGU 3’ is involved in the initiation of translation However, in eukaryotes, genes are often interrupted: exons are protein coding segments, introns are intervening (non-coding) segments The presence of introns in eukaryotic genes was discovered in the 1970s The removal of introns along with additional RNA processing steps are required to form the mRNA that will be translated into a polypeptide Summary of the three main processing steps in eukaryotic nuclear pre-mRNA 1. Addition of 7-Methyl Guanosine Cap o Linked to pre-mRNA by a unique 5’-5’ phosphate linkage 3’ 5’ Intron removal
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2. Addition of PolyA Tail o Pre-mRNA is cleaved and then, a long string of A residues is added by Poly A polymerase 3. Removal of Introns o Introns in pre-mRNA are removed by a specialized process called RNA splicing Details of the three pre-mRNA processing steps 1. The 7-Methyl Guanosine (7-MG) Cap o Addition of 7-MG cap occurs early in the elongation process 2. Addition of the 3’ PolyA Tail o Eukaryotic pre-mRNA is cleaved 11-30 nt following the 5’ AAUAAA 3’ sequence in pre-mRNA o Poly A polymerase adds a string of ~200 A residues at the cleaved end 3. Removal of Introns from pre- mRNA o Removal of introns must be precise in order to properly fuse the 3’ end of one exon to the 5’ end of the next exon o Every intron has two conserved sequences that are required for its removal 5’ and 3’ splice sites: GU and AG sequences, respectively Intron Branch Point: conserved A residue o Accomplished by an RNA/protein complex known as the spliceosome RNA/protein structure Five small nuclear RNAs (snRNAs): designated U1, U2, U4, U5, and U6 The snRNAs associate with about 40 small proteins to form small nuclear ribonucleoproteins (snRNPs) snRNPs U1, U2, U4/6, and U5 assemble to form a complete spliceosome Lariat formation involves unique linkage between the 5’ snRNP assembly
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phosphate of the G and the 2’ OH of the A So, the introns of nuclear pre- mRNA transcripts are carried out by spliceosomes The introns of some rRNA precursors are removed autocatalytically by reaction of the RNA molecule itself The introns of tRNA precursors are excised by precise endonucleolytic cleavage and ligation reactions The presence of introns allows for alternative modes of splicing and
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