lec 2 notes.docx

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Stavroula Andreopoulos

SPLICINGEUKARYOTESlec2 Splicing occurs posttranscriptionally in eukaryotes but not in prokaryotes Other RNA besides mRNA rRNA and tRNA are components of nuceloproteins with a variety of functions eg Splice introns and process rRNAtRNA RNAP I is found in the nucleolus and produces 3 rRNAs in tandem ie 18S 58S 28S1Primary transcript prerRNA is 45S which undergoes nt modifications via ribonucleoproteins to add CH3Pseudouridine groups2Cleavage of prerRNA by small subunit processome SSU to form mature rRNAsRNAP II found in the nucleoplasm produces mRNAs 7Primary transcript 5end is enzymatically capped with mG via a 55 cap to stabilise st mRNA against endonucleases degradation and phosphatise degradation of the 1phosphate Eukaryotes only have 3 caps 012iP of GTP attacks P of ATP using guanylyl transferase to release PPi GPi AGMPADPstiiGMP with a ribose sugar upside down forms the 1 cap ie Cap 0 to allow 55 link formation7iiiCap 0 N7 is methylated using Sadenosyl methionine to form mGivSubsequent methylations of ribonucleotides downstream occur using the same enzyme ie 2OH of ribose is methylatedAAUAAA3Polyadenylylation of mRNA produces a consensus sequenceaka polyA addition site in order for transcription termination to occur PolyA tail increases stability of transcript and protects against exonucleases1RNAP II synthesises the consensus sequence with a GU bp just downstream2Cleavage polyA specificity factor CPSF recognises the consensus sequence to bind mRNA3Cleavage factor CF downstream on consensus sequence joins to CPSF to 3form looplike structure in transcript to stop transcription ie GU AAUAAA4PolyA polymerase PAP polyadenylates the 3end 1035nts down from consensus by adding a string of 80250 A residues
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