Lecture 6 RNA Processing (Splicing)

Lecture 6 RNA Processing (Splicing) - IPHY 3060 Lecture 6...

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IPHY 3060 Lecture 6: RNA Processing II —Splicing
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mRNA Splicing
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Key Points from Last Time: After transcription of an RNA from the DNA, the RNA is not finished and has to be processed further
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Key Point #1: The coding region of a newly transcribed mRNA has protein coding regions interspersed with regions that do not code for a protein
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Introns and Exons Exon —sequences of DNA in the coding region of a gene that code for a protein Introns —sequences of DNA in the coding region of a gene that do NOT code for a protein Gene Pre-mRNA 1 2 3 Introns mRNA 1 2 3
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RNA Splicing: The removal of the non-coding intron sequences found in the RNA after transcription
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RNA Splicing Function: to produce a mature mRNA that codes for a functional protein Occurs in the nucleus during transcription
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Key Point #2: RNA splicing requires special sequences within the intron
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Intronic Splice Site Sequences GU on 5’ end of intron Branch point A residue C/U-rich region AG on 3’ end of intron CU-rich region 5’ Exon 3’ Exon Intron G U A G 5’ splice site 3’ splice site A Branch point A 20-50 bp
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U RNPs Consist of both protein and RNA Bind to intronic splicing sequences Catalyze the cleavage of the intron from the exons and fusion of the exons together Form a complex known as the spliceosome Spliceosome
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Splicing
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Splicing Step 1: U1 and U2 bind to intron 5’ end and branch point A, respectively
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Splicing Step 1: U1 and U2 bind to intron 5’ end and branch point A, respectively Step 2: U 4, 5, and 6 bind to U1 and 2, form spliceosome
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Splicing Step 3: release of some U RNPs
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Splicing Step 4: hydroxyl on branch point A attacks phosphate bond linking last nucleotide of exon to first nucleotide of intron Step 5: 5’ end of intron is “free”; intron is in lariat formation
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Splicing Step 6: Hydroxyl on last nucleotide of 5’ exon attacks phosphate bond linking last nucleotide of intron to first nucleotide of exon, freeing intron and linking exons together Step 7: U snRNPs dissociate, intron is degraded
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First Transesterification—forms lariat Lariat
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Second Transesterification—ligates exons together, cleaves lariat intron
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Splicing seems complicated:
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Splicing seems complicated: Why is it done?
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