Lec24 - Lecture 24 Non-coding RNAs & RNA interference...

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Lecture 24 Non-coding RNAs RNA interference (RNAi) (Lodish 6th ed. Chapter 8.4)
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We refer to “non-coding RNAs” all RNAs in the cell that do not code for protein These include: rRNAs tRNAs small splicing/processing RNAs (snRNAs, snoRNAs) discussed previously the more recently discovered microRNAs or small interfering RNAs (discussed here) that mediate gene silencing by RNA interference (RNAi) Other less well understood “long non-coding RNAs” (lncRNAs) with transcription regulatory functions Not discussed here (note: most of the genome in mammals appears to be transcribed at low levels - not only the genes - generating many lncRNAs with poorly understood functions!!!)
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RNA interference (RNAi) RNA interference (RNAi) is a eukaryotic gene-regulatory pathway that “ silences the expression of specific genes in response to homologous small double-stranded RNAs . Gene X DNA mRNA X Protein X Small 21-23 bp double-stranded RNAs Transcription Stability Translation Transcriptional Gene Silencing: Post-Transcriptional Gene Silencing: - mRNA degradation - inhibition of translation 1. 2. homologous/complementary to target gene/mRNA X inhibition
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Two types of small silencing RNAs: A. miRNAs (micro-RNAs) Derived from endogenous genes transcribed by Pol II as independent transcription units (miRNA genes) or from introns in pre-mRNAs of protein-coding genes. B. siRNAs (small interfering RNAs) Originate from exogenous long double-stranded RNAs (e.g., RNA viruses) or endogenous “aberrant” transcripts from repeated DNA and transposons From long double-stranded RNAs From hairpin RNA structures within Pol II transcripts Ribonucleases specific for double-stranded RNA (Dicer) and RNA hairpins (Drosha) miRNA siRNA
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siRNAs direct cleavage of target mRNAs by the RISC complex ATP A multiprotein complex containing an endo- ribonuclease (Ago2) that associates with siRNAs and cleaves target mRNAs complementary to the associated siRNA RISC : R NA-I nduced S ilencing C omplex Target release and degradation (exonucleases) Recycling of Activated RISC viral dsRNAs or dsRNAs from repeated DNA/transposons Post-transcriptional gene silencing by siRNAs: “Activated RISC”
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miRNAs inhibit translation of target mRNAs in association with RISC Post-transcriptional gene silencing by miRNAs: AAAA ORF ribosome RISC complexes usually base-pairing of miRNAs with 3’ untranslated region (3’ UTR) of mRNAs 3’ UTR Inhibition of translation AUG STOP Open-reading frame (ORF) / coding region CAP
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Lec24 - Lecture 24 Non-coding RNAs & RNA interference...

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