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

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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)
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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: - modifications (methylation) of DNA & chromatin Post-Transcriptional Gene Silencing: - mRNA degradation - inhibition of translation 1. 2. homologous/complementary to target gene/mRNA X inhibition 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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Why do miRNAs most often inhibit translation but generally do not induce mRNA degradation like siRNAs?
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