Chapter 8 Post Transcriptional gene control Ways to control translation after transcription 1 miRNAs repress translation by stalling the ribosome 2 siRNAs cause mRNA degradation both are 20 26 bp short RNA s believed to be ancient defense mechanisms against viruses MicroRNA s present as genes humans have at least 1 000 of them can be transcribed by RNA Poly II once transcribed it is a primary microRNA 70 80 bp psedocomplimentary not perfectly complementary but folds back on itself into a hairpin loop o allows it to be found in the nucleus by DGCR8 double stranded RNA binding protein o second protein Drosha binds a ribonuclease cuts part of the primary miRNA Once in the cytoplasm TRBP protein binds double stranded RNA binding protein Dicer protein cuts the pre miRNA and left with a mature microRNA 20 26 bp One of the 2 final strands enters the RISC complex where microRNA is loaded miRNA RISC complex base pairs the miRNA complementary regions of the 3 untranslated regions of target mRNAs o stalls the ribosome so translation can t be completed siRNA s RNA Interference come from viruses or manufactured in the lab bind to coding region of the mRNA need a perfect match in the 21 23 bp long siRNA not just pseudocomplimentary dicer cuts at hairpin and left with siRNA that is complementary to the target RNA binds to coding regions of mRNA activates argonaute and cuts mRNA every 10 11 bp o net effect mRNA is degraded
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