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Joe Kim (16)

Module 9(1) - March 21 - BIO 2C03.docx

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Joe Kim

BIO 2C03 2013 Module 9 Lecture 1 – MicroRNAs (miRNAs) and the Control of Translation (March 21) siRNA vs miRNA  MicroRNA (miRNA) o Large family of small (circa 21 nucelotides long) regulatory, non-coding RNAs (ncRNA) o miRNAs bind to and attenuate the expression of target mRNAsby causing translational repression or degradation o miRNAs bind (i.e. “base pair”) generally to the 3’ UTR of one or a few different mRNA species o it is predicted that the human genome can express as many as 1000 different miRNA species controlling the expression of about 30% of al protein-coding genes of the genome  siRNA and mirRNA share many features but fulfill different functions Introduction to small (short) non-coding RNAs (sncRNAs)  Small interfering RNAs (siRNAs) and microRNAs (miRNAs) are short RNA species characterized by extensive region of secondary structure  They are generated by their own machinery but several factors and processes are common in the biosynthesis of these non-coding RNA species  siRNAs function in “genome surveillance” limiting the propagation of viruses or transposons and controlling chromatin structure  mirRNAs are regulatory molecules limiting gene expression through diverse mechanisms including the inhibition of translation in animal cells Overview of Translation Initiation  Translation Initiation in Prokaryotes 1 BIO 2C03 2013  Summary of Translation in Prokaryotes o Four steps in translation 1. tRNA charging 2. Initiation 3. Elongation 4. Termination and peptide release 2 BIO 2C03 2013  Translation Initiation in Eukaryotes o No Shine-Delgarno sequence on mRNA o Initiation generally depends on the Cap o The Cap is recognized by elF4E o Several “eIF” are required for initiation  eIF = eukaryotic Initiation Factor o Interaction of the “PolyA Binding Protein” at the 3’ end of the mRNA enhances the recruitment of the small ribosomal subunit  Functions of the CAP o mRNA stability, coupling to splicing, mRNA export, mRNA translation  Cap-Dependent and Cap-Independent Translation Initiation o Initiation can take place at an “internal ribosome entry site” or IRES o Several viruses use IRES to promote the translation of their own mRNAs  eIF-4G mediates the interaction with PABP and EIF-4E (Cap-binding protein)  The 43S pre-initiation complexmust unwind the mRNA and scan the mRNA for an AUG o The translation initiation codon is often found within a consensus sequence called the Kozak sequence (named after Marilyn Kozalk)  Translation Initiation in Eukaryotes – Summary o Formation of the pre-initiation complex requires several eIFs o Initiation is generally dependent on the interaction of eIF-4E with the Cap o The interaction of PABP with eIF-4G promotes the formation of the pre-initiation complex o An RNA helicase activity, present in the pre-initiation complex, unwinds the secondary structure of the RNA 5’ UTR o The pre-initiation complex, which includes the small ribosomal subunit, scans the 5’ UTR and looks for the f
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