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Exam 4 Study Guide (Got A+ on the test)

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Louisiana State University
Biological Sciences
BIOL 3090

BIOL 3090 Sect 1 Spring 2011Study Guide for Exam 4 Everything we covered in unit 4 is fair game but this reviewstudy guide covers topics that I feel are most important Use the study guide as a check list once you know one topic move on to the nextPace yourselvesRemember I write new questions for every test so old exams wont be of much helpyou have to know the materialStarting with Lecture 26 on April 1 reading assignments pick up with Page th340 in the new 6 editionChapter 8 PostTranscriptional Gene Control RNA ProcessingOur last topic on test three was Group II self splicing introns1 RNAEditing What is a kinetoplast and where do you find it and how do guide RNAs edit its premRNAs pp 340341How prevalent is RNA editing in mammals Fig 819A kinetoplast is one large mitochondrion that is found in the protozoan species Trypanosoma Kienetoplast mRNAs are edited by the insertion mostly and deletion less frequently of U nucleotides Editing is controlled by small guide RNAs These RNAs eventually encode a protein so the sequence must be correct Kinetoplast premRNA editing moves from 35 Once one region is edited another guide RNA anneals to edit more sequences further upstream Notice that the guide RNA has As and Gs that direct the incorporation of Us in the premRNA Incredibly after editing the ORF encodes a functional mitochondrial protein Only two examples of mRNA editing are known in mammals In one example C is deaminated to U by a special enzyme found in intestinal cells but not in hepatocytes 2 TF Certain mRNAs are specifically localized to distinct regions of the cell True 3 The steady state concentration of a mRNA depends on its rate of synthesis and its rate of degradation4 Whats the main pathway for mRNA degradation in the cytoplasm Fig 829 What are P bodies page 353There are specific mechanisms to degrade mRNA Eventually the PolyA tail will become so short that the mRNA is highly unstable and will degrade 35 exonucleolytic decay or decapping will degrade the mRNA A special enzyme is used for decapping There is also 53 exonuclease activity once the 5 cap is removedThe P body is the processing body This is where mRNAs are being degraded15 What are microRNAs and how do they regulate translation of particular mRNAs Fig 826 and the left half of Fig 825MicroRNAs miRNAs block translation Humans encode 1000 miRNAs These are transcribed by RNA Polymerase II Drosha is a doublestranded RNA specific endonuclease that works with DGCR8 Pasha to cleave the premiRNA to 70 nucleotides Drosha and Pasha recognize the hairpin structure and cleavetrim it down to size It is the premiRNA that is being trimmed The mature trimmed version is exported to the cytoplasm Once in the cytoplasm the premiRNA hairpin is processed by DICER and RISC DICER recognizes the dsRNA and trims it even further to 21 nucleotides in length DICER then passes the 21 nucleotide fragments to RISC to look for complementary RNA miRNAs bound by RISC anneal to complementary 3 UTRs of certain mRNAs to inhibit translation perhaps by localizing mRNA to cytoplasmic P bodies which are sites of mRNA degradation miRNA is not a perfect hybrid because there are gaps The important thing is that you are blocking translation siRNA will induce RNAi if there is a perfect hybrid 6 What are translationally dormant mRNAs And how do they become active for translation Fig 828mRNAs are found in oocytes eggs or very early embryos are held in a quiescent state that prevents them from interacting with translating ribosomes Maskin binds to CPEB a binding protein with a RRM and zinc finger that is found to the cytoplasmic polyadenylation element CPE upstream of the PolyA tail Maskin also binds to the initiation factor eIF4E that is bound to the 5 cap This creates a loop and effectively prevents translationWhen CPEB becomes phosphorylated Maskin is released Then cytoplasmic versions of polyadenylation factors then extend the PolyA tail PABPI then binds the PolyA tail and stabilizes translation factors The mRNA is now primed and activated for translation 7 What are iron response elements and how do they regulate translation of the transferrin receptor mRNA Fig 831bIron response elements IREs along with the IREBinding Protein IREBP regulate the transferrin receptor TfR In the 3 UTRs of short lived mRNAs proteins bind for 35 degradation and TfR is produced The TfR binds transferring a vertebrate serum protein that carries iron to deliver iron into cells 2
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