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RNA Processing I SG

RNA processing I

QuestionAnswer
True or False: Primary transcript is unable to be fully utilized for its function unless it undergoes RNA splicing True
Describe the 5'UTR non-coding region of mRNA 5' UTR= 210 nts. Allows the binding of 3 to 4 different ribosomes. Determines the efficiency of binding ribosomes on Euk mRNA
Internal Ribosome Entry Site For mRNA that have IRES, the ribosome does not have to bind the from the 5' end inward. Can bind internally
Fragile X syndrome Relates to the size of the the 5' UTR CCG. Should be no greater than 44 repeats. Over 200=full blown.
Describe the 3' UTR region of mRNA Generally dictates the stability of mRNA. There are 3' features that enforce the degradation of mRNA in Euk systems
All systems in Euk and Pro undergo processing except for mRNA of bacteria. Coupled transcription-translation
What kind of processing does tRNA undergo? At 5' and 3' end giving rise to 4S product. All functioning tRNA must have CCA at 3' end. The last A allows binding of Amino acids
What kind of processing does rRNA undergo? Ensure stoichiometry of the 16S, 23S and 5S at constant ratio. 23S and 5S go into 50S ribosome. 16S goes into 30S ribosome.
Describe the 5' cap structure of mRNA in Euk Has guanine nucleotide at 5' end. linked by a 5'-5' triphosphate bridge. Methyl group at 7th position (added co-transricptionally) Cap 1 and cap 2 methlyation
What is the purpose of the 5' cap? Protects the 5' end from exposure to degradation by 5' exonuclease. Averts instability due to cap 1 and cap 2 methylation.(chance of cis-diol cleavage is lost) Allows the end to be recognized by initiation factors.
Describe the 3' polyadenylation Addition of poly A tail. Endonuclease cleaves primary transcript shortly downstream of AAUAAA= recognized by poly A polymerase which is responsible for adding poly A tail.
What is the purpose of poly A tail? Protects the 3' exonuclease from attacking it. Poly A binding protein in complex with poly A.
What % of Euk mRNA have 5'cap? 98%. 2% are translated by IRES feature= allows them to use some initiation feature.
What % of Euk mRNA have poly A tail? 50%. Poly A tail is smaller in brain. Histone 3 does not have poly A tail.
Which mRNA has dual feature? p53. 5'cap and IRES, plays an important role in gene regulation.
When does RNA editing occur? Post-transcriptionally. Deamination occurs. Deaminases called ADARs require impact on structure
B100 vs. B48 Tissue-specific differential expression of ApoB100 vs. ApoB48 due to RNA editing mechanism. RNA editing gives rise to a product that has slightly functional twist compared to parent molecule.
Function of PABP Brings Poly A into juxtaposition to 5'cap structure and is mediated by a large # of initiation factors that is found within eIF4F.
rRNA processing in Euk Involves synthesis of a long primary transcript. 45S-28S, 5s, 18s. Cleavage of 45S generates 32S and 20S. 32S cleaved into 2 products that go into 60S. 20S cleavage=18S that goes in 40S.
Splicing No more than 20% tRNA undergo splicing using endonuclease and ligase. rRNA undergoes splicing and mRNA undergoes many splicing reactions using splicesome.
How is the 3'CCA created in tRNA? Catalyzed by nucleotidyl transferase.
tRNA splicing events IV sequences at 5' and 3' site is removed by endonuclease cleavage. Ligation occurs and requires ATP. Maintained in final mature form by intramolcular hydrogen bonds
Splicing of tetrahymena rRNA group I intron. Initiator presented in form of guanosine residue. 3'OH initiates transesterfication reaction at 5' splice site and 5'OH undergoes 2nd transesterfication.
 

 



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