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RNA Processing I SG
RNA processing I
Question | Answer |
---|---|
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. |