RNA Processing Flashcards
What functional RNAs can be generated by processing Primary RNA transcript?
- mRNA
- tRNA
- rRNA
What does the 5’ cap consist of that protects the 5’ end of RNA transcript?
7-methylguanosine attached in a 5’, 5’-Triphosphate linkage
How is the 5’ cap tethered to Pol II carboxy terminal domain?
By Cap Binding Complex (CBC)
How many classes of Introns are there?
4
Defined by how they’re spliced
What are Class I & II introns?
Self-splicing
- No proteins involved
- RNA acts as an enzyme: Ribozyme
- Found in some nuclear, mitochondria, chloroplast genes, archaea and rare bacterial genes
What are Class III introns?
Not self splicing: splicesomal introns
- Largest group: represent most mRNAs
- Require small nuclear ribonucleoproteins (snRNPs or snurps)
- Spliceseosome activity is tightly coordinated with transcription
What are Class IV introns?
tRNAs require endonuclease and ATP
What are the steps of the Transesterification Reaction used by Class I introns?
- 3’ OH of guanosine cofactor acts as a nucleophile
- 3’ OH of 5’ splice site acts as a nucleophile
What are the steps in splicing Class II introns?
- 2’ OH Adenosine in intron acts as a nucleophile
- Forms intermediate lariat structure with 2’, 5’ link
- 3’ OH of 5’ splice site acts as nucleophile
Which SnRNA binds to the complementary sequence in 5’ splice site in the intron?
U1
What are the FIVE steps in Spliceosome activity?
- Assembly
- Activation
- Catalysis
- Products
- Recycling
What is involved in the Assembly phase of Spliceosome activity?
- U1 snRNA 5’ end binds to complementary site in 5’ end of intron
- U2 snRNP base pairs with 3’ end
- Complex forms to generate catalytic activity
Where is the splicing apparatus tethered to?
Carboxyl Terminal Domain (CTD) of RNA Pol II
Introns remain in nucleus
What is the function of the Poly A tail mRNA in Eukaryotes?
- Regulates stability of mRNA
- Protects it from nucleases
- Proteins can bind here
How is a Poly A tail formed on mRNA?
- Polymerase extends RNA past cleavage signal sequence AAUAA
- Endonuclease cleaves at AAUAA
- Polyadenylate polymerase adds (AAAA)n tail
What are TWO mechanisms of alternative processing of mRNA in Eukaryotes?
- Poly A site choice: more than one polyadenylation and cleavage site
- Alternative 3’ splicing: Different 3’ splice sites available
How many mechanisms does the Calcitonin gene use?
2
- Different Poly A sites predominate in brain or thyroid
- Alternative splicing
What are the steps for processing pre-rRNA transcripts in bacteria?
- RNA precursor (30s pre-RNA)
- RNA is methylated at specific bases
- Cleavage by RNase II, RNase P, RNase E and nucleases
