Transcription And RNA Processing II Flashcards
What three things does mRNA processing include in eukaryotes?
Capping
Tailing
Splicing
What is added to the 5’ end of mRNA? And what is significant about this bond?
7-methylguanosine and methyl groups to the next few bases
5’-5’ phosphodiester linkage, most enzymes can’t recognize this linkage therefore it serves to stabilize the molecule and protect it from ribonucleases.
What is added to the 3’ end of the pre-mRNA?
A poly A tail via polyadenylation.
What is the function of an AAUAAA sequence upstream of the poly A site and a GU-rich sequence downstream?
Serve as binding sites for endonucleases to cleave at a CA dinucleotide to allow polyadenylation.
What is RNA splicing?
The processing step where introns are removed and exons are linked together.
What is a spliceosome?
An RNA and protein complex where RNA splicing takes place.
Describe the roles of U1snRNP and U2snRNP.
U1 binds the 5’ splice site
U2 binds to the branch point in the intron
U1 and U2 bind to one another, bending the intron and bring the two exons close together.
What is the next step in splicing after U1 and U2 bind each other?
U4/U6 and U5 enter as a complex.
U5 binds to the first exon and “holds” it.
What happens after U4 and U1 leave the splice site?
U6 forms a complex with U2 causing:
- the first exon to be cleaved at the 5’ splice site
- the newly freed 5’ end of the intron joins to an adenine via a 2’-5’ phosphodiester bond creating the lariat structure.
What is the final step in splicing?
U5 binds the 3’ splice site, leading to the cleavage and removal of the lariat intron and linkage of the two exons.
What is the function of SR proteins?
Recruit snRNPs to initiate spliceosome assembly.
Also bind the C terminal domain of RNA Pol to maintain the correct order of splicing by coupling it to transcription
What is the function of U2AF?
Binds to the 3’ site and recruits U2 to the branch point within the intron.
What is RNA editing?
The process of changing single bases in RNA to alter the amino acid sequence of the protein product.
How are lariats targeted by enzymes for degradation?
The enzymes recognize the 2’-5’ phosphodiester bond
What areas of the mRNA are targeted for degredation?
3’ and 5’ naked ends
