Pre mRNA Processing Flashcards
What are the three ways that most mRNAs are processed? (Primary Transcript - mature mRNA)
Capping, Splicing, and Addition of the Poly-A Tail
What is Capping of an mRNA primary transcript?
On the 5’ end of the primary transcript, a phosphate is removed, a guanosine monophosphate is added, and a methyl is added to the guanosine at the 7’ position. This is a 5’-5’ linkage with triphosphate inbetween.
How does capping assist mRNA?
5’ Cap protets mRNA from ribonucleases and assists in ribosome binding (through eIF4E) in Eukaryotes. It also assists in splicing/processing through its cap binding complex.
What are the reactions necessary to create the 5’ cap?
- Cut off the last 5’ phosphate, add GTP as GMP (losing PiPi) in a 5’ to 5’ linkage, and finally methylate the 7’ position of the guanosine.
What is splicing, and how is it done?
This is the removal of introns through a lariat intermediate. The 3’ exon is brought close via binding between the branch point A (2’-OH) and the 5’ exon.
What are the conserved sequences on an intron, and where are they?
GU (5’ end), AG (3’ end), an A (branch point A) and AAUAAA (at poly site).
What methods can be used to create more than one mRNA from a primary transcript?
Exon skipping, mutually exclusive exons, truncation at the 3’/5’ ends of any given exon, or retained introns.
What diseases are associated with splicing defects?
Marfan’s Syndrome (Fibrillin Gene is mutated causing long, tall features and a protruding chest), Abnormal splicing of CD44 causes tumor metastasis.
What binds to the GU at the 5’ end of an intron?
U1snRNA
What binds to the AG at the 3’ end of an intron?
U2AFsnRNA (U2 Associated Factor)
What binds at the branch point A on an intron?
U2snRNA
Explain the Cleavage/PolyAdenylation process?
RNA is cleaved at 3’ end 10-30bp past AAUAAA. Polyadenylate Polymerase then adds 150-200bps.
What is the function of the PolyA Tail?
It protects mRNA from enzymatic destruction and acts in exporting the mRNA from the nucleus.
How does immunoglobulin M use alternative PolyAdenylation to create multiple proteins?
Two chains can be created, one transcribes much past the poly-A site to create a longer protein.
How does the calcitonin gene create multiple proteins?
The thyroid version, CGRP, is the full length version, whereas the brain version, calcitonin, excludes exon #4.
