Lecture 14 Flashcards
How is rRNA processing different from something like mRNA processing?
mRNA processing undergoes splicing (specifically alternative splicing) whereby introns are spliced out and exons are brought together in a particular arrangement. rRNAs undergo cutting by RNases, but the exons are not “spliced” together.
Which Pol RNA products, Pol I, II, and/or III products, undergo processing?
ALL OF THEM
How does the Pol II tail aid in processing?
It helps recruit splicing and processing enzymes AS THE pre-mRNA IS BEING MADE.
What is the 5’ cap (step 1) of pre-mRNA processing, and why is it specific to mRNA?
It is a 7-methylgunaylate cap added by Guanyly Transferase, and it’s specific to mRNA because RNA Pol I and III don’t have the “tail” that facilitates capping.
How does the 3’ poly A tail get put onto the pre-mRNA, and what step in mRNA processing does this represent?
The sequence AAUAAA at the 3’ end of the transcript recruits certain proteins that facilitate cutting of the RNA at this point. Then, Poly A Polymerase (PAP) generates the poly A tail (with an OH at this 3’ end.) This represents Step 2 in mRNA processing.
What is the final step in mRNA processing?
Step 3, the final step, is splicing, to generate a mature mRNA that can be translated.
Which processing events help facilitate the generation of different mRNA, and thus different proteins, from the same gene?
Alternative splicing (cell specific splicing events) and alternative polyadenylation (different poly A sites on pre-mRNA recognized in different cells/tissues.) For example, Calcitonin and CGRP are made from the same primary transcript (calcitonin = exons 1-4 whereas CGRP = exons 1-3,5-6)
What are two sequences within an intron that help define its boundaries, and at which ends are they located?
There’s a GU sequence at the 5’ end and an AG sequence at the 3’ end.
How do Spliceosomes recognize the intron boundary sequences?
The U1 snRNA of the spliceosomes contain complementary sequences that bind these intron boundary sequences. Without the complementarity, the spliceosome can’t recognize the splicing site.
What occurs/allows for exons to be brought together?
There are 2 transesterification reactions that take place while spliceosomes “hold the whole thing together.”
What are the 3 pathways for mRNA decay?
- Decapping
- Deadenylation
- Endonuclease activity (cutting the transcript somewhere in the middle.)
What are miRNAs and siRNAs responsible for?
miRNAs affect mRNA processing by base-pairing with it. siRNAs specifically silence an mRNA.
Beta-thalassemia affects the ___ globin protein, leading to symptoms like anemia, pale complexion and sometimes splenomagly. It can arise from a mutation in the ______, resulting in less bet-globin production, or it can arise from a mutation in both alleles that gives rise to issues with the __ and __ dinucleotide regions that define _______ which leads to no beta-globin.
It affects the beta-globin protein (part of the hemogolbin complex) and can lead to symptoms like mild anemia, pale complexion, or in some cases, splenomegaly.
It can arise from a mutation in the promoter –> less beta-globin produced. Or, it can arise from a mutation in both alleles that cause issues with the GU and AG dinucleotide regions that define splicing sites –> no mature mRNA from the pre-mRNA, so no beta-globin (more severe.)
How does Retinitis Pigmentosa arise?
It is caused by an issue with the PPRF protein of the spliceosome. It leads to aggregation of “bone spicules” in the retina.
What is the cause for Spinal Muscular Atrophy (SMA)?
It is caused by a mutation in the splicing of SMN1 and SMN2. Low levels of these proteins cause motor neurons to atrophy, and thus the muscle they innervate atrophies as well. The real issue is, although SMN1 and 2 are duplicate genes, SMN2 protein is normally produced at low levels because of exon skipping (exon 7) in most SMN2 RNAs. When there is a mutation in SMN1, the naturally low levels of SMN2 can’t compensate.
