RNA Replication II Flashcards
What are 3 processing steps of mRNA and where does processing take place?
Processing events are coupled to transcription. The phosphorylated CTD of RNApol2 acts as a binding site for enzyme complexes involved in mRNA processing.
Describe 5’ capping of mRNA.
GTP is added in reverse orientation (i.e., 5’ to 5’). There are RNases (enzymes that degrade RNA) that bind to and degrade RNA from the 5’ end. The 7-methylguanosine cap on the 5’ end of mRNA prevents it being degraded by these enzymes.
Describe 3’ tailing.
3’ tailing enzymes associate with phosphorylated CTD of RNApol2, and travel with the polymerase all the way from the transcription initiation site AAUAAA is a highly conserved signal sequence in mammalian cells. Lies 10-30 nucleotides upstream of the polyadenylation site
What is the functin of splicing?
In mammals, most genes contain multiple introns (noncoding sequences), which typically account for about ten times more pre-mRNA sequences than do the exons. Splicing must be highly specific to yield functional mRNAs.
T or F. Does the splicing of exons occur at the same time as the ligation of exons?
TRUUU.
-Specific RNA sequences indicate splice sites, i.e., the end of one exon and beginning of an intron
What are Spliceosomes?
U1, U2, and U5 snRNPs each contain a single snRNA molecule, whereas U4 and U6 snRNAs are complexed to each other in a single snRNP.
What is the first step of mRNA splicing?
At what poing does U2 bind?
(3’ spliceosome site)
Other proteins recognize the 3’ spliceosome site and cause the intron to “bulge.” U5 holds the first exon in place
What base does the 5’ end of the intron bind to and what structure is formed?
U6 snRNA catalyzes both steps in pre-mRNA splicing.
What do SR and U2AF proteins do?
Also mediate alternative splicing
-associate with CTD of RNApol2 - couple splicing to transcription - which may be important to keeping the correct order of splicing
What is alternative splicing and why is it important?
It is estimated that nearly all human genes produce multiple transcripts through alternative splicing, with up to several thousand mRNAs arising rom some genes.
Increase the diversity of proteins that can be encoded by the approximately 21,000 genes in humans
What is an examle of alernative splicing?
What is RNA editing and give an example?
E.g. deamination of cytosine to uridine, adenosine to inosine.
The apolipoprotein B (ApoB) gene is edited to produce two distinct proteins in the liver and the small intestine. ApoB100 (4536aas) is produced in the liver by translation of unedited mRNA. ApoB48 (2152aas) is produced in the intestine from mRNA that a C has been changed to a U by deamination, creating a stop codon (CAA->UAA).
Describe mRNA degragation.
Processed mRNAs are protected by capping and polyadenylation. Premature termination codons are identified as termination codons that occur more than about 50 nucleotides upstream of an exon-exon junction in spliced mRNAs. This halts translation and leads to degradation of defective mRNA.
What is the half life for mRNAs?
rRNAs and tRNAs account for more than 90% of all RNA in cells.
Short half life = rapid response to environmental stimuli (e.g., often transcription factors have short half-life)
picture of mRNa degragation
