Lecture 10 Flashcards
Difference between prok and euk mRNA?
eukaryotes:
- 7-methylguanosine cap on the 5’ end
- Poly-adenylation at the 3’ end
prokaryotes:
1. mRNA is polycistronic or contains multiple gene sequences
When does pre-mRNA become mRNA?
modification of the 5 prime end (methylguanosine)
addition of the poly-A tail on the 3 prime end
splicing of the exons and the removal of the introns
What RNA processing proteins does RNA poly II have?
- capping factors
- splicing factors
- polyadenylation factors
What does phosphatase do?
removes one phosphate from the 5’ end of the RNA
What does Guanylyl transferase do?
adds a GMP in a reverse linkage (5’ to 5’ instead of 5’ to 3’)
What does Guanine-7-methyl transferase do?
adds a methyl group to the 7 position of the terminal guanine
What does 2’-O-methyl transferase do?
adds a methyl group to the 2’-O position to the next to last base on the 5’ end
why is the 5’ methyl cap important in mRNA regulation?
It helps with processing, transport, and translation
What is the function of the 5’methyl cap?
- regulates export of mRNA out of the nucleus
- required for the efficient translation of mRNA into protein
- prevents 5’ degradation
3’ mRNA processing-cleavage
1) Cleavage and polyadenylation specificity factor (CPSF) binds to the hexamer AAUAAA (the polyadenylation signal).
2) Cleavage stimulating factor F (CstF) binds the GU-rich element beyond the cleavage site.
3) Cleavage factors bind to the CA sequence at the cleavage site.
how does polyadenylation occur?
- Poly-A-polymerase (PAP) adds approximately 200 A nucleotides to the 3’ end produced by the cleavage
- Poly-A Binding Proteins (PABP) binds to the poly-A tail and assist in directing translation by the ribosome
- The cleaved fragment of the RNA is degraded in the nucleus.
What is the function of the 3 prime adenylation?
- increases the half life of the mRNA
- protects from enzyme attack
- transcription termination
- aids in mRNA export out of the nucleus
How is intron removal done?
- 5 snRNAs and other splicing factors mediate splicing
- The snRNA’s are U1, U3, U4, U5, U6 (bind with the consensus sequences)
- U1 and U4 leave, thus activating the splicosome
- U2, U5 and U6 interact to bring neighboring exons
into correct alignment allowing for two transesterification reactions to occur
What are the consensus sequences?
3 prime = AG
5 prime = GU
Systemic Lupus Erythematosus
This is an autoimmune disorder where the body attacks its own U1 RNA component of the splicosome, histone, and topoisomerases
symptoms:
fatigue, arthritis, fever skin rashes and kidney problems
How can biodiversity be increased in mRNA?
alternative splicing and RNA editing
What is alternative splicing?
Splicing the same mRNA strand in different ways to get a different protein
explain negative and positive alternative splicing?
positive - an activator protein binds to the mRNA that supports splicing
negative - a repressor bind to the mRNA that inhibits splicing
beta-thalassemia
Many individuals with thalassemia have
mutations in the beta-globin gene that generate additional splice sites within the mRNA
Limb Girdle Muscular Dystrophy
Some patients with Limb Girdle Muscular Dystrophy possess a mutation in the calpain-3 gene that generates a new splice site within exon 16
shorter mRNA and thus defective protein
Chemical modification of mRNA is known as what?
RNA editing
What are the two types of RNA editing?
- Adenosine to Inosine
2. Cytidine to Uridine
cytidine deaminase enzyme?
edits the apolipoprotein B, thus their are two forms
adenosine deaminase.?
Editing of the glutamate receptor involves a single base change from A to I (inosine). This editing results in a codon change within the mRNA from glutamine to arginine
This editing is required for proper brain development
this editing impacts calcium permeability
mutation in intron 1 splice acceptor site?
shorter length mRNA
mutation in intron 1 intorduces a splice acceptor site?
longer mRNA
Mutation in intron 2 splice donor site?
longer mRNA
