DLA post-transcription Flashcards
What are the 3 important modification of mRNA?
- Addition of 7- methyl-guanosine cap of the 5’ end of mRNA
- Addition of poly-A tail at 3’end of RNA
- Removal of introns and splicing together of exons
Where does 7-methyl-guanosine capping occur? and by what protein? and when?
in nucleus by a group of proteins that are initially bound to the RNA polymerase II enzyme, prior to completing transcription.
What is capping important?
It is required for the efficient binding of the ribosome to the mRNA and translation
What kind of enzymes are involved in capping? (4)
- Phosphatase
- Guanylyl transferase
- Guanine-7-methyl transferase
- 2’O-methyl transferase
Where and when does polyadenylation occur?
It occurs in nucleus following transcription.
What are the three functions of poly A tail?
- Aiding mRNA transport out of the nucleus to cytoplasm
- Stabilize mRNA in the cytoplasm so that they can serve as messages for translation for a longer time
- increase efficiency of the initial steps of translation
What kind of enzymes are involved in polyadenylation? (4)
- Cleavage and polyadenylation specificity factor (CPSF)
- Cleavage stimulating factor F (CstF)
- Cleavage factor
- Poly-A-polymerase (PAP)
Cleavage and polyadenylation specificity factor (CPSF)
Binds to the polyadenylation
signal (AAUAAA) in the 3’ UTR of the mRNA
Cleavage stimulating factor F (CstF)
binds to a GU-rich region located in the 3’ UTR
past the point where the 3’UTR is cleaved
Cleavage factors
bind to a CA sequence located after the polyadenylation signal and
cleaves the mRNA at this location. The small cleaved fragment is degraded and the
remaining mRNA is polyadenylated.
Poly-A-polymerase (PAP)
adds approximately 200 A nucleotides to the new 3’ end of the
mRNA produced by the cleavage of the 3’ UTR.
Poly-A binding protein (PABP)
binds to the poly-A tail and assists in directing
translation by the ribosome.
Where does splicing occur?
at splice donor and splice acceptor sites located on the 5’ and 3’ ends of introns.
What sequence does 5’junction site possess?
GU sequence
What sequence does 3’ junction site possess?
AG sequence
What is the name of conserved sequence located 18-38 nucleotides upstream of 3’ junction site, where it always possess an A nucleotides?
The branch point sequence
What are name of proteins that are involved in splicing?
small nuclear ribonucleoproteins (snRNP). They form spliceosome.
Steps of splicing (4)
1) Cleavage of the pre-mRNA occurs at the 5’ splice junction site.
2) The G nucleotide located at the 5’ of the intron folds back and forms a bond with the A
nucleotide of the branch point sequence. This produces a lariat structure.
3) Cleavage of the pre-mRNA occurs at the 3’ splice junction site thus removing the intron.
4) The two exons are ligated together.
What is the purpose of alternate splicing?
Alternative splicing ultimately increases the level of molecular diversity of both mRNA and proteins
in the cell.
How is alternate splicing regulated?
by binding of activator or repressor proteins at the splice junction site.
What is RNA editing?
change of genetic information at the level of mRNA
2 examples of RNA editing in mammal
single base editing of apolipoprotein-B mRNA and the glutamate receptor mRNA
Which enzyme carries out RNA editing of apolipoprotein-B mRNA?
Cytidine deaminase
Which enzyme carries out RNA editing of glutamate receptor mRNA?
Adenosine deaminase
For parasites, what kind of enzymes are involved in RNA editing?
- Guide RNA binding
- enzyme complex that has endonuclease activity,
- a terminal uridyltransferase (inserts uracil bases)
- RNA ligase
What is the clinical relevance of abnormal splicing of mRNA to lupus erythematosus?
Lupus patients
possess auto-antibodies that cross react with the U1 RNA component of the spliceosome. This may
prevent the normal splicing of the mRNA of some genes.
How is beta thalassemia and limb girdle muscular dystrophy related to abnormal splicing?
mutations in genes destroy splice donor/acceptor
sites or generate new splice sites.
