Pre-mRNA processing LO Flashcards
What are the 3 major ways that mRNAs are processed?
- capping
- splicing
- cleavage/polyadenylation
what is capping?
addition of 7’methylGPPP cap to protect the exposed phosphate
what is splicing?
removal of introns by excision
ligation of exons together to make complete readable sequence
what is cleavage/polyadenylation?
removal of portion of 3’ end of by cleavage and then addition of polyadenosine tail to improve stability
what is pre-mRNA?
the primary transcript from DNA, needs to be processed before it can be translated, does not have 5’ cap or poly A tail. contains introns
what is mRNA in regards to pre-mRNA?
processed mRNA that is ready for translation into a protein
what binds the 5’ cap to the pre-mRNA?
cap binding proteins (CBP)
what is the functions of the 5’ cap?
- promote splicing of proximal intron to cap
- signal that mRNA is ready for export to cytoplasm
- prevent degredation of mRNA
what signals that mRNA is ready for degredation?
removal of the 5’ cap
what is the function of eIF4E?
(eukaryotic initiation factor 4E)
promotes translation initiation
what are the 3 steps required to place a 5’ cap to a pre-mRNA?
- triphosphatase removes gamma phosphate from 5’ end
- Guanylyltransferase adds GTP to terminal phosphate losing 2 phosphates from GTP in the process
- 7 nitrogen of guanine methylated by methyl transferase
what type of linkage is formed by guanylyltransferase?
5’ to 5’ linkage between phosphates of RNA and GTP
what is the methyl donor in the methylation of 7 nitrogen of guanine?
S-adenosylmethionine
what is the conserved consensus sequence in the 5’ end of an intron?
G U
what is the conserved consensus sequence in the 3’ end on an intron?
A G
what is the poly A site consensus sequence?
A A U A A A
what is alternative splicing?
removal of different parts of a pre-mRNA to form different mRNA that will code for different proteins. Many pre-mRNA transcripts can code for multiple proteins but not until they are spliced at specific sites
what are 2 diseases caused by splicing defects?
- marfans syndrome
2. spinal muscular atrophy
what is the presentation of Marfan’s syndrome?
very tall stature, long limbs, protruding sternum, weakness or defects of the aorta and heart valves
what type of inherited disease is Marfan syndrome in regards to genetics?
dominant autosomal disease
Marfan is a result of disruption of splicing of which gene transcript?
fibrilin
what is fibrilin?
component of connective tissues
mutations of the splicing of the fibrilin gene interfers with the binding of which molecules?
U1 and U2 with snRNAs which regulate splicing causing marfans
what is the function of U1 snRNA in splicing?
recognized the 5’ splice sequence by base pairing with it (base pairs between the exon and the intron)
what is the function of U2 snRNA?
recognizes the branch point between introns and exons
what causes recognition of exons by the splicesome?
binding of U1 snRNA with U2 snRNA upstream of exons
what is a branch point on a pre-mRNA?
series of nucleotides between exons
what are the 2 reactions that make the mature 3’ end of mRNA?
- cleavage: 20 bp downstream of AAUAAA is cut by endonuclease exposing 3’ hydroxyl group
- at 3’ OH, polyadenylation occurs
what is the function of cleavage/polyadenylation specificity factor (CPSF)?
- associated with RNA pol II and polyadenylate polymerase
- when poly A tail gets around 250 bp long, polyadenylate polymerase cannot bind to CPSF anymore and this signals termination of RNA pol II from transcribing
What are the 3 major functions of an mRNA’s poly A tail?
- makes mRNA stable and prevents premature degredation
- tail is bound by poly A binding protein (PAB I) which boosts translation in the cytoplasm by recruiting mRNA to ribosome
- signals for transport from the nucleus to the cytoplasm
how does alternative poly A sites cause more than one protein to be made from a single gene?
early termination of transcript by addition of the poly A tail can leave exons from being transcribed thus making a different protein
what is an example of different poly A sites resulting in different proteins?
- IgM has 2 poly A sites
- MuS and MuM are 2 sites
what type of IgM occurs from cleavage at the muS site?
antibody form that is secreted from plasma cells
what type of IgM from cleavage at the muM site?
membrane bound antigen receptor on B cells
