mRNA processing: 5' cap and poly (A) tail
polycistronic mRNA
- encode multiple polypeptide chains - have multiple start and stop signals - prokaryotic mRNAs
monocistronic mRNA
- encodes a single polypeptide chain - single start site for protein synthesis - almost all eukaryotic mRNAs
5' capping: role in nuclear pore docking
- mRNP= messenger ribonuclear protein complex; RNA and protein - RNA-binding protein package mRNA for export - some shuttling RNA-binding proteins removed and cytoplasmic proteins bind through the process - RNA helicase in NPC cytoplasmic filaments remodels mRNPs - ribosomes can bind during process (5' end transports first)
Functions of 3' polyadenylation
- protects the mRNA from nuclease attack - aids ribosome recognition of mRNA as molecule to be translated - induces proteins that export the mRNA from the nucleus - used to isolate mRNAs from rRNA or tRNA in the lab (want to examine gene expression -> cRNA)
Draw out a comparison between the processes for synthesis of functional mRNA in prokaryotes and Eukaryotes - what are the biggest differences
- separation b/w processes - 5' cap - polyA tail - intron removal
Prokaryotic RNA processing...why are they the exception to the generalization that RNA requires processing before it can be used
- synthesized in a form read for translation immediately - transcription and translation are not separated by a membrane (may have an influence on each other; could happen simultaneously)
Polyadenylation process
1. CPSF (cleavage and polyadenylation specificity factor) binds AAUAAA 2. CSF binds GU/U to form stable loop by CPSF 3. CFs bind near cleavage site 4. CPSF and CstF form an RNA loop 5. cleavage 6. poly (A) polymerase uses ATP to add tail 7. poly (A) binding proteins; later role in stimulating translation
Roles for 5' caps
1. signaling proper splicing 2. regulation of ribosome binding for translation 3. docking with nuclear pore during export
polyA tail: length
200-250 nts long
polyA tail: name for process and the enzyme that does it
3' polyadenylation - poly (A) polymerase or polyadenylate polymerase - uses ATP as a substrate to add A's to 3' end of pre-mRNA
What sequence must pre-mRNA have to get a tail
AAUAAA
polyA tail: when
Added AFTER transcription at 3' end - known because tail not encoded by the gene; not in DNA
Eukaryotic RNA processing: what does transcription yeild
a precursor mRNA (pre-mRNA) called a primary transcript
5' capping: consist of
a terminal nucleotide, 7-methylguanylate (m7G) in a 5' - 5' linkage with the initial nucleotide of mRNA chain
eIF4E
binds 5' cap in cytoplasm...eukaryotic initiation factor 4E, translation initiation factor
What does pre-mRNA undergo before it can become functional in the cell
chemical changes
Prompts for poly A tail
cleavage of pre-mRNA 10-35 bp downstream addition of adenines to 3' end
splicing
hydrolytic removal of pre-mRNA portions (introns)
when would there not be a polyA tail
mRNAs for major histones (DNA packaging proteins) lack tails (form stem loops) tRNA and rRNA
Definition of RNA processing and "phases"
made up of all chemical modifications necessary to make a mature RNA product 1. addition of a 5' cap (ALL) 2. addition of a 3' poly (A) tail (most at both ends; long) 3. splicing
What is the 5' cap
methylated guanosine at position 7 on the purine ring
what are the two types of RNA
monocistronic mRNA polycistronic mRNA
Draw out the pathway for gene expression in eukaryotic cells. What does this large pathway indicate
mult. ways and levels from beginning to end to influence gene expression
5' capping: protects
protects mRNA from degradation by nucleases that attack RNA at the 5' end
5' capping: serves to
serves in positioning mRNA on ribosome for initiation of translation
5' capping: when
shortly after initiation of RNA synthesis
What is an important distinction between eukaryotic and prokaryotic RNA transport
transcription and translation are kept separate by nuclear membrane. RNA processing in nucleus required to convert pre-mRNAs to mature RNAs
