Molecular Biology (10) Flashcards
Describe the process of elongation in prokaryotic translation
Aminoacyl-tRNA binds to A site
Peptide bond forms between adjacent AAs - in P site and A site
Translocation of ribosome - A site empty
Describe the process of termination of prokaryotic translation
Stop codon in A site recognised
RF1 and RF2 - bind to stop codon
RF3-GTPase - proofreads - stimulates release of RF1 and RF2
What is the effect of the release factors?
Force ribosome to accept water molecule instead of AA
Polypeptide released - ribosomal subunits and release factors dissociate
What is the composition of a prokaryotic ribosome?
50S + 30S - 70S ribosome
What is the composition of a eukaryotic ribosome?
60S and 40S - 80S ribosome
What prevents the 60S ribosomal subunit binding to the 40S subunit in eukaryotic translation initiation?
40S subunit has eIF1A in A site
eIF1 and eIF3 in E site
Where does eIF4 bind?
5’ methyl cap of mRNA
Name the subunits of the eIF4 complex and their roles
4E - recognises 5’ methyl cap
4B - hydrolytic ATP capacity
4G - binds polyA tail - circularises mRNA
4A - helicase - activated by 4B - cleaves secondary structures out of mRNA - allows complex to move along for initiation codon
What is the role of eIF2?
Has GTPase activity
Recognises P site of 40S - binds - does not initially base pair
eIF5 simulataneously binds to eIF1 and eIF3 in E site
Forms pre-initiation complex
When is the initiation complex formed?
When P site methionine lined up over AUG
What is the consensus translation start sequence in eukaryotes?
Kozak sequence - surrounds AUG initiation codon
5’-ACCAUGG-3’
What is eIF2?
Ternary complex - met-tRNA with 2 molecules of GTP
What is the result of the formation of the initiation complex?
Codon and anticodon base pairing
2 GTP on met-tRNA hydrolysed to 2 GDP - releases most initiation factors - leaves eIF1A in A site
60S subunit with eIF5B GTPase binds to 40S
What is the effect of eIF5B hydrolysing bound ATP?
Causes eIF1A and eIF5B-GDP to dissociate from A site
Translation starts
How is the polyA tail bound the the 5’ end of mRNA?
PolyA tail interacts with eIF4G - eIF4G bound to eIF4E - eIF4E bound to 5’ end
Where and how does actinomycin D act?
Binds to DNA - blocks movement of RNA polymerase - prevents RNA synthesis
Prokaryotes and eukaryotes
Where and how does puromycin act?
Adds to growing end of polypeptide chain - nascent polypeptide released prematurely
Prokaryotes and eukaryotes
Where and how does alpha-amanitin act?
Binds preferentially to RNA polymerase II - blocks mRNA synthesis
Eukaryotes
Where and how does cycloheximide act?
Blocks ribosome translocation
Eukaryotes
Where and how does tetracycline act?
Blocks binding of aminoacyl-tRNA to A site
Prokaryotes
Where and how does streptomycin act?
Blocks transition from translation initiation to elongation
Prokaryotes
Where and how does chloramphenicol act?
Blocks peptide bond formation - blocks peptidyl transferase reaction
Prokaryotes
Where and how does erythromycin act?
Blocks E site of ribosome
Prokaryotes
Where and how does rifamycin act?
Binds to RNA polymerase - blocks initiation of RNA synthesis
Prokaryotes
