Translation Flashcards
mRNA role
carries codon
tRNA role
adaptors between codon + aa
rRNA role
associates with a set of proteins to form ribosomes
RNA polymerase characteristics
RNA polymerase synthesises RNA in a 5’-3’ direction
• RNA polymerase does not need a primer.
• The RNA product does not remain base-paired to the
template DNA strand
Polycistronic transcripts
mRNA which encodes two or more proteins.
Role of aminoacyl tRNA sythetase
Loading -> high fidelity
Initiation in bacteria
RNAP recognises starting AUG by shine-dalgarno motif 6-8 nt upstream AUG
- > pairs with polypyrimidine motif in small ribosome sub-unit rRNA
- > position AUG in P site
Initiation in eukaryotes
5’ to 3’ scanning for Kozak sequence which contains AUG
Elongation general mech:
Ribosome places aa-tRNA in A site facilitated by GTPase elongation factor
GTPase elongation factor in bacteria v eukaryotes
EFTu (bacteria)/ eEFIA (eukaryotes)
What happens in the initial selection phase?
aatRNA in complex with EFtu is rejected/accepted by ribosome
- > if accepted GTP hyrolysed
- > EFTu leaves ribosome - proofreading will reject wrong tRNA
What happens after selection?
Peptide bond formation in large sub-unit
-> growing polypeptide chain from P site to nucleophilic amine of A site
How are tRNA’s translocated to the Exit site?
GTPase couples GTP hydrolyses to translocate tRNA to E site
How do the GTPases used for exit translocation in bacteria and eukaroytes differ?
EFG (bacteria)/ eEF2 (eukaryotes)
General mechanism of termination
Stop codon recognised by release factors
- > 1 end A site, the other changes conformation of active site in large sub-unit
- > hydrolytic reaction -> growing polypeptide released
How do release factors differ in bacteria from eukaryotes?
Bacteria - RF1/RF2 -> recognise UAA, UGA, UAG with overlapping specificity
RF1: UAA/UAG and RF2: UAA/UGA
Eukaryotes - eRF1 - recognises all 3
Core polymerase: a2bb’w
What role do these sub-units have?
b’: core polymerase subunit (DNA binding)
b: core polymerase subunit (polymerase active
site)
a: scaffolding role
w: unknown role
What is the sigma factor?
role in transcription initiation (promoter
recognition), main type is s70
