Protein synthesis Flashcards
What do ribosomes comprise of?
RNA and Protein
What are the 2 types of ribosomes?
○ Free in cytosol
○ Bound to the endoplasmic reticulum
Where does protein synthesis occur?
In a cleft between the large and small sub-unit
How many amino acids are added per second to the polypeptide chain?
20
What subunits do mammalian ribosomes consist of and why is it not 100s?
○ Consist of a 40s and 60 s subunit together being 80s ribosome
○ Not 100s because S stands for Svedberg which is a unit for the sedimentation rate(not linear)
Translation(Initiation)
Initiation
○ Assembly of the ribosomes on mRNA together with the first initiator tRNA
○ mRNA in cytoplasm
Multiple initiation factors bind to cap and poly A tail of mRNA
○ EIf1 and EIF3 bind to 40s subunit and separate it from 60s
○ EIF2 binds to tRNA
○ The 40s binds to the tRNA with EiF factors 1,2 and 3 forming 48 pre initiation complex
○ EiF4 binds to the cap structure at 5' end of mRNA
○ 48 pre initiation complex then binds to the cap structure
○ Complex scans for start codons AUG (mediated by initiation factors)
-1 ATP molecule used for each translocation of base when scanning
○ AT the start codon the 60S subunit joins onto the 40S and EIF s are released
Translation(Elongation)
○ Initiator tRNA is in P site
○ tRNA with complimentary anticodon to codon goes to A site
○ Due to close proximity of amino acids of amino acids in site P and A, a peptide bond is formed between them
○ Ribosome moves along by another codon
○ Empty tRNA goes to E site and is released
○ The other tRNA which was at the A site now holds onto peptide chain and is in the p site which is now known as peptidal RNA
Translation(Termination)
○ There’s a stop signal UAG in A site
○ No tRNA will have a complimentary anticodon to UAG codon
○ This codon is recognised by a protein called a release factor
○ Hydrolysis of terminal peptidyl-tRNA
Polypeptide is released
