L18-19: Translation Flashcards
What are features of the genetic code?
Non-overlapping
Degenerate
Universal
What is the primary structure of tRNA?
Small nucleic acids of 70-90nts
Have a 5’ monophosphate
Contain modified bases:
Ribothymidine
Pseudouridine
Dinydroïridine
Inosine
What are features of tRNA secondary structure?
D loop: dinydrouridine
T loop: pseudouridine
Variable arm
Anticodon loop
Amino acid acceptor site
How are tRNAs aminoacylated?
Through tRNA synthetases:
Amp added to give aminoacyl adenylate
Which reacts with appropriate uncharged tRNA to give aminoacyl tRNAand AMP
What are the differences between the a different classes of aminoacyl tRNA synthetases?
Specific enzymes for 10/20 essential amino acids so bind different faces on tRNA
How are tRNA molecules distinguished?
Using identity elements
How does proofreading by aminoacyl tRNA synthetases take place?
Acylation site rejects amino acids that are larger than the correct one
Editing site hydrolytic cleaves amino acids that are smaller than the correct one
How do codons and anticodons interact?
In an anti-parallel fashion (tRNAs are able to recognise more than one codon)
What is the Wobble Hypothesis?
The codon and anticodon pair with usual rules (A-T, G-C)
The base at the 5’ end of the anticodon can form a non-standard H bond with the base at the 3’ end of the codon
What can the different bases pair up with according to the Wobble Hypothesis?
A-U
C-G
U-A/G
G-C/U
I (Inosine)-A/C/U
What is the composition of prokaryotic ribosomes?
50s:
31 different proteins (L), 23s rRNA & 5s rRNA
30s:
21 different proteins (S) & 16s rRNA
What does the s stand for on ribosomes?
Svedberg unit - measure sedimentation rate in a centrifuge and thus size and shape
What are the stages of translation and what do they entail?
1) Initiation:
Initiation factors 1-3 & GTP
Initiator tRNA
2)Elongation:
Elongation factors & GTP
3) Termination
Stop codon
Release factors 1&2
What is the initiator tRNA?
fMet - N-formylmethionine
How is protein synthesis initiated?
A special tRNA brings formylmethionine to the ribosome at the AUG start codons
How is initiator tRNA formed?
tRNAf combines with methionine to form formylmethionine then transformylase forms Formylmethionyl-tRNAf (fMet-tRNAf)
How does initiation take place?
30s subunit of the ribosome binds to the ribosome binding site at near the start codon (AUG) then the initiator tRNA (fMet-tRNAf) binds then the 50s subunit binds which forms the 70s initiation complex (sites A,P & E)
What are the A,P and E sites composed of?
A: Amino acyl site
P: Peptidyl site
E: Exit (empty) site
What is the Shine Dalgarno sequence?
It properly positions the 30s ribosomal subunit by base pairing with the Shine Dalgarno sequence and 16s rRNA
What are the roles of the initiation factors?
IF1&3 bind to a free 30s
IF2 binds to GTP
30s attaches to mRNA
Charged initiator tRNA binds and IF3 released
50s binds displacing IF1&2 GTP is hydrolysed
What is the process of elongation?
-Delivery of aminoacyl tRNA to the A site
-Peptide bond formation
-Translocation
What factors are required during the steps of elongation?
Delivery requires:
EF-Tu, EF-Ts & GTP
Translocation requires:
EF-G & GTP
How are the elongation factors used in the delivery step?
EF-Tu allows GTP to get hydrolysed to GDP which is the regenerated back to GTP using EF-Ts
How are the elongation factors used in the translocation step?
EF-G allows the hydrolysis of GTP to GDP which then produces EF-G on its own
How is the peptide bond formed in translation?
Amino group attacks carbonyl group of the ester linkage which forms the peptide bond and releases the deacylated tRNA
What factors are used in termination?
Release factors
How does the termination process take place?
RF1 recognises UAA and UAG
RF2 recognises UAA and UAG
RF£.GTP helps the 2 release factors carry out termination RRF & EF-G promote dissociation of the ribosome
