Translation and The genetic code (lecture 8) Flashcards
(17 cards)
name 3 stop codons in homo sapiens
TAA
TAG
TGA
first position 5’
third position 3’
structure of tRNA
clover leaf structure:
D loop
anticodon loop
T loop
amino acids attach at the 3’ end
5’ end and 3’ end hybridize and build the acceptor stem
what’s wobble base pairing?
A wobble base pair is a pairing between two nucleotides in RNA molecules that does not follow Watson-Crick base pair rules
wobble pairs?
U - G, I (hypoxanthine), A
C - G, I
what does it mean: the genetic code is redundant?
several codons code for one tRNA/ aa
because some tRNAs allow a mismatch
(wobble) at the third position
and: mutiple tRNAs for a given aa
Translation Requires two Sequential Adaptors
First adaptor:
Aminoacyl-tRNA synthetase
couples aa to correct tRNA (ATP dependent)
(one for each aa)
Second adaptor:
tRNA anticodon pairs with codon (energy independent)
(redundant)
name pro- and eukaryotic ribosomes
pro:
70S -> 50S -> 5S, 23S rRNA
-> 30S ->16S rRNA
eu:
80S -> 60S -> 5S, 28S, 5.8S rRNA
-> 40S -> 18S
name the steps of elongation
- tRNA binding at the A-site while old tRNA loaded with peptide chain sits at the P-site
- peptide bond formation (still at A-site) mediated by ribosomal RNA
(peptidyl transferase activity of the large SU) - large SU translocation -> tRNA bound to peptide chain is now at P-site and A-site is free, old/empty tRNA is at E-site
- small SU translocation
- exit of tRNA at E-site and binding of loaded tRNA at A-site
cofactors of translation-elongation?
what are their functions?
energy required?
Elongation factors drive translation directionally ‘forward’ and improve accuracy
• EF1(Tu):
(aa-tRNA match; codon-anticodon match (+rRNA)) -> proofreading
• EF2 (G) (transfer of peptide chain to new aa and translocation of small subunit)
both are GTP dependent
initiation of translation
what is required and why?
• Requires initiator tRNA (Met bound)
-> searches for the start codon AUG along the mRNA (it is bound to small SU)
• eIFs (eukaryotic initiation factors)
• Interaction of 5’ cap with 3’ polyA
(+polyA binding proteins)
name the eIFs and their function
eiF2 loads the initiator tRNA to the small SU
eiF4E and eiF4G bind the 5’ cap and the 3’polyA in bring them in proximity
when initiatior tRNA has found the AUG, the eiFs dissociate and the large SU binds so that initiation tRNA is at the P-site
how are the tRNAs named which are bound to the peptide chain and to a single aa?
peptide chain: peptidyl tRNA
new aa: aminoacyl-tRNA
this same the tRNA molecules, not the tRNA + aa/peptide chain
how is the translation terminated?
when the stop codin UAG is in A-site, release factors bind to the stop codon in A-site
which reaction happens during termination?
Peptidyl transferase adds a water molecule to the
peptidyl tRNA resulting in the formation of the Cterminal COOH
This reaction frees the peptide chain from the peptidyl tRNA
• Ribosome releases mRNA and disassembles
how can the output of translation be pumped up?
polyribosomes:
Multiple initiations on individual mRNA
As soon as preceding ribosome has translated enough to move
out of the way (min. 25aa), the 5’ end of the mRNA is threaded into
the next ribosome
With synthesis rates up to 2aa/sec most proteins are produced
within 20 sec to several minutes
give an example for the Variability and Flexibility of Translating the Genetic Code
A suppressor is a mutation allowing to overcome effects of another mutation
• The suppressor tRNA system is based on a 1st mutation in the mRNA that can be overcome
by a 2nd mutation in the tRNA (recognizing the mutant codon) or vice versa.
A non-sense suppressor tRNA contains a
mutated anticodon that recognizes a Stop
codon
• An aa is inserted at the Stop codon
• Can result in (a) suppression of nonsense
mutation or (b) read-through at a natural
Stop codon
A miss-sense suppressor tRNA contains a mutated anticodon that recognizes a wrong codon • Another aa is inserted at a given codon • Results in suppression of miss-sense mutation
stop codon in ?e.coli?
UAG
