Translation Flashcards
How does mRNA reach ribosomes?
Moves through nuclear pores to cytoplasm
Where are ribosomes found?
Free in cytoplasm
On cytoplasmic surface of RER
What happens to proteins made on RER ribosomes?
Secreted /targeted to organelles or inserted into plasma membrane
What happens to protein made in free ribosomes in cytoplasm?
Remain in cytoplasm
What 2 components does ribosome consist of?
rRNA and protein
What forms the ribozyme (key reactive site in ribosome)?
rRNA
What does each ribosome consist of?
80S complex with ribonucleoprotein subunits
large 60s subunit contains 3 RNAs
small 40s subunmit
Where are ribosomal subunits made?
Nucleolus
What do tRNAs do?
Carry specific amino acids to ribosomes (sequence determined by mRNA template)
What does aminoacyl-tRNA synthetase do?
aminoacyl-tRNA syntheses recognises tRNA and catalyses transfer of correct amino acid to the 3’ end of the correct tRNA (formation of aminoacyhl-tRNA).
What end of the tRNA does the AA join?
3’ end
Describe the accuracy of aminoacyl tRNA synthetase, why?
Enzyme is highly specific
tRNA synthetase only engages particular tRNAs and proof reading ensures accuracy in process
What is an anticodon?
Region of tRNA, Sequence of 3 bases that regonises codon on mRNA
How can some tRNAs recognise more than one codon?
Wobble hypothesis: often a triplet of 2 specific bases followed by any base codes for same amino acid.
3rd base more tolerant with the number of H bonds that form - wobble position
What initiates translation?
Cap at 5’ end recruits 40s subunit
40s scans mRNA for start codon (AUG)
Initiation factor proteins (IFP), GTP and met-tRNAi are bound to 40s subunit
Describe structure of ribosome 60s subunit, how does peptide bond form?
3 tRNA binding sites
E (exit) site
P (met-tRNAi) site p
A(tRNA) site
Peptide bond forms between amino acids by peptidyl transferse when correct tRNA in A site
Describe what happens initially as movement of ribosome in translation?
Moves along RNA towards 3’ end by one codon (driven by GTP and elongation factors)
met-tRNA (methionine tRNA first one) released through E site
tRNA with growing chain in P site
A site free to bind next tRNA
What is met-tRNAi?
Initiator tRNA conjugated to methionine
What is the start codon?
AUG
What are the stop codons?
UAA, UGA, UAG
How can you remember the stop codons?
U are annoying, U are gross, U go away
What bond joins an AA to a tRNA?
Ester
What is required to form AA-tRNA bond?
ATP
What does the 40s do vs the 60s?
40s - scans mRNA
60s - joins AA
What are the three tRNA binding sites of the 60s unit?
E (exit site)
P (met-tRNAi) site p
A(tRNA) site
How does the polyA tail recruit mRNA to ribosome?
Associates with proteins (poly-A binding protein) recruiting the mRNA to the ribosome.
How is the 5’ cap involved in translation ?
Key in initiation
Small ribosomal subunit is recruited to the cap and then scans the mRNA for AUG start codon.
How is the code degenerate when base pairing is specific?
4^3 possible codons can be formed by AUGC, this would require 61 t-RNA molecules as there is 3 possible stop codons.
In reality there are less than 45 t-RNAs meaning some tRNAs can recognise more than one codon (degenerate code).
tRNAs usually differ in the third base, as the rules at the third position are less strict and it can still base pair with a wobble position
What enzyme catalyses peptide bond formation?
Peptidyl transferase
In the assembled ribosome, there are two binding sites, designated A and P. Progress of the mRNA through the ribosome involves…
transfer of the growing polypeptide from peptidyl-tRNA in the P site to the aminoacyl-tRNA in the A site
For translation of mRNA into a protein product, tRNA molecules react with…
specific aminoacyl-AMPs
