Translation Flashcards
What is Transfer RNA?
A transfer RNA is a small RNA molecule that:
- binds a specific amino acid
- recognises the corresponding codon in messenger RNA by base pairing (anticodon)
What is the start codon? (Bases)
AUG
What does the start codon do?
Specified the beginning of translation. Codes for methionine.
What are the 3 stop codons?
UAG, UGA, UAA
What do the stop codons do?
Specify the end of translation
Which directions do anticodons go?
5’ to 3’
Which direction do codons go?
3’ to 5’
What is the secondary shape of tRNA?
Clover leaf- in reality an L-shape molecule- 3 loops folded on top of each other with a stem (loop because they do not base pair to one another
What does aminoacyl-tRNA synthetase do?
Joins the correct amino acid to the tRNA
Why can some tRNAs recognise more than one codon?
Because of imperfect “wobble” base pairing
Where does the imperfect “wobble” occur?
At the third position of the codon
How are T & C, and A & G similar?
Are chemically similar
Where does protein synthesis take place?
Ribosomes.
What are ribosomes?
Complex structures containing RNA and protein
What are ribosomes made up of?
Has two subunits- 1 large and 1 small. Has binding sites for mRNA and for tRNA (E,P,A sites)
What is the difference in colour between the large and small subunits of a ribosome?
The large is lighter green
Describe protein synthesis (the basics)?
. Messenger RNA binds to ribosome
. Aminoacyl-tRNAs recognise and bind to base triplets in mRNA
. The ribosome transfers the growing polypeptide to the new amino acid
. Protein is extended from amino terminus to carboxy terminus
. Initiation, elongation and release factor proteins assist the process
What is the initiation mechanism?
Process involves initiation factor proteins:
- Methionyl-tRNA (in eukaryotes) or formylmethionyl-tRNA (in bacteria) binds to P site in small subunit of ribosome
- mRNA binds to the small subunit; start codon binds to tRNA anticodon
- Large subunit of ribosome binds
- Aminoacyl-tRNA for second amino acid binds A site on ribosome
- Protein synthesis proceeds as for elongation
Describe the elongation mechanism
- Messenger RNA is bound to the small subunit of the ribosome. The growing polypeptide is attached to tRNA in the P site of the ribosome
Aminoacyl-tRNA for the next amino acid recognises and binds to the mRNA bade triplet in the A site of the ribosome - The ribosome transfers the growing polypeptide from the peptidyl-tRNA in the P site to the new aminoacyl-tRNA
- The large subunit moves relative to the small subunit
- The ribosome is “reset” so that the next tRNA can bind
- The “spent” tRNA is ejected from the E site and the next aminoacyl-tRNA binds
(If the wrong tRNA comes along it will be rejected)
What is the P site?
Where the growing polypeptide chain is
Where does the energy for protein synthesis come from?
The hydrolysis of GTP
Describe how GTP is used to provide energy in protein synthesis
- EF-Tu (in bacteria; called EF1 I’m eukaryotes) carries aminoacyl-tRNA to the ribosome. EF-Tu also binds GTP. If bade pairing is correct, EF-Tu releases aminoacyk-tRNA and hydrolyses GTP to GDP and phosphate
- EF-G (I’m bacteria;called EF2 in eukaryotes) with bound GTP binds to the A site. It hydrolyses GTP and resets the ribosome ready for the next aminoacyl-tRNA
Describe the termination mechanism of translation
Translation ends at the first in-frame stop codon (UAA or UAG or UGA).
- Release factor protein binds to A side instead of tRNA
- Peptidyl-tRNA bond is hydrolysed, releasing newly synthesised protein
- Ribosome dissociates into large and small subunits (so that the process can start again)
Give examples of model organisms
. Escherichia coli (bacteria) . Saccharinyces cerevisiae (yeast) . Fruit flies . Mice . Humans