L2 - Translation

Question 1

what type of bond are amino acids joined by?

Answer 1

peptide bonds (protein = polypeptide)

Question 2

Importance of sequence of base pairs

Answer 2

the sequence of base pairs in mRNA defines the sequence of amino acids in a protein, defining the final shape and function of the protein.

Question 3

what is a triplet codon?

Answer 3

3 bases that code for one amino acid

Question 4

what are START and STOP codons?

Answer 4

3 bases that signal the start or end of a polypeptide chain, AUG = start codon (codes for Methionine)
UAG, UGA, UAA = stop codons

Question 5

what is Transfer RNA?

Answer 5

tRNA is a small RNA molecule that binds a specific amino acid by recognising the corresponding codon by base pairing

Question 6

tRNA structure

Answer 6

tRNAs contain modified bases, amino acid covalently bonded to 3’ end, chain in 3 leaf clover shape, with T loop, anticodon loop and D loop. 5’ end next to 3’ as chain loops back round. Anticodon on tRNA binds to codon in mRNA.

Question 7

what is the idea of ‘wobble base pairing’?

Answer 7

some tRNAs can recognise more than one codon because imperfect ‘wobble’ base pairing can occur at the 3rd position of the codon, which is less critical than the base pairing at first 2 positions (G-U only has 2 H bonds therefore weaker)

Question 8

which enzyme joins the correct amino acid to the tRNA?

Answer 8

aminoacyl-tRNA synthetase

Question 9

what is the main structure of ribosomes?

Answer 9

consists of a large and a small sub unit, has binding sites for both mRNAs and tRNAs called E, P, A sites

Question 10

what happens in protein synthesis after the mRNA binds to the ribosome?

Answer 10

Aminoacyl-tRNAs recognise and bind to base triplets on the mRNA, the ribosome then transfers each new amino acid to the growing polypeptide. Protein is extended from AMINO terminus to the CARBOXY terminus.

Question 11

what 3 things assist the process of protein synthesis by helping the ribosome?

Answer 11

INITIATION, ELONGATION and RELEASE FACTOR proteins

Question 12

what are the steps in the INITIATION MECHANISM?

Answer 12

1. Methionyl-tRNA (eukaryotes) or formylmethionyl-tRNA (bacteria) binds to P site (middle) in small subunit. (tRNA carrying start codon)
2. mRNA binds to small subunit, start codon binds to tRNA anticodon
3. large subunit of ribosome binds
4. aminoacyl-tRNA for the 2nd amino acid binds to A site on ribosome
5. protein synthesis proceeds as for normal elongation

Question 13

what are the steps in the ELONGATION MECHANISM?

Answer 13

1. mRNA bound to small subunit and growing polypeptide chain is attached to tRNA in the P site
2. the ribosome transfers the growing polypeptide FROM the peptidyl-tRNA in the P site TO the new aminoacyl-tRNA
3. Large subunit moves relative to the small subunit (in A site) - large moves first to free A site
4. ribosome ‘reset’ so next tRNA can bind with next amino acid
5. used tRNA ejected from E site as next tRNA binds

Question 14

what are the Elongation factors?

Answer 14

1. EF-Tu (called EF1 in eukaryotes)

2. EF-G (EF2 in eukaryotes)

Question 15

what is the TERMINATION MECHANISM of translation?

Answer 15

1. release factor protein binds to A site instead of tRNA
2. peptidyl-tRNA bond is hydrolysed releasing newly synthesised protein
3. ribosome dissociates into large and small subunits

Question 16

what does EF-Tu (bacteria) / EF1 (eukaryotes) do?

Answer 16

carries aminoacyl-tRNA to the ribosome
binds GTP EF-Tu
releases aminoacyl-tRNA
hydrolyses GTP to GDP + Pi

Question 17

where does energy for protein synthesis come from?

Answer 17

hydrolysis of GTP to GDP + Pi

Question 18

what does EF-G (bacteria) / EF2 (eukaryotes) do?

Answer 18

with bound GTP = binds to the A site 
hydrolyses GTP (creating energy)
resets the ribosome ready for the next aminoacyl-tRNA

Question 19

when does translation terminate?

Answer 19

Translation ends at the first in-frame stop codon (in a group of 3 that will be translated):
UAA / UAG