NAGE 7 - Translation & Modification

Question 1

What is the difference between a codon and a triplet?

Answer 1

A triplet is in the DNA, while a codon is in the mRNA.

Question 2

What is a UTR in mRNA.

Answer 2

An untrqnslated region - involved in recognition of translation or mRNA stability. Present at both ends.

Question 3

Describe the structure of tRNA.

Answer 3

A cross with three loops, the base has an anticodon bound. The amino acid binds to the 3’ end.

Question 4

What is the function of aminoacyl tRNA synthetases?

Answer 4

An enzyme which produces adenylated amino acids that bind with the tRNA. The reaction uses ATP, making AMP. The enzyme carries the amino acid to the tRNA - when the right one is found, ANP and the enzyme dissociate.

Question 5

Describe the intiation stage of translation.

Answer 5

1) Ribosome subunits dissociate.
2) A pre-initiation complex assembles (eIFs, met tRNA and the 40s subunit). eIFs used include eIF2 which binds alongside GTP, and eIF4E & eIF4G which bind to the the cap and are recognised by the preinitiation complex.
3) mRNA binds to the preinitiation complex.
4) The 60s subunit binds. GTP is hydrolysed to ensure correct base pairing. eIF2 and GDP dissociate

Question 6

Describe the process of elongation in translation.

Answer 6

1) Met tRNA is bound to the peptidyl (P) site. A new tRNA binds to the A site in frame with Met.
2) Peptidyl transferase catalyses the formation of a peptide bond (present in the 60s subunit.
3) Peptidyl tRNA then translocates to the P site. Elongation factors promote the movement of the ribosome using GTP.
4) The first tRNA dissociates

Question 7

How does use of GTP ensure accuracy of translation?

Answer 7

They enhance the efficiency and accuracy of translation by providing pauses, allowing the incorrect base pairs to dissociate.

Question 8

Describe the termination stage of protein translation.

Answer 8

1) Stop codons are recognised (UAG, UAA and UGA) by release factors, proteins that bind to the empty A site. Peptidyl transferase catalyses the transfer of the protein chain to water.
2) The peptide chain releasesfollowing hydrolysis.
3) Dissociation of release factors and ribosomes.

Question 9

What are polyribosomes?

Answer 9

Ribosomes dont work alone - they all form a string of ribosomes along the mRNA simultaneously synthesising proteins.

Question 10

How does protein synthesis occur in the RER?

Answer 10

• The mRNA strand binds to a free ribosome.
• A signal recognition pariticle is recognised, causing the ribosome to move to the SRP receptor on the surface of the RER.
• Translation resumes and the growing polypeptide enters the lumen of the RER through a protein channel.
• The signal sequence is cleaved and the protein folds

Question 11

How are transmembrane proteins synthesised differently?

Answer 11

They encode one or more extra hydrophobic sequences, which holds them in the membrane.

Question 12

What are the different possible methods of post-translational modification?

Answer 12

• Disulfide bond formation
• Proteolytic cleavage
• Glycosylation (addition of a carbohydrate)
• Phosphorylation (addition of a phosphate)
• Addition of lipid groups (prenulstion and acylation)
• Hydroxylation

Question 13

What post-translational modification occurs in insulin biosynthesis?

Answer 13

• Preproinsulin is converted to proinsulin by release of the signal sequence.
• Disulfide bonds are formed in the ER/golgi
• Proteolytic cleavage in the secretory vesicle occurs which releases chain C. Occurs in the secretory vesicle.