7 - Protein translation and post-translation modification

Question 1

What is the triplet code?

Answer 1

Three nucleotides encode one amino acid, hence the name “triplet code”; a group of three nucleotides is called a codon

Question 2

What does protein synthesis start and stop with?

Answer 2

Protein synthesis always starts with Met =AUG and finishes with a stop codon =UAA, UAG or UGA

Question 3

How does tRNA work?

Answer 3

Transfer RNAs are the transporters of amino acids (bound to the 3’ terminus) to the ribosome; they contain an anticodon loop that binds to the corresponding codon on the mRNA

Question 4

4 steps of translation initiation?

Answer 4

1. Dissociation of ribosome subunits (40S + 60S)
2. Assembly of preinitiation complex containing Met-tRNA + Initiation Factors + 40S subunit
3. mRNA binds to pre initiation complex
4. Binding of 60S subunit.

Question 5

3 steps of translation elongation?

Answer 5

1. Binding of tRNA to new A site.
2. Catalysis of peptide bond between two amino acids by peptide transferase.
3. Translocation of tRNA to P site and dissociation of first tRNA

Question 6

3 steps of translation termination?

Answer 6

1. Recognition of stop codon
2. Release of peptide chain
3. Dissociation of release factors and ribosomes.

Question 7

What are antibiotics?

Answer 7

Natural products of bacteria or fungi that can selectively inhibit prokaryotic protein synthesis because the translational machinery is complex and easily disrupted.

Question 8

What are the 4 stages occurring on the RER of synthesis of proteins destined for the secretory pathway to the cell surface?

Answer 8

1. Recognition of hydrophobic N-terminal signal sequence by signal recognition particle (SRP)
2. Binding of SRP to a receptor at the RER surface
3. Translocation of the growing protein into the lumen of RER
4. Cleavage of signal sequence and protein folding

Question 9

What extra feature do transmembrane proteins have ?

Answer 9

Additional hydrophobic sequences that stick in the membrane of the RER.

Question 10

How are proteins modified after translation?

Answer 10

Proteolytic cleavage
Disulphide bond formation
Addition of carbohydrate (Glycosylation)
Addition of phosphate (Phosphorylation)
Addition of lipid groups (Acylation, Prenylation)

Question 11

What is Glycosylation?

Answer 11

Addition of carbohydrate

Question 12

What is phosphorylation?

Answer 12

Addition of phosphate

Question 13

What is Acylation/prenylation?

Answer 13

Addition of lipid groups

Question 14

Which post translational modifications does insulin undergo?

Answer 14

Signal sequence removal.
Disulphide bonding
Proteolytic cleavage