Protein translation and post-translational modification

Question 1

How mRNA sequence are read

Answer 1

AAs read in triplets
Each triplet is a frame
Translation starts at first start codon and continues in frame
Stops at first in-frame stop codon

Question 2

Start codon sequence

Answer 2

AUG

Question 3

Stop codon sequences

Answer 3

UAA
UAG
UGA

Question 4

Location of translation and RNA involved

Answer 4

Occurs in ribosomes

tRNA main component

Question 5

How is the fidelity of the genetic code maintained?

Answer 5

Aminoacyl tRNA synthetases

• AA gets stuck onto correct tRNA by synthesise
• One aminoacyl tRNA per amino acid
• Hydrolyses incorrect amino acid-tRNA pairings and inserts correct one

Question 6

Process of translation

Answer 6

INITIATION
- Dissociation of ribosome subunits
- Assembly of pre-initiation complex
- Binding of mRNA to pre-initiation complex
- Binding of 60s subunit
ELONGATION
- Binding of new tRNA carries AA to A site
- Peptide bond is formed between the 2 amino acids via peptide transferase
- Translocation of tRNA to P site and dissociation of first tRNA
- Cycle repeats for each new AA added to the sequence
TERMINATION
- Stop codon recognised by release factors
- Peptidyl transferase release peptide chain
- Dissociation of ribosomes and release factors

Question 7

Mechanisms of antibiotic protein inhibition

Answer 7

• Translational machinery is complex and easily disrupted, common target for antibiotics
• Antibiotics exploit differences between prokaryotic and eukaryotic ribosomes and translation factors
• Antibiotics selectively inhibit prokaryotes

Question 8

Examples of antibiotics that inhibit protein synthesis

Answer 8

• Streptomycin, inhibits initiation
• Tetracycline, inhibits aa-tRNA binding
• Erythromycin, inhibits translocation
• Chloramphenicol, inhibits peptidyl transferase
• Puromycin, terminates elongation prematurely

Question 9

Method of identifying proteins destined to be secretory or transmembrane

Answer 9

These proteins have a special ‘signal sequence’.

This sequence is recognised by a ‘signal recognition particle’.

Question 10

Process and steps of pathway for newly-synthesised protein to enter secretory pathway

Answer 10

• Recognition of signal sequence by signal recognition particle, halts translation
• Binding of SRP to receptor on the surface of RER, translation resumes
• Translocation of growing polypeptide into RER lumen
• Transmembrane proteins have one or more extra hydrophobic sequences which hold them in membrane
• Cleavage of signal sequence in protein, protein folds inside ER

Question 11

Ways newly-synthesised proteins can be post-translationally modified

Answer 11

• Disulphide bond formation
• Proteolytic cleavage
• Glycosylation
• Phosphorylation
• Prenylation, acylation
• Hydroxylation

Question 12

Explain glycosylation

Answer 12

Addition of carbohydrate to a fully synthesised protein

Question 13

Explain phosphorylation

Answer 13

Addition of phosphate to a fully synthesised protein

Question 14

Explain prenylation and acylation

Answer 14

Two types of addition of lipid groups to a fully synthesised protein