Protein translation

Question 1

How does a ribosome know when to start and end translation?

Answer 1

It scans for a START and STOP codon respectively

Question 2

In which direction does translation occur?

Answer 2

5’ to 3’

Question 3

How many subunits does a ribosome have and how are they classified?

Answer 3

2, 40S and 60S

Question 4

How is tRNA used in translation?

Answer 4

They form aminoacyl-tRNA complexes that bind to complimentary codons on the mRNA, forming the required amino acid.

Aminoacyl group is added to the 3’ end

Question 5

What are the steps of translation?

Answer 5

Initiation
Elongation
Termination

Question 6

What occurs in initiation?

Answer 6

1. Disassembly of ribosome units
2. Formation of pre-initiation complex
   
   • eIF4E binds to cap, 40S subunit which is bound to
     eIF2, GTP and met-tRNA recognises this and moves
     towards
3. Binding of pre-initiation complex to mRNA
   
   • Initiator MEt binds to AUG
4. 60S subunit binds to mRNA, met-tRNA is at P site
   
   • GTP hydrolysed

Question 7

What occurs in elongation?

Answer 7

1. New tRNA binds to adjacent site ‘A’ on ribosome - closer to 3’ end
2. Peptide bond between to AAs on adj. tRNAs is catalysed by PEPTIDYL TRANSFERASE
3. New tRNA is moved to ‘P’ side of ribosome, and met-tRNA disassociates - BY ELONGATION FACTORS
4. Repeat till STOP CODON

Question 8

What occurs in termination?

Answer 8

1. Stop codon is recognised, RELEASE FACTORS bind to ‘A’ site - no tRNA for stop codon. Pep transf catalyses final transfer - water thus hydrolysis and release.
2. Chain is released
3. Release factors and ribosomes disassociate.

Question 9

Do ribosomes work singly on mRNA?

Answer 9

No, do as polyribosomes - string of beads

Question 10

How do antibiotics only inhibit prokaryotic protein synthesis?

Answer 10

Exploit the differences - 70s v 80s ribosomes

• Streptomycin - inititiation inhibitor
• Erythromycin - inhibits translocation

Question 11

How are proteins transported across organelle membranes?

Answer 11

Presence of SIGNAL SEQUENCES - direct to rER - where secretory and transmembrane proteins are synthesised

• Signal-recognition particle recognises this, halting translation
• SRP binds to rER, allowing translation to continue
• THen, mRNA is transLOCated into the rER
• FInally, signal peptide is cleaved, leaving the protein in the rER

Question 12

What occurs to the protein once inside the rER?

Answer 12

Post-translational modification

• Disulphide bonds -INSULIN
• Proteolytic chain removal -INSULIN
• Carbohydrate, phosphates and lipids addition