Molecular Biology 10 : Translation in prokaryotes (elongation + termination), tranlsation in eukaryotes

Question 1

What happens during the elongation phase of translation in prokaryotes ?

Answer 1

• an aa-tRNA binds to the A site
• a peptide bond is formed
• the ribosome is translocated along the mRNA strand and the cycle is repeated until a Stop codon is reached

Question 2

What are the 3 release factors responsible for translation termination in prokaryotes ?

Answer 2

• RF1 and RF2 recognise and bind to stop codons

* RF3-GTP : proofreads to ensure stop codon is correctly recognised and stimulates release of RF 1 and 2

Question 3

Which tRNA can recognize the first AUG in eukaryotes ?

Answer 3

Only Met-tRNA

Question 4

What are 8 steps of translation initiation in eukaryotes ?

Answer 4

1. eIF2 ternary complex formation : 2-GTP + Met
2. 43S pre-initiation complex formation : eIF2 + 40 S subunit bound to 3, 1 and 1A + 5
3. mRNA activation : mRNA + eIF4 complex (4E, 4G + 4A), + 4B, and ATP –> ADP + Pi
4. Attachment of 43S pre-initiation complex to mRNA
5. 5’ to 3’ scanning
6. Initiation codon recognition, hydrolysis of eIF1-bound GTP and Pi release –> leads to formation of 48S initiation complex
7. Subunit joining and factor displacement
8. Hydrolysis of eIF5B-bound GTP and release of eIF5B anf eIF1A –> leads to formation of 80S initiation complex

Question 5

What is the Kozak sequence ?

Answer 5

The Kozak sequence is a sequence which occurs on eukaryotic mRNA and has the consensus (5)’ ACCAUGG (3’). The Kozak consensus sequence plays a major role in the initiation of the translation process.

Question 6

What are the 4 steps of elongation in eukaryotic translation ?

Answer 6

1. Entry of next aa-tRNA at A site : aa-tRNA is bound to EF1-alpha (Elongation Factor 1 alpha) and GTP
2. GTP hydrolysis (EF1-alpha-GTP –> EF1-alpha-GDP + P), ribosome conformational change
3. Peptide bond formation
4. Ribosome translocation (EF2-GTP –> EF2-GDP + Pi)

Question 7

How is translation terminated in eukaryotes ?

Answer 7

• the ribosome meats a stop codon (UAA, UAG, UGA)
• eRF1, bound to eRF3-GTP, binds to UAA in the A site
• hydrolysis of eRF3-GTP to eRF3-GDP leads to peptidyl-tRNA cleavege and the formation of the post-termination complex, which dissassembles (ATP –> ADP + Pi)

Question 8

Name some transcription/translation inhibitors that act only on bacteria ?
How do they work ?

Answer 8

Tetracycline : blocks binding of aa-tRNA to the A site of the ribosome
Streptomycin : prevents the transition from translation initiation to chin elongation and also causes miscoding
CHloramphemicol : block the peptidyl transferase reaction on the ribosome
Erythromycin : binds in the exit channel of the ribosome and thereby inhibits elongation of the peptide chain
Rifamycin : blocks initiation of RNA chains by binding to RNA Pol (prevents RNA synthesis)

Question 9

Name some transcription/translation inhibitors that act only on eukaryotes ?
How do they work ?

Answer 9

Cyclohexamide : blocks the translocation reaction on the ribosome
Anisomycin : blocks the peptidyl transferase reaction on the ribosomes
Blocks mRNA synthesis by binding preferentially to RNA Pol II

Question 10

Name some transcription/translation inhibitors that act only both on bacteria and eukaryotes ?
How do they work ?

Answer 10

Puromycin : causes the premature release of nascent polypeptide chains by its addition to the growing chain end
Actinomycin D : binds to DNA and blocks the movement of RNA Pol (prevents RNA synthesis)