Translation 2 (Week 5)

Question 1

How is translation begun in prokaryotes?

Answer 1

A consensus sequence

Question 2

What are bacterial genes grouped into?

Answer 2

Often bacterial genes are grouped into operons (encoding proteins required for a key activity)

Question 3

Which occurs first, translation or transcription?

Answer 3

Translation begins before transcription finishes

Question 4

What happens sometimes to formyl-methionine after translation?

Answer 4

In some cases this amino acid is cleaved off after translation

Question 5

What do initiation factors 1 and 3 combine with?

Answer 5

Initiation factors (IFs) 1 and 3 combine with the 30S ribosome subunit.

Question 6

What is the main component reaction in translation?

Answer 6

The main reaction in translation is the addition of an amino acid to the carboxyl terminus of the polypeptide.

Question 7

What occurs during elongation? (1)

Answer 7

The incoming tRNA binds at the A site
Peptidyl transferase catalyses the transfer of the peptide onto the incoming amino acid
The uncharged amino acid leaves the E site

Question 8

What are the elongation reactions driven by?

Answer 8

The elongation reactions in the previous slides are driven by several elongation factors, which deliver the aminoacyl-tRNAs, allow proof-reading and catalyse the translocation of the ribosome

Question 9

Which elongation factors are involved in proofreading and translocation?

Answer 9

The elongation factor (EF-Tu) - GTP binds to the incoming aminoacyl-tRNA

In GTP-bound state base pairing of anticodon/codon can occur, but peptide bond cannot form

Question 10

What happens during proofreading? (1)

Answer 10

incorrectly bound tRNAs dissociate from the ribosome
A correct match triggers hydrolysis of GTP and release of Pi.
This causes a conformational change in EF-Tu releasing it from the tRNA

Question 11

What happens in translocation? (1)

Answer 11

After peptide bond formation EF-G/GTP binds to the EF-Tu binding site on the 50S subunit

Question 12

What happens in the termination of translation? (1)

Answer 12

The 3 stop codons do not have an associated tRNA.

Instead a release factor binds to the stop codon in the A site

Question 13

What do common antibiotics inhibit?

Answer 13

Protein synthesis

Question 14

How does the consensus sequence start translation?

Answer 14

The purine rich sequence (Shine-Dalgarno sequence) interacts with rRNA, denoting the adjacent AUG as the start site

Question 15

How do operons encode mRNA?

Answer 15

These operons are transcribed as a single “polycistronic” mRNA, with each open reading frame having its own ribosome binding site.

Question 16

What does the ribosome bind to before RNA polymerase has finished transcribing?

Answer 16

In prokaryotes, the ribosome binds to the mRNA before RNA polymerase has finished transcribing

The processes of transcription and translation are coupled

Question 17

What is the name of the first amino acid in Prokaryotes that has its own tRNA?

Answer 17

The first amino acid in prok proteins is formyl-methionine

Question 18

What does the fact that formyl-methionine has its own tRNA distinguish it from?

Answer 18

internal methionine codons.

Question 19

What does initiation factor 2-GTP deliver?

Answer 19

IF2-GTP delivers the fMet-tRNAf (formyl-methionine tRNA) and all the components assemble at the start site (16S rRNA interacts with Shine-Dalgarno sequence).

Question 20

What happens to the initiation factors if a tRNA pairs with AUG codon?

Answer 20

IFs 1 and 3 dissociate, IF2 then recruits 50S subunit and GTP is hydrolysed releasing IF2-GDP and Pi.

Question 21

How is the addition of an amino acid to the carboxyl terminus of the polypeptide carried out?

Answer 21

This is done by the peptidyl transferase activity of the large ribosomal subunit and involves the transfer of the existing chain onto the incoming amino acid

Question 22

What occurs during elongation? (2)

Answer 22

After the transfer, the large ribosome subunit moves forming hybrid sites (A in small subunit is now P in large subunit)

Question 23

What occurs during elongation? (3)

Answer 23

The small subunit then moves, pulling the mRNA through to create a new A site, ready to accept the incoming tRNA

Question 24

What occurs during elongation? (4)

Answer 24

The cycle is complete when the incoming aminoacyl-tRNA binds at the A site and the outgoing tRNA exits the E site.

Question 25

What occurs during proofreading? (2)

Answer 25

EF-Tu GDP is released from the ribosome.
There is another chance for incorrectly paired tRNAs to be released
tRNA position moves to allow peptide bond formation

Question 26

What occurs in translocation? (2)

Answer 26

GTP hydrolysis induces a conformational change which forces the tRNAs and the mRNA to move a 3 base distance through the ribosome, freeing up the A site and releasing the uncharged tRNA

Question 27

What happens in the termination of translation? (2)

Answer 27

As there is no amino acid to add to the growing chain, the peptidyl transferase catalyses the addition of an H2O, releasing the peptide from the tRNA

Question 28

What happens in the termination of translation? (3)

Answer 28

After the peptide is released, the ribosome disassembles ready to reassemble on another mRNA