Mod7 - RNA to Protein

Question 1

Describe the two mechanisms of Wobble Base Pairing

Answer 1

G-U pairing
I (inosine) with C, A or U

Question 2

Where does Wobble Base Pairing occur?

Answer 2

ONLY between the third base of the codon and the first base of the anticodon

Question 3

Why do Svedberg values of rRNA not always seem to add up?

Answer 3

Svedbergs are not just a measure of mass, but of SIZE - so the 3-D SHAPE of the rRNA matters

Question 4

Describe the process of Amino Acid activation, before they can be joined to tRNA

Answer 4

The amino acid is coupled with ATP via an esterification to form Aminoacyl-AMP, releasing two phosphates
Aminoacyl-AMP is then joined to the tRNA by the enzyme Aminoacyl-AMP Synthase, releasing AMP

Question 5

On what arm of the tRNA is the Amino Acid added?

Answer 5

The acceptor arm

Question 6

What is the role of the Shine-Dalgarno sequence in prokaryotes?

Answer 6

It interacts with the 16S rRNA of the 30S subunit, to identify the site of initiation of protein synthesis

Question 7

What is the first stage of Translation Initiation?

Answer 7

Initiation Factors (IF1, IF2-GTP and IF3) bind to the 30S subunit

Question 8

What occurs after the three initiation factors bind to the 30S?

Answer 8

Initiator Methionine tRNA and the mRNA join the complex

Question 9

What is the final stage of Translation Initiation?

Answer 9

The large (50S) subunit joins the complex, causing IF2 to dissociate and GTP to be converted to GDP. IF3 also dissociates

Question 10

What are the functions of IF1?

Answer 10

Blocks the A site to tRNAiMet, ensuring it only slots into the P site; also inhibits premature 30S-50S interaction

Question 11

Roles of IF3?

Answer 11

Inhibits premature 30S-50S interaction, stabilises free 30S, and performs an accuracy check to ensure tRNAiMet is bound in the right place

Question 12

In what form is the next Amino Acid brought to the complex during Elongation?

Answer 12

As an Aminoacyl-tRNA coupled with the Elongation Factor EF-Tu-GTP (this is called the Ternary Complex)

Question 13

How is the second amino acid added to the polypeptide chain in Elongation?

Answer 13

The Aminoacyl-tRNA binds to the A site of the 50S, while EF-Tu-GDP is released. A peptide bond then forms between the first and second Amino Acids

Question 14

What is the final step in Elongation?

Answer 14

The first amino acid is released from the P-Site, and the Ribosome Translocates, mediated by EF-G, with a conversion of GTP to GDP

Question 15

When does Termination occur during Translation?

Answer 15

When the A-Site is lined up with a stop codon (UAA, UAG or UGA)

Question 16

What is the first step in Termination of Translation?

Answer 16

Instead of a new AA-tRNA, the vacant A-Site is taken up by a Release Factor

Question 17

What are the roles of the different Release Factors?

Answer 17

RF1 and RF2 recognise the stop codons (RF1 = UAA/UAG, RF2 = UAA/UGA), while RF3 helps RF1 or RF2 to bind to the ribosome

Question 18

What occurs once a Release Factor is bound to the ribosome?

Answer 18

Hydrolysis of the polypeptide chain from the last tRNA, releasing the polypeptide and tRNA. The Ribosome then dissociates