Part 2: The Translation Cycle

Question 1

Polysome:

Answer 1

• a very large mRNA/ribosome complex
• Initiation and elongation occur simultaneously yielding multiple ribosomes on a single mRNA molecule

Question 2

The three steps of translation initiation:

Answer 2

1. assembly of ribosome and mRNA
2. positioning of the small ribosome subunit on the start codon
3. Joining the large ribosome subunit to the preinitiation complex

Question 3

The initiator tRNA (AUG; methionine) is “special.”

Why?

Answer 3

• it recognizes the ribosome independently of all the other tRNAs
  
  • recognizes the ribosome in its split-up form
  • binds directly to the small subunit with the help of eIF2

Question 4

What protein binds and delivers initiator Met-tRNA to the small subunit of a ribosome to begin the initiation step?

Answer 4

eIF2

GTP-dependent

Question 5

What protein binds to the 5’ cap of an mRNA strand?

Answer 5

eIF4E

Question 6

What protein binds eIF4E and is required for assembly of pre-initiation complex?

Answer 6

eIF4G

(a scaffold protein)

Question 7

eIF2

Answer 7

• protein that binds and delivers initiator Met-tRNA to the small subunit of a ribosome to begin the initiation process

Question 8

eIF4E

Answer 8

• protein that binds to the 5’ cap of an mRNA strand

Question 9

eIF4G

Answer 9

• scaffold protein that binds eIF4E
• required for assembly of pre-initiation complex
• provides bridge between 5’ cap of mRNA and the 3’ poly-A tail

Question 10

The small subunit of the ribosome recognizes and binds to:

Answer 10

• the mRNA strand with eIF4E on the 5’ cap bound to eIF4G that bridges to 3’ end of mRNA.

Question 11

The preinitiation complex consists of:

Answer 11

• the small ribosome subunit (40s) bound to the 5’ end of an mRNA strand
  
  • eIF4E on the 5’ cap of mRNA
  • eIF4G bound to eIF4E that bridges to 3’ end of mRNA

Question 12

The only step in translation that utilizes ATP:

Answer 12

• during the initiation phase when the ribosome of the preinitiation complex scans the attached mRNA for the start codon (AUG)
• ATP-dependent
• uses a helicase

Question 13

How does the large ribosomal subunit (60s) bind to the small ribosomal subunit of the preinitiation complex?

Answer 13

• GTP-hydrolysis reaction
  
  • releases eIF2
  • binds large subunit (60s)

Question 14

Elongation:

Answer 14

• movement of the ribosome down the mRNA coordinated with aminoacyl tRNA delivery

Question 15

The four steps of elongation:

Answer 15

1. delivery of aa-tRNA to A site and E site release
2. GTP hydrolysis and eEF1A release
3. eEF2 binding to catalyze translocation
4. GTP hydrolysis and eEF2 release - completion of cycle

Question 16

eEF1A

Answer 16

• binds all canonical tRNAs (i.e. all tRNAs except the initiator tRNA and the selenocysteine tRNA)
  
  • brings to the A-site

Question 17

eEF2

Answer 17

• protein required for ribosome translocation
  
  • translocates the peptide chain from:

A-site → P-site

Question 18

During translation, where does proofreading occur?

Answer 18

• during elongation
• occurs in the ribosomal A-site where codon/ anticodon pairs are “checked” by ribosome conformation

Question 19

Is there a tRNA for termination?

Answer 19

• No, there is no “terminator” tRNA.
• eRF1 binding cause peptide hydrolysis and release, terminating translation

Question 20

What is the single step of translation termination?

Answer 20

• eRF1 binds to the A-site of the ribosome when the A-site is over a stop codon
• catalyzes peptide hydrolysis and release from ribosome
• ribosome recycling signals released