RR12

Question 1

Describe the two events that initiation of protein synthesis can be divided into?

Answer 1

1) (43S complex)
tRNAmet interacts with EIF2 (in GTP bound state). The complex interacts with small ribosomal subunits and other pre-initiation factors to ensure stays separate from large subunits.

2) mRNA activation - 5’ end cap is recognized, eIF4 interacts with 43s pre-initiation complex, through bridging of eIF4G. Brings down small subunits to 5’ end of mRNA, eIF4G has association with PAPBC forming a loop.

Question 2

What happens when the scanning complex hits AUG?

Answer 2

The start codon EIF2 - GTP is hydrolyzed to EIF2 GDP. (when the large subunit arrives to interact with the small one). Forms 80s ribosome which is capable at elongating a polypeptide chain.

Question 3

When does the elongation cycle begin?

Answer 3

It begins when an amino acid charged tRNA enters the A site. The complex EFI(alpha), GTP and trnaaa that matches next mRNA codon is allowed to enter the ribosomal A site. This begins the elongation phase. It required EF - bound to GTP. Specific EF (elongation factors) interact with all charged tRNAs in the cytoplasm.

Question 4

What do interactions between 5’ and 3’ end of translated mRNA favour?

Answer 4

Favours the re initiation of translation.

Question 5

What happens when the A site is bound?

Answer 5

EFI alpha is released and GTP hydrolysis causes a conformational change in the ribosome. This brings P and A site closer in proximity.
GTP is hydrolyzed again, causing forward translocation by 1 codon. Moving tRNAs into E and p sites, this is monitored by EF2-GTP. tRNA in E site is forced out during the NEXT conformational change. Growing polypeptide chain is always covalently linked to one of the tRNAs in the P or A site.

Question 6

What is a ribozyme?

Answer 6

It is a polypeptidyl transferase that is responsible for peptide bond formation. It is an enzymatically active RNA molecule. It does not require a protein to make a peptide bond.
- 23s rRNA catalyzes peptide bond
- 28s rRNA in eukaryotic ribosomes

Question 7

How is translation terminated?

Answer 7

A ribosome encounters a stop codon in the A site, it is recognized by eRF1, which goes to the site.
Because there is no remaining amino acid present, it will now be cleaved.
eRF3 releases polypeptide chain, hydrolyzes GTP to cleave last amino acid. Now there is tRNA present at the E site.
The complex breaks apart, leaving a large and small subunit.

Question 8

Are highly translated mRNAs highly stabilized?

Answer 8

Yes