Translation and post-translational modification

Question 1

Describe the structure of typical mRNA.

Answer 1

mRNA has a 7-methylguanylate cap at the 5’ end followed by a 5’ untranslated region (UTR) You then get the coding region in the middle
At the end you get a 3’ UTR and the poly-A tail

Question 2

What are the stop codons?

Answer 2

UAA, UGA, UAG

Question 3

What is the Methionine codon?

Answer 3

AUG

Question 4

What is the significance of Methionine?

Answer 4

It is the first amino acid in virtually all polypeptides

Question 5

What enzyme is involved in the transfer of an amino acid to tRNA?

Answer 5

Aminoacyl tRNA synthetase

Question 6

How does this enzyme work?

Answer 6

Aminoacyl tRNA synthetase gets activated (by ATP hydrolysis) to form adenylated amino acid which binds with a molecule of AMP and binds to the relevant amino acid
This adenylated amino acid complex then binds to the tRNA and transfers the amino acid to the 3’ end of the tRNA The adenylated amino acid and AMP dissociate

Question 7

What are the three stages of translation?

Answer 7

Initiation – Elongation - Termination

Question 8

What components are make up the preinitiation complex?

Answer 8

40S ribosomal subunit Methionine tRNA eIF2 (initiation factor) GTP

Question 9

Describe initiation.

Answer 9

The preinitiation complex recognises initiation factors (eIF4E and G) on the 7MeG and bing to the mRNA
It moves along the mRNA in a 5’ to 3’ direction until it reaches the first in-frame AUG (methionine)
Here the GTP is hydrolysed providing energy to ensure correct base pair matching
The GDP and eIF2 then dissociate causing a conformational change, which allows the binding of the 60S ribosomal subunit to form a full ribosome

Question 10

Describe elongation.

Answer 10

The next charged tRNA comes and binds to the A site of ribosome Peptidyl transferase catalyses the formation of a peptide bond between the two amino acids
The tRNA from the P site then dissociates and the ribosome moves along

Question 11

What proteins facilitate elongation?

Answer 11

Elongation factors

Question 12

Describe termination.

Answer 12

When the A site of the ribosome moves over the first in-frame stop codon, a release factor (a type of protein) binds instead of a tRNA
The release factor transfers the growing polypeptide chain to water thus terminating the polypeptide and allowing it to detach from the ribosome

Question 13

Where is the signal sequence found and what does it consist of?

Answer 13

It is found at the N terminus It mainly consists of hydrophobic amino acids

Question 14

Describe how proteins enter the ER.

Answer 14

The signal sequence binds to a signal recognition particle (SRP) and this binding halts translation
The SRP then binds to an SRP receptor on the ER membrane and translation resumes
The binding of SRP to the SRP receptor triggers the assembly of a protein channel through which the polypeptide is threaded into the ER

Question 15

What extra feature do proteins that are destined to be transmembrane have?

Answer 15

They have an extra signal sequence

Question 16

What happens once the protein has entered the compartment?

Answer 16

The signal sequence is cleaved

Question 17

Describe, in full, the post translational modification involved in the production of insulin.

Answer 17

You begin with preproinsulin, which has an N-terminus signal sequence Once it enters the ER the signal sequence is cleaved and it 3 disulphide bonds form
This makes it proinsulin
Then the C chain is cleaved making it into insulin
You get an A and B chain held together by disulphide bonds