Nucleic acids 6-Protein Translation and post-translation modification

Question 1

What is the Structure of a typical mRNA

Answer 1

5’ CAP - 5’ UTR – Coding Region – 3’ UTR – poly A
CAP: 7- Methyl guanosine (entry site for ribosome)

(UTR: untranslated region
Poly A: protects mRNA)

Question 2

How many possible codons are there and how many AA?

Answer 2

64 potential codons, only 20 amino acids
therefore each amino acid is coded for by more
than one codon. This is degenerate

Question 3

How is mRNA read

Answer 3

 Starts by finding 5’ CAP- recognises it then scans 5’ – 3’
 Translation starts at first AUG, which sets the “frame” in succeeding triplet codons to bring amino acids
 Translation stops at first “in frame” termination codon

Question 4

Describe tRNA (4)

Answer 4

 The transporter of amino acids to the ribosome for translation (at least one per amino acid)
 Hydrogen bonds within itself to form a compact structure (represented by a clover shape)
 Anti-parallel binding- like DNA
 3 bases (anticodon) are complementary to codon on mRNA, i.e. mRNA = 5’-3’, tRNA= 3’- 5’

Question 5

Mechanism by which Aminoacyl tRNA Synthetases work:

Answer 5

1. Using the energy from ATP, the enzyme forms an intermediate known as an adenylated amino acid. The intermediate has the enzyme bound to AMP (derived from ATP which loses a pyrophosphate molecule) and the AMP is bound to the correct amino acid.
2. The adenylated amino acid then binds an appropriate tRNA. The AMP comes off and the enzyme dissociates.
3. The amino acid is transferred to the 3’ hydroxyl of the tRNA.

Question 6

Describe the initiation of translation (4)

Answer 6

Step 1: dissociation of ribosome subunits (40S + 60S)
 Step 2: assembly of preinitiation complex containing Met-tRNA+ Initiation factors (eIFs) +40S subunit + GTP
(Guanosine triose phosphate). Only initiator Met-tRNA can bind to 40S subunit alone. 40S subunit is
primarily involved in tRNA and mRNA recognition
 Step 3: Preinitiation complex recognises mRNA and binds to it.
 Step 4: Assembly of full ribosome - 60S subunit binds

Question 7

Describe the elongation stage of translation (3)

Answer 7

 Step 1: Binding of new tRNA carrying second amino acid to A site.
 Step 2: Catalysis of peptide bond formation between the two amino acids by PEPTIDYL TRANSFERASE
 Step 3: Translocation of tRNA to P (peptidyl) site and dissociation of the first tRNA. This shifts the ribosome down by one codon.

Question 8

What are elongation factors

Answer 8

Elongation Factors (EFs) are proteins that promote movement of ribosome along mRNA using GTP. They use the energy of GTP to enhance the efficiency and accuracy of translation by providing “pauses” (e.g. the time taken for GTP hydrolysis) that allow incorrect base pairs to dissociate.

Question 9

Describe the termination stage of translation (3)

Answer 9

Step 1: Recognition of Stop Codon

Step 2: Release of Peptide Chain

Step 3: Dissociation of Release Factors and Ribosomes

Question 10

Define polyribosomes

Answer 10

• Ribosomes do not work singly on a mRNA but in multiple copies on the mRNA – a polyribosome – like a
string of beads
• This makes translation very efficient
• Translation speed of each ribosome = 15 amino acids/sec
• Multiple ribosomes processing simultaneously a 300 amino acid long protein, i.e. one ribosome every 30a.a.
of synthesized protein - the number of protein molecules produced in 1 min is ~4000

Question 11

How do antibiotics work in prokaryotics

Answer 11

• Translational machinery is complex, easily disrupted – common target for antibiotics
• Antibiotics exploit differences between prokaryotic and eukaryotic ribosomes and translation factor
 Antibiotics selectively inhibit prokaryotes
 Antibiotics are natural products of bacteria or fungi to give them a selective advantage over other microbes

Question 12

What is the signal Sequence

Answer 12

Proteins that are destined to be secretory or transmembrane have a special sequence (first 20-24 amino acids) called a SIGNAL SEQUENCE.

The signal sequence is present at the N-terminus of the polypeptide (first sequence that is synthesised).

The signal sequence is rich in HYDROPHOBIC AMINO ACIDS.

Question 13

How do you get the proteins into the compartments?

Answer 13

Step 1: Recognition of Signal sequence by a protein- RNA complex “Signal-Recognition Particle” (SRP) halting translation
Step 2: Binding of SRP to a receptor at RER surface, translation resumes
Step 3:Translocation into the lumen of RER
Step 4: Cleavage of signal sequence by signal peptidase (co-translational) and folding

Question 14

6 Types of Post-Translational Modification

Answer 14

1. Disulphide bond formation
2. Proteolytic cleavage
3. Glycosylation (addition of carbohydrate)
4. Phosphorylation (addition of phosphate)
5. Prenylation, Acylation (addition of lipid groups)
6. Hydroxylation

Question 15

Describe the Post-Translational Modification of Insulin:

Answer 15

1. Signal sequence is removed and degraded
2. Disulphide bonds form between cysteine residues within the polypeptide chain - 3 Disulphide Bonds form while the protein folds to form PROINSULIN (single chain polypeptide with 3 disulphide bonds).
3. Before it is packaged into secretory vesicles, it is proteolytically cleaved in two positions to release the C chain.
4. This leaves you with active, fully-functional insulin which consists of the A and B chains held together by the three disulphide bonds.

Question 16

What is the start codon and corresponding amino acid?

Answer 16

AUG- Met