Protein Translation and Post-Translational modification

Question 1

5’ 7-methyl guanosine cap of mRNA

Answer 1

entry site for ribosome

Question 2

5’ UTR of mRNA

Answer 2

untranslated region (non-coding region)
may be involved in regulation of translation or mRNA stability

Question 3

What follows the 5’UTR on mRNA?

Answer 3

Coding region
3’ UTR
PolyA tail

Question 4

What is the function of the polyA tail on mRNA?

Answer 4

To protect the 3’end from degradation

Question 5

Why is the genetic code described as degenerate?

Answer 5

Common AAs have several codons

Question 6

What are the stop codons?

Answer 6

UAA, UAG, UGA

Question 7

What is the Methionine codon?

Answer 7

AUG

Question 8

What is the significance of Methionine?

Answer 8

It is the 1st AA in nearly all polypeptides

Question 9

Transfer RNA function

Answer 9

Transport AA’s to ribosome

Question 10

How many tRNA per AA?

Answer 10

> ,1

Question 11

Where is an AA attached to on tRNA

Answer 11

3’ end

Question 12

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

Answer 12

Aminoacyl tRNA synthetase

Question 13

How many Aminoacyl tRNA synthetases per AA and why is this important?

Answer 13

1 per AA

Important in fidelity of translation - ensure correct AA is bound to correct tRNA

Question 14

How does Aminoacyl tRNA synthetase work?

Answer 14

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

Question 15

What are the 3 stages of translation?

Answer 15

Initiation
Elongation
Termination

Question 16

Initiation in translation

Answer 16

Dissociation of ribosome subunits (eukaryotic 40S + 60S)
Assembly of preinitiation complex containing Met-tRNA + eukaryotic translation initiation factors (eIFs) + 40S subunit
Binding of mRNA to preinitiation complex
Binding of 60S subunit

Question 17

Elongation in translation

Answer 17

Binding of new tRNA to immediately adjacent A site on ribosome in frame with initiator Met
Catalysis of peptide bond formation between the 2 amino acids by peptidyl transferase (PT) on 60S subunit
Translocation of tRNA to P site and dissociation of 1st tRNA
Ribosome slides along so previous tRNA is now in the P site and the used tRNA falls off- facilitated by Elongation Factors (EFs) using GTP

Question 18

Why do elongation factors use GTP?

Answer 18

To enhance efficiency and accuracy of translation by providing “pauses” (e.g. GTP hydrolysis) that allow incorrect base pairs to dissociate

Question 19

Termination in translation

Answer 19

Recognition of stop codon
Release of peptide chain
Dissociation of release factors and ribosomes

Question 20

What occurs in recognition of stop codon in termination of translation?

Answer 20

Release factors (proteins) bind to empty A site (no tRNA for stop codons)
Peptidyl transferase catalyses transfer of the completed protein chain to water (hydrolysis) and releases it from the ribosome

Question 21

Polyribosome

Answer 21

several ribosomes work on each strand at once

Allows much more protein to be produced in the same amount of time

Question 22

Many antibiotics inhibit protein synthesis in prokaryotes

Answer 22

Antibiotics exploit differences between prokaryotic and eukaryotic ribosomes and translation factors
Allows selective inhibition of prokaryotes

Question 23

What does Streptomycin inhibit?

Answer 23

Inhibits initiation

Question 24

What does Tetracycline inhibit?

Answer 24

Inhibits AA-tRNA binding

Question 25

What does Erythromycin inhibit?

Answer 25

Inhibits translocation

Question 26

What does Chloramphenicol inhibit?

Answer 26

Inhibits peptidyl transferase

Question 27

What does Puromycin do?

Answer 27

Terminates elongation prematurely

Question 28

Where does protein synthesis occur and why do cells need a mechanism to transfer proteins across membranes?

Answer 28

Occurs in Cytoplasm

Cellular organelles are bound by a membrane

Question 29

Where are secretory and transmembrane proteins synthesised?

Answer 29

In rough endoplasmic reticulum

Question 30

Signal sequence

Answer 30

Proteins destined to be secretory or transmembrane have a special sequence (1st 20-24 AAs)
Found at N terminus of polypeptide
Rich in hydrophobic amino acids (lipophilic so can cross membranes)

Question 31

Describe how proteins enter the RER

Answer 31

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 RER 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 lumen of the RER

Question 32

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

Answer 32

They encode 1 or more extra hydrophobic sequences that hold them in the membrane

Question 33

What happens once the protein has entered the RER?

Answer 33

The signal sequence is cleaved by signal peptidase and is degraded.
Result: main protein, which is folded.

Question 34

Why are most proteins further modified before becoming fully functional?

Answer 34

Only 20 amino acids

Cell uses post-translational modifications to increase diversity

Question 35

Give 3 examples of post-translational modifications

Answer 35

Disulphide bond formation
Proteolytic cleavage
Addition of carbohydrate/ phosphate/ lipids

Question 36

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

Answer 36

Preproinsulin enters the ER: signal sequence is cleaved and 3 disulphide bonds form
This makes it proinsulin
The C chain is cleaved making it into insulin: A and B chain held together by disulphide bonds