Protein synthesis

Question 1

Describe the structure of mRNA at the start of translation

Answer 1

5' 7-methylguanosine cap
5' UTR (untranslated region)
Start codon (AUG)
Stop codon 
3' UTR
Poly-A tail

Question 2

What is the most common start codon, and what does it code for?

Answer 2

AUG

Methionine

Question 3

How many codons are there for how many amino acids?

What can DNA be described as as a result?

Answer 3

64 for 20

Degenerate

Question 4

What are the 3 stop codons?

Answer 4

UAA
UAG
UGA

Question 5

What direction is mRNA translated?

Answer 5

5’ to 3’

Question 6

What is the entry site for a ribosome?

Answer 6

7-methylguanosine cap

Question 7

What is the difference between ribosomes in eukaryotes/prokaryotes?

Answer 7

Eu: 80s ribosomes, 49 proteins
Pro: 70s ribosomes, 33 proteins

Question 8

What does the difference in eu/pro ribosomes allow the use of?

Answer 8

Antibiotics secreted from bacteria/fungi which only act on prokaryotes
They give a selective advantage.

Question 9

Name the three stages of translation

Answer 9

1. Initiation
2. Elongation
3. Termination

Question 10

Where does an amino acid bind to tRNA?

Answer 10

3’

Question 11

Which enzyme ‘picks up’ an amino acid, and binds it to tRNA?

How does it work?

Answer 11

aminoacyl tRNA synthetase - specific to each amino acid.
Amino joined to enzyme using ATP. Forms E-AMP-Aminoacid
tRNA removes enzyme and AMP

Question 12

What is the first stage of initiation?

Answer 12

40s + 60s subunits dissociate into cytoplasm looking for mRNA

Question 13

What happens when the 40s subunit locates mRNA?

Answer 13

Preinitiation complex assembles.

Question 14

What does the preinitiation complex consist of?

Answer 14

Met-tRNA, eIF’s 40s subunit

Met as its always the first AminoA

Question 15

What is eIF?

Answer 15

An initiation factor

Group of proteins

Question 16

What happens to the preinitation complex?

Answer 16

The 60s subunit binds.

A full ribosome is formed.

Question 17

What do eIFs do? 
Name them (3)

Answer 17

eIF 4E + 4G bind to 5’ cap

eIF2 contains GTP which is hydrolysed to bind 60s when AUG is found

Question 18

Name the 2 sites of a ribosome in elongation

Answer 18

P site - on the left (peptidal)

A site on the right of the P site (amino acyl)

Question 19

Describe elongation (3 steps)

Answer 19

1. Next tRNA binds to a site
2. Peptide bond formed between 2 amino acids on P and A sites by peptidyl transferase.
3. Translocation of tRNA from A to P, (previous P dissociates)
   Repeat

Question 20

How quick is elongation?

Answer 20

15 codons/second

Question 21

Describe termination (3 steps)

Answer 21

1. Stop codon recognised
2. Release factors bind to empty A site.
3. Protein dissociates, ribosome dissociates

Question 22

What is a polyribosome?

Answer 22

Several ribosomes working on a single strand of mRNA.

Question 23

Protein synthesis takes place in the cytoplasm with which exception?

Answer 23

Mitochondrial protein synthesis.

Question 24

What is the name for the first 20-24 amino acids?

Answer 24

Signal sequence

Question 25

What does the signal sequence bind to?

Answer 25

Signal reception particle on ER

Question 26

What happens when a signal sequence is recognised by a SRP?

Answer 26

Translation halts, SS binds to SRP, translation resumes

Protein channel forms in ER, and protein enters.

Question 27

What happens when the preproprotein has entered the ER?

Answer 27

The signal sequence is cleaved by signal peptidase creating a proprotein.

Question 28

What will be present is the protein is destined to become a transmembrane protein?

Answer 28

Extra hydrophobic sequence at C terminus which integrates into membrane

Question 29

What allows the signal sequence to easily travel across membranes?

Answer 29

It is hydrophobic. (Lipophilic)

Question 30

Name the ways a proteins tertiary structure can be modified!

There are 6/7

Answer 30

Disulphide bond formation
Proteolytic cleavage
Glycosylation	(addition	of	carbohydrate)
Phosphorylation (addition of phosphate)
Prenylation, Acylation (addition of lipid groups)	
Hydroxylation