Molecular Biology (10)

Question 1

Describe the process of elongation in prokaryotic translation

Answer 1

Aminoacyl-tRNA binds to A site
Peptide bond forms between adjacent AAs - in P site and A site
Translocation of ribosome - A site empty

Question 2

Describe the process of termination of prokaryotic translation

Answer 2

Stop codon in A site recognised
RF1 and RF2 - bind to stop codon
RF3-GTPase - proofreads - stimulates release of RF1 and RF2

Question 3

What is the effect of the release factors?

Answer 3

Force ribosome to accept water molecule instead of AA

Polypeptide released - ribosomal subunits and release factors dissociate

Question 4

What is the composition of a prokaryotic ribosome?

Answer 4

50S + 30S - 70S ribosome

Question 5

What is the composition of a eukaryotic ribosome?

Answer 5

60S and 40S - 80S ribosome

Question 6

What prevents the 60S ribosomal subunit binding to the 40S subunit in eukaryotic translation initiation?

Answer 6

40S subunit has eIF1A in A site

eIF1 and eIF3 in E site

Question 7

Where does eIF4 bind?

Answer 7

5’ methyl cap of mRNA

Question 8

Name the subunits of the eIF4 complex and their roles

Answer 8

4E - recognises 5’ methyl cap
4B - hydrolytic ATP capacity
4G - binds polyA tail - circularises mRNA
4A - helicase - activated by 4B - cleaves secondary structures out of mRNA - allows complex to move along for initiation codon

Question 9

What is the role of eIF2?

Answer 9

Has GTPase activity
Recognises P site of 40S - binds - does not initially base pair
eIF5 simulataneously binds to eIF1 and eIF3 in E site
Forms pre-initiation complex

Question 10

When is the initiation complex formed?

Answer 10

When P site methionine lined up over AUG

Question 11

What is the consensus translation start sequence in eukaryotes?

Answer 11

Kozak sequence - surrounds AUG initiation codon

5’-ACCAUGG-3’

Question 12

What is eIF2?

Answer 12

Ternary complex - met-tRNA with 2 molecules of GTP

Question 13

What is the result of the formation of the initiation complex?

Answer 13

Codon and anticodon base pairing
2 GTP on met-tRNA hydrolysed to 2 GDP - releases most initiation factors - leaves eIF1A in A site
60S subunit with eIF5B GTPase binds to 40S

Question 14

What is the effect of eIF5B hydrolysing bound ATP?

Answer 14

Causes eIF1A and eIF5B-GDP to dissociate from A site

Translation starts

Question 15

How is the polyA tail bound the the 5’ end of mRNA?

Answer 15

PolyA tail interacts with eIF4G - eIF4G bound to eIF4E - eIF4E bound to 5’ end

Question 16

Where and how does actinomycin D act?

Answer 16

Binds to DNA - blocks movement of RNA polymerase - prevents RNA synthesis
Prokaryotes and eukaryotes

Question 17

Where and how does puromycin act?

Answer 17

Adds to growing end of polypeptide chain - nascent polypeptide released prematurely
Prokaryotes and eukaryotes

Question 18

Where and how does alpha-amanitin act?

Answer 18

Binds preferentially to RNA polymerase II - blocks mRNA synthesis
Eukaryotes

Question 19

Where and how does cycloheximide act?

Answer 19

Blocks ribosome translocation

Eukaryotes

Question 20

Where and how does tetracycline act?

Answer 20

Blocks binding of aminoacyl-tRNA to A site

Prokaryotes

Question 21

Where and how does streptomycin act?

Answer 21

Blocks transition from translation initiation to elongation

Prokaryotes

Question 22

Where and how does chloramphenicol act?

Answer 22

Blocks peptide bond formation - blocks peptidyl transferase reaction
Prokaryotes

Question 23

Where and how does erythromycin act?

Answer 23

Blocks E site of ribosome

Prokaryotes

Question 24

Where and how does rifamycin act?

Answer 24

Binds to RNA polymerase - blocks initiation of RNA synthesis

Prokaryotes