Control of Gene Expression in Prokaryotes

Question 1

Are genes always on or off unless controlled?

Answer 1

On

Question 2

What dictates levels of gene expression?

Answer 2

Strength of a promoter

Question 3

What do repressors bind to?

Answer 3

Operators

Question 4

What does the operator overlap with?

Answer 4

RNA polymerase binding sites

Question 5

Where do activators bind?

Answer 5

To sites that do not overlap with RNA binding sites

Question 6

How do activators help increase gene expression?

Answer 6

They help RNA polymerase to bind to the promoter

Question 7

In the prescence of both glucose and lactose which sugar will a prokaryotic cell use first?

Answer 7

Glucose

Question 8

What is an operon?

Answer 8

A unit of genomic DNA that contains a cluster of genes under the control of a single promoter which act together as part of a unified pathway

Question 9

Give an example of how an activator might work

Answer 9

• in the prescence of a ligand the activator is turned on
• this makes protein-protein interactions with RNA polymerase which helps binding

Question 10

Give an example of how a repressor may work

Answer 10

• if ligand is absent the repressor can bind to the operator
• this means RNA polymerase cannot bind

Question 11

What negative control mechanism is used by lac operon?

Answer 11

Lac repressor

Question 12

Which positive control mechanism is the lac operon controlled by?

Answer 12

CAP-cAMP complex

Question 13

Which enzyme does lac z code for?

Answer 13

β-galactosidase

Question 14

What does β-galactosidase do?

Answer 14

Cleaves lactose into glucose and galactose

Question 15

What enzyme does lac y code for?

Answer 15

β-galactoside permease

Question 16

What does β-galactoside permease do?

Answer 16

It is a membrane bound protein which pumps lactose into the cell

Question 17

What enzyme does lac A code for?

Answer 17

β-galactoside transacetylase

Question 18

What does β-galactoside transacetylase do?

Answer 18

• Transfers an acetyl group from acetyl-CoA to β-galactosides
• Activates lac z

Question 19

What is the structure of the lac repressor?

Answer 19

Tetramer

Question 20

What is allolactose?

Answer 20

A product of metabolism of lactose

Question 21

What occurs to the lac repressor in the prescence of allolactose?

Answer 21

• allolactose binds to the lack repressor
• Binding of allolactose prevents the lac repressor from binding to the operator
• RNA polymerase is allowed to bind and transcribe the operon

Question 22

What chemical does CAP sense?

Answer 22

cAMP

Question 23

What kind of molecule is CAP?

Answer 23

A protein

Question 24

What happens to cAMP levels in the prescence of glucose?

Answer 24

They reduce

Question 25

What happens in the cell when glucose levels reduce?

Answer 25

* Adenylate cyclase is activated * cAMP levels rise

Question 26

What kind of promoter is the lac promoter?

Answer 26

A weak promoter

Question 27

How is the lac operon activated by CAP?

Answer 27

* When glucose levels fall, cAMP levels rise (due to activation of adenylate cyclase) * cAMP binds to CAP * CAP can then bind to the operon and recruit RNA polymerase * produces strong gene expression

Question 28

What is the dormant state of a bacterium infected with a virus called?

Answer 28

Lysogenic growth

Question 29

What does the Cro repressor turn off?

Answer 29

Expression of the lambda repressor

Question 30

Which operator does the lambda repressor have the highest affinity to?

Answer 30

OR₁

Question 31

Which operator does the cro repressor have the highest affinity for?

Answer 31

OR₃

Question 32

What does the lambda repressor turn off?

Answer 32

Expression of all phage genes, including the gene for the Cro repressor

Question 33

Which operator does the lambda repressor first bind to?

Answer 33

OR₁

Question 34

Which operator is nearest to the promoter for the lambda repressor gene?

Answer 34

OR₃

Question 35

Which operator is nearest to the promoter for the Cro repressor gene?

Answer 35

OR₁

Question 36

What happens when the lambda repressor binds to OR1?

Answer 36

Due to the proximity of OR1 to the promoter for the Cro repressor, it blocks cro expression

Question 37

What kind of binding occurs between lambda repressor molecules on operators?

Answer 37

Cooperative binding

Question 38

What occurs when lambda repressor binds to OR₂?

Answer 38

* It produces enchanced lambda repressor expression by enhancing RNA polymerase binding * Now acts as a lambda activator

Question 39

What occurs when lambda repressor binds to OR₃?

Answer 39

It blocks lambda repressor expression by blocking binidng of RNA polymerase

Question 40

How does the lambda repressor maintain good levels of expression?

Answer 40

It alternates between binding to OR₂ and OR₃

Question 41

What prompts breakdown of the lambda repressor?

Answer 41

The cell entering a stressful situtation e.g. DNA damage

Question 42

What happens when the cell enters a stressful situation?

Answer 42

* The cell produces lots of proteases * The lambda repressor is broken down by proteolysis * Cro represor is no longer blocked

Question 43

What happens when the cro repressor binds to OR₃?

Answer 43

* Blocks lambda repressor expression * Allows for the expression of the remaining 50 phages genes * Lytic growth occurs