Chapter 6: 6.10 Prokaryotic Operons: Lactose (Lac) Operon [detailed]

Question 1

True or False:

Lactose metabolism is under the control of a single promoter

Answer 1

True

Question 2

List:

What the enzymes the following codes for:
1. lacZ
2. lacY
3. lacA
4. lacI

Answer 2

1. β-galactosidase
2. Permease
3. Transacetylase
4. Lac repressor

Question 3

When lactose is present:

What is the role of permease?

Answer 3

A lactose transporter
* Transports lactose into the cell

Question 4

When lactose is present:

What is the role of β-galactosidase?

Answer 4

Converts lactose to allolactose

Question 5

Why are permease and β-galactosidase still present even when there is no lactose?

Answer 5

Low levels of transcription from repressor dissociation

Question 6

When lactose is present:

What happens to the allolactose?

Answer 6

Binds the the allosteric site on the Lac repressor
* Changes repressor conformation = Does not allow it to bind to the operator

Question 7

When lactose is present AND glucose is absent:

What do permease and β-galactosidase do?

Answer 7

• Permease transports lactose into the cell
• β-galactosidase converts lactose to allolactose

Question 8

When lactose is present AND glucose is absent:

What does low glucose levels result in?

Answer 8

Activation of adenylyl cyclase
* Synthesizes cAMP

Question 9

When lactose is present AND glucose is absent:

What does cAMP do?

Answer 9

Activates the CAP protein

Question 10

When lactose is present AND glucose is absent:

What is CAP protein?

Answer 10

• CAP: An activator protein
• cAMP: An inducer

Question 11

When lactose is present AND glucose is absent:

What does an active CAP protein do?

Answer 11

Binds to the CAP sequence in the Lac promoter
* Promotes RNA polymerase binding and transcription

Question 12

True or False:

CAP binding will result in lower transcription levels

Answer 12

False, it will result in higher transcription levels

Question 13

When lactose is present AND glucose is present:

What happens to adenylyl cyclase/cAMP/CAP protein levels?

Answer 13

• Inactivates adenylyl cyclase
• Decreases cAMP levels
• Inactivates CAP protein

Question 14

When lactose is present AND glucose is present:

The resulting inactive CAP protein will result in…

Answer 14

Lower transcription levels, as it doesn’t bind to the transcriptor

Question 15

State:

• Lac operon expression level
• Mechanism

When…
* Lactose Present
* Glucose Absent

Answer 15

• Very high
• Inactive Lac Repressor
• Active CAP

Question 16

State:

• Lac operon expression level
• Mechanism

When…
* Lactose Present
* Glucose Present

Answer 16

• Medium
• Inactive Lac Repressor
• Inactive CAP

Question 17

State:

• Lac operon expression level
• Mechanism

When…
* Lactose Absent
* Glucose Absent

Answer 17

• Very low
• Active Lac Repressor
• Active CAP

Question 18

State:

• Lac operon expression level
• Mechanism

When…
* Lactose Absent
* Glucose Present

Answer 18

• Very low
• Active Lac Repressor
• Inactive CAP

Question 19

Will lactose be broken down for energy if glucose is present?

Answer 19

No