module 6 - 19.5 the Lac operon

Question 1

what is an operon?

Answer 1

• a group of genes transcribed at the same time
• made up of a promoter, an operon, and structural gene(s)

Question 2

where is the Lac Operon found?

Answer 2

bacteria

Question 3

what does the Lac Operon contain?

Answer 3

the genes needed to digest lactose

Question 4

what is the Lac Operon made up of?

Answer 4

• lactose inhibitor/repressor gene (lacI)
• promoter
• lac operator gene/operator (lacO)
• structural genes

Question 5

what do the structural genes in the Lac Operon do?

Answer 5

code for enzymes involved in digestion of lactose

Question 6

what happens if there is no lactose in a bacterium?

Answer 6

the Lac Operon is inactive

Question 7

how does the Lac Operon inhibit the expression of structural genes and stay inactive? (no lactose present)

Answer 7

1. regulatory gene produces repressor protein
2. repressor protein binds to operator/lacO
3. RNA polymerase unable to bind to promoter
4. structural genes for digesting lactose cant be transcribed

Question 8

how does the Lac Operon stay active? (lactose present)

Answer 8

1. regulatory gene produces repressor protein
2. lactose in cell binds to repressor protein so protein changes tertiary structure
3. repressor protein cant bind to operator
4. RNA polymerase binds to promoter so structural genes for digesting lactose can be transcribed

Question 9

when is the lac operon only active?

Answer 9

when lactose is present

Question 10

what is the lac repressor?

Answer 10

• acts as lactose sensor
• blocks transcription of the operon but stops acting as a repressor when lactose is present

Question 11

what is a catabolite activator protein (CAP)?

Answer 11

• acts as glucose sensor indirectly though cAMP
• binds to promoter (left)
  activates transcription of the operon if glucose levels are low

Question 12

what 3 genes does the Lac Operon contain?

Answer 12

• lacZ
• lacY
• lacA
  all are transcribed as a single mRNA

Question 13

what is the CAP binding site in the Lac Operon?

Answer 13

• a negative regulatory site that is bound by CAP
• when CAP binds, it promotes transcription by helping RNA polymerase bind to the promoter

Question 14

what is the operator in the Lac Operon?

Answer 14

• is a negative regulatory binding site bound by the lac repressor protein
• operator overlaps with promoter
• when lac repressor is bound, RNA polymerase cant bind to promoter & start transcription

Question 15

when is cAMP produced?

Answer 15

• when glucose levels are low
• low glucose = cAMP attaches to CAP, allowing it to bind to DNA, CAP helps RNA polymerase to bind to promoter

Question 16

when does the Lac Operon really turn on?

Answer 16

• glucose must be unavailable
• lactose must be available