Lac Operon

Question 1

What is an operon

Answer 1

Series of sequential genes transcribed together into polycistronic mRNA
One promoter, one transcriptional terminator
Each gene has its own RBS and start and stop codons

Question 2

Operon control

Answer 2

Activator binding site:Regulatory DNA sequence before promoter + operator that helps sigma factor/RNA polymerase attach
Operator:Regulatory DNA sequence after promoter that determines whether RNA pol can bind/proceed

Question 3

Types of genes

Answer 3

Constitutive-always on
Inducible-only turned on when needed
Repressible-only turned off as needed

Question 4

Controlling transcription

Answer 4

Up regulated-more mRNA +protein
Down regulated-less mRNA and protein

Question 5

Positive

Answer 5

Activator protein-Binds at activator binding site not in promotor
promotes transcription when bound
helps recruit sigma factor/RNA pol recognize +bind to DNA

Question 6

Negative

Answer 6

Repressor protein
binds at operator
Inhibits transcription when bound
blocks RNA polymerase from transcribing

Question 7

Effector

Answer 7

Any molecule that can regulate the activity of a protein
small molecule that binds to a protein
changes shape-changes activity

Question 8

Inducible genes

Answer 8

Inducer-inducible genes
positive: Binds activator, attaches to activator binding site, sigma factor +RNA
Negative: binds repressor, makes it detach from the operator, RNA pol proceeds

Question 9

Repressible genes

Answer 9

Inhibitor-repressible genes
positive: Binds activator, makes it detach from activator binding site
Corepressor-repressible
Negative: Binds repressor, makes it attach to operator

Question 10

The lac operon

Answer 10

The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source

Question 11

the lac repressor inhibits transcription

Answer 11

Lacl gene encodes the Lac repressor, under control of its own promoter
expressed separately
under constitutive promoter
Absent:lac repressor binds tightly to the operator,preventing transcribing by RNA polymerase

Question 12

When lactose is present

Answer 12

lac repressor loses its ability to bind DNA
clears the way for RNA pol to transcribe the operon
The effector allolactose causes this change
Lactose will be converted to allolactose inside cell

Question 13

RNA pol doesn’t bind well to lac operon

Answer 13

Needs extra help from the Catabolite activator protein
Cap binds to DNA before lac operon promoter-helps rna pol attach to promoter
cap encodes by a gene separate from lac operon

Question 14

Catabolite activator protein

Answer 14

CAP can’t bind to DNA
regulated by cyclic AMP
camp binds to cap it can bind to DNA and promote transcription
can only transcribe when glucose is absent

Question 15

Glucose present,lactose absent

Answer 15

No transcription of lac operon
No cAMP
Lac repressor remains bound to the operator-blocks RNA polymerase
cAMP levels are low because glucose is high
CAP is inactive

Question 16

Glucose+lactose present

Answer 16

Lac repressor is released from the operator
inducer (allolactose) is present
cAMP levels are low
Low transcription of the lac operon

Question 17

Glucose absent,lactose absent

Answer 17

No transcription
cAMP levels are high +Glucose levels are low
CAP is active
Lac repressor will be bound to the operator
due to absence of allolactose
prevents RNA pol from transcribing

Question 18

Glucose absent,lactose present

Answer 18

Lac repressor is released from the operator
Inducer (allolactose)
cAMP levels are high
CAP is active +bound to the DNA
CAP helps RNA pol bind to promotor
Strong transcription

Question 19

Proteins produced from lac operon

Answer 19

lacZ=enzyme called beta-galactosidase
breaks down glucose, splits lactose into glucose and galactose
lacY=membrane protein called lactose permease transmembrane pump that allows the cell to import lactose
lacA=enzyme called transacetylase, adds acetyl group to galactose