Lac Operon AP Biology PhET Simulation
The goal of this simulation is to help you understand how prokaryotes (note that this is for prokaryotes and NOT
eukaryotes) control gene expression. Specifically, this activity will simulate an inducible operon called the lac operon
used to control the expression of genes that control lactose levels in bacteria. Again, this is for bacteria only and
has nothing to do with lactose catabolism or lactose intolerance in humans.
Step 1: Drag the lac promoter to the stretch of DNA. Do NOT drag the lacZ gene to the DNA. What happens? Why is
this?
Without the lacZ gene being present, the lac promoter alone will not result in the production of the lac enzyme in the prokaryote.
The lac promoter is responsible for controlling the expression of the lacZ gene, but the gene itself encodes the information for the
production of the enzyme. Without the gene, the promoter cannot initiate transcription and expression of the lac enzyme.
Step 2: Now try dragging the lacZ gene to the DNA and note what happens.
Dragging the lacZ gene to the DNA will result in the expression of the lac enzyme in the prokaryote. The lac promoter will bind to the
lacZ gene, initiating transcription and expression of the gene. This will result in the production of the lac enzyme, which is involved in
lactose metabolism.
Step 3: Inject some lactose (about 25 molecules should do it) into the simulation. Note what happens. Specifically, what is
lactose being converted into?
Injecting lactose into the simulation would activate the lac operon and result in the production of the lac enzyme, which would then
catalyze the breakdown of lactose into glucose and galactose. The increased levels of lactose would cause an increase in the amount of
lac enzyme produced, allowing for more efficient metabolism of lactose.
Step 4: Note that the lac enzyme continues to be produced even in the absence of lactose. Why is this a problem? Try
dragging the lac operator gene onto the stretch of DNA. What is the result?
The continued production of the lac enzyme even in the absence of lactose is a problem because it wastes energy and resources for
the prokaryote. The continued production of the enzyme is inefficient and can result in decreased growth and survival of the
prokaryote.
Dragging the lac operator gene onto the stretch of DNA would result in the repression of the lac operon and the decreased production
of the lac enzyme. The lac operator gene acts as a "switch" to control the expression of the lac operon. When the lac operator gene is
present, it blocks the binding of the lac promoter to the lacZ gene, preventing transcription and expression of the lac enzyme. This
allows the prokaryote to conserve energy and resources when lactose is not available.
Step 5: Now try adding the lacI promoter and gene to the stretch of DNA. What happens?
Adding the lacI promoter and gene to the stretch of DNA would result in the production of the LacI repressor protein, which would
bind to the lac operator gene and repress the expression of the lac operon. This would prevent the expression of the lacZ gene and the
production of the lac enzyme, even in the presence of lactose. The lacI promoter and gene function together to control the expression
of the lac operon by producing a repressor protein that regulates the binding of the lac promoter to the lacZ gene. The lacI promoter
and gene allow for tight regulation of the lac operon, allowing the prokaryote to efficiently manage its energy and resources.
Step 6: Again, add some lactose (and again, 25 molecules should work well) into the simulation. What is the INITIAL
result of adding lactose when both genes are activated?
The initial result of adding lactose when both the lacI promoter and gene and the lac promoter and lacZ gene are present would be the
de-repression of the lac operon. The presence of lactose would induce the formation of a lactose-LacI complex, causing the LacI
repressor protein to release from the lac operator gene. This would allow the lac promoter to bind to the lacZ gene, initiating
transcription and expression of the lac enzyme. The production of the lac enzyme would then allow for the metabolism of lactose into
glucose and galactose. This process allows the prokaryote to efficiently respond to changes in lactose levels in the environment.
Step 7: Do not add any more lactose and watch what transpires. Note what happens and why this occurs. How could
you re-activate the lacZ gene?
When no more lactose is added, the lacI repressor protein would re-bind to the lac operator gene, repressing the expression of the lac
operon. This would prevent the transcription and expression of the lacZ gene, and the production of the lac enzyme. The repression of
the lac operon would conserve energy and resources for the prokaryote when lactose is no longer available.
To re-activate the lacZ gene, lactose would need to be added to the simulation again. This would induce the formation of a lactose-LacI
complex, causing the LacI repressor protein to release from the lac operator gene and allowing the lac promoter to bind to the lacZ
gene. This would initiate transcription and expression of the lac enzyme and allow for the metabolism of lactose.
Step 8: Now try the lactose transport tab and insert all of the promoters and genes. Add some lactose and watch to see
what transpires. What is the role of the lacY gene? How does this help the system?
The lactose transport tab includes the lacY gene, which codes for the lactose permease protein. This protein is responsible for
transporting lactose across the prokaryotic cell membrane, allowing the prokaryote to take up lactose from the environment and
utilize it as a source of energy.
The addition of lactose in the presence of the lacY gene and other components of the lac operon would allow for the efficient
transport and utilization of lactose. The presence of lactose would induce the formation of a lactose-LacI complex, causing the LacI
repressor protein to release from the lac operator gene. This would allow the lac promoter to bind to the lacZ gene, initiating
transcription and expression of the lac enzyme. The lactose permease protein would transport lactose into the cell, where the lac
enzyme would metabolize lactose into glucose and galactose.
The inclusion of the lacY gene in the lactose transport system helps to ensure the efficient utilization of lactose as a source of
energy for the prokaryote.