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The Lac Operon quiz #2

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  • How does the cell recognize that lactose needs to be metabolized?
    The cell recognizes that lactose needs to be metabolized when lactose is present in the environment. Lactose is converted to allolactose, which inactivates the lac repressor protein (lacI), allowing RNA polymerase to initiate transcription of the lac operon genes.
  • What type of operon is the lac operon?
    It is an inducible operon.
  • When is gene expression blocked in the lac operon system?
    Gene expression is blocked in the lac operon system when lactose is absent. The active repressor protein lacI binds to the operator, preventing RNA polymerase from transcribing the operon genes.
  • Which region(s) of the lac operon affect the binding of RNA polymerase?
    The operator region of the lac operon affects the binding of RNA polymerase. When the repressor protein lacI is bound to the operator, it blocks RNA polymerase from binding to the promoter and initiating transcription.
  • Under what conditions is the lac operon most likely to be transcribed?
    Presence of lactose and absence of glucose.
  • Which two of the following are characteristics of the lac repressor protein? A) It binds to the promoter. B) It binds to the operator. C) It is inactivated by allolactose. D) It enhances transcription.
    B) It binds to the operator. C) It is inactivated by allolactose.
  • When are the genes involved in lactose metabolism in E. coli expressed and when are they turned off?
    The genes involved in lactose metabolism in E. coli are expressed when lactose is present and glucose is absent. They are turned off when lactose is absent, as the repressor protein lacI binds to the operator, blocking transcription.
  • How is the lac operon affected by negative control?
    The lac operon is affected by negative control through the action of the lac repressor protein (lacI). In the absence of lactose, lacI binds to the operator, preventing transcription. When lactose is present, allolactose inactivates lacI, allowing transcription to proceed.
  • What is the lac operon in E. coli a classic example of?
    The lac operon in E. coli is a classic example of an inducible operon, which is a gene regulatory system that is turned on in response to the presence of a specific substrate, in this case, lactose.
  • Why is the transcription rate of the lac operon low when both glucose and lactose are absent?
    The transcription rate of the lac operon is low when both glucose and lactose are absent because the lac repressor protein (lacI) is active and bound to the operator, blocking RNA polymerase from initiating transcription. Additionally, the absence of lactose means there is no allolactose to inactivate the repressor.
  • What is the function of the lac operon in E. coli, and how is its expression regulated in the presence and absence of lactose?
    The lac operon in E. coli encodes three genes (lacZ, lacY, and lacA) responsible for metabolizing lactose. In the absence of lactose, the active repressor protein lacI binds to the operator, blocking transcription. When lactose is present, it is converted to allolactose, which inactivates lacI, allowing RNA polymerase to transcribe the operon. This ensures the operon is only active when lactose is available, conserving energy.