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PCAT Biological Processes – Lac operon in E. coli

  • by Sam
  • Apr 19, 2018
  • PCAT Question of the Day

In an E. coli cell with abundant lactose present:

A. the repressor is bound to the operator.
B. gene transcription of the structural lac genes does not take place.
C. allolactose is bound to the operator.
D. allolactose is bound to the repressor.

Click for Explanation

D is correct. When lactose is present, transcription of structural lac genes takes place so the bacterium can metabolize the lactose sugar. Allolactose (an isomer of lactose) binds to the repressor, causing it to dissociate from the operator region. With the operator now available, transcription can take place. Remember that bacteria want to express the genes for lactose metabolism when lactose is present so they can use it as a food source.

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