For an E. coli strain with the lac operon genotype I⁺ P⁺ O⁺ Z⁺ Y⁺, identify the level of transcription of the operon genes in each growth medium listed. Specify transcription as 'none,' 'basal,' or 'activated' for each medium, and provide an explanation to justify your answer.
Growth medium contains glucose but no lactose.

Question

For an E. coli strain with the lac operon genotype I⁺ P⁺ O⁺ Z⁺ Y⁺, identify the level of transcription of the operon genes in each growth medium listed. Specify transcription as 'none,' 'basal,' or 'activated' for each medium, and provide an explanation to justify your answer.
Growth medium contains glucose but no lactose.

Growth medium contains glucose but no lactose.

Accepted Answer

Hey, everyone. Let's take a look at this question together. In what instance does the lac repressor protein bind to the operator site and block the transcription of the operon genes? Is it answer choice? A in the absence of lactose, answer choice B in the absence of glucose, answer choice C in the absence of lactose and glucose or answer choice D in the presence of lactose and glucose work this problem out together to try to figure out which of the following answer choices is the correct answer. So we have this lack repressor protein and we also have the operator site and we know that the lack repressor protein relies on the presence or the absence of lactose to determine if it will or will not bind to that operator site. Which another name for the operator site is the lack O which is that operator region of the lac operon. So that means that the lac repressor protein binding to the operator site to block the transcription of the operon genes relies on either the presence or the absence of lactose. And so when we have lactose, meaning that lactose is present, we know that R N A limes is what binds to, to lack O which is that operator region of the lac operon. And transcription occurs since R N A polymerase is what binds to the operator site and transcribes the opera on genes. However, when lactose is absent, the lac repressor protein vines instead of R N A polymerase, and this blocks the transcription of those operon genes. Since the lac repressor protein is binded to the operator site and not R N A polymerase. So that means that in the absence of lactose, the lac repressor protein binds to the operator site and blocks the transcription of the operon genes. So answer choice. A in the absence of lactose is the correct answer. Since the process of the transcription of the operon genes relies on R N A polymerase binding to that operator region of the lac operon, which is LAC O and R N A polymerase can only bind to that region. When lactose is present, this lactose is an inducer molecule which disables the repressor. And so when it is present, the lac repressor protein cannot bind. And when it is absent, the lac repressor protein can bind and block the transcription of the operon genes. So answer choice. A is the correct answer. I hope you found this video to be helpful. Thank you and goodbye.

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Key Concepts

Lac Operon Structure

Glucose Effect

Transcription Levels