Question:
Read the following passage and answer the questions that follow :
Gene expresses itself in a cell system as a protein/enzyme. How an expression of gene occurs in a cell system, when it needs to occur and how the gene expression is regulated in prokaryotic cell system was studied by the combined efforts of Francois Jacob, a geneticist and Jacques Monod, a biochemist. For their work on lactose metabolism in E. coli and introducing the concept of "lac operon", they were awarded the Nobel Prize in 1965.
Why is lac operon said to be a transcriptionally regulated system ?
Read the following passage and answer the questions that follow :
Gene expresses itself in a cell system as a protein/enzyme. How an expression of gene occurs in a cell system, when it needs to occur and how the gene expression is regulated in prokaryotic cell system was studied by the combined efforts of Francois Jacob, a geneticist and Jacques Monod, a biochemist. For their work on lactose metabolism in E. coli and introducing the concept of "lac operon", they were awarded the Nobel Prize in 1965.
Why is lac operon said to be a transcriptionally regulated system ?
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The switch works on mRNA synthesis by RNA polymerase.
Updated On: Oct 22, 2026
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Solution and Explanation
Step 1: Find the step of gene expression that is controlled:
Gene expression has transcription (DNA to mRNA) and translation (mRNA to protein) as its main steps.
In the lac operon, the control acts before the mRNA is formed.
Step 2: Show the control:
In the absence of lactose, the repressor binds the operator and RNA polymerase cannot transcribe the structural genes, so no mRNA is made.
In the presence of lactose, the inducer inactivates the repressor, and RNA polymerase makes the mRNA.
Final Answer:
Because the repressor and inducer decide whether the structural genes are transcribed into mRNA, the lac operon is regulated at the level of transcription.
Gene expression has transcription (DNA to mRNA) and translation (mRNA to protein) as its main steps.
In the lac operon, the control acts before the mRNA is formed.
Step 2: Show the control:
In the absence of lactose, the repressor binds the operator and RNA polymerase cannot transcribe the structural genes, so no mRNA is made.
In the presence of lactose, the inducer inactivates the repressor, and RNA polymerase makes the mRNA.
Final Answer:
Because the repressor and inducer decide whether the structural genes are transcribed into mRNA, the lac operon is regulated at the level of transcription.
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