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what is the lac operon in e. coli a classic example of?
gene expression occurring at transcription
gene expression leading to dna mutations
gene expression occurring by changing protein shape
antibiotics blocking protein synthesis

Explanation:

The lac operon in E. coli is a well - known model in molecular biology. It controls the expression of genes related to lactose metabolism. Gene expression regulation at the transcription level is a key concept here. When lactose is present, it acts as an inducer. It binds to the repressor protein, changing the repressor's shape so that it can no longer bind to the operator region of the lac operon. As a result, RNA polymerase can bind to the promoter and transcribe the genes (lacZ, lacY, lacA) in the operon. This is a classic example of gene expression being regulated at the transcription step.

• The lac operon is not about gene expression leading to DNA mutations. DNA mutations involve changes in the DNA sequence itself, which is not the main function of the lac operon's regulatory mechanism.
• It is not about changing protein shape for the sake of gene expression (the protein shape change of the repressor is a means to an end of regulating transcription).
• Antibiotics blocking protein synthesis is a different biological process (e.g., some antibiotics like tetracycline bind to the ribosome and prevent tRNA from binding, but this is not related to the lac operon's function).

Answer:

gene expression occurring at transcription