Question

In the lac operon, the structural genes are switched off when repressor binds to the

• A. Operator
• B. Promoter
• C. Regulator
• D. Inducer
• E. Allolactose

Answer 1

The correct option is A Operator

A molecule, that regulates gene expression by binding to repressor to prevent the transcription, is inducer. Allolactose serves as inducer in the lac operon and binds to the repressor and decreases the repressor's affinity for the operator site; thereby facilitating transcription. Promoter is a DNA segment that serves to initiate transcription of a particular gene. They are present upstream near the transcription start sites of genes. Regulator gene regulates the expression of structural genes by its protein product that are mostly transcription factors. These transcription factors bind to the operator region which is located just upstream the biding site of the RNA polymerase and hence prevent the binding and transcription. Operator is a DNA sequence present between the promoter region and the first coding gene. Thus, operator region serves as repressor protein binding site. Binding of repressor to operator prevents the binding of RNA polymerase at promoter sequence, thereby not allowing gene expression. Option A is the correct answer.