Lac Operon

The lac operon is a cluster of genes in Escherichia coli controlled coordinately to enable lactose metabolism, comprising a promoter, an operator, and three structural genes (z, y, a). The repressor protein, encoded by the regulatory i gene, binds to the operator in the absence of lactose and physically obstructs RNA polymerase from transcribing the structural genes, switching the operon off. The option that it enhances RNA polymerase binding is incorrect because the repressor acts negatively, hindering rather than promoting transcription. The claim that it mutates the promoter is wrong since the repressor only binds reversibly and causes no permanent change to the DNA sequence. The idea that it degrades existing mRNA confuses transcriptional control with post-transcriptional decay, which is not the repressor's role. When lactose is present, the inducer (allolactose) binds the repressor, altering its shape so it can no longer attach to the operator, thereby turning the operon on. As described in NCERT Class 12, Chapter 6 (Molecular Basis of Inheritance), this is the classic example of negative regulation. As a logical check, blocking the operator stops transcription initiation, which is exactly the immediate effect of repressor binding.