Answer :
Option B ; Inducer is the answer.
A substance that controls the expression of genes is known as an inducer in molecular biology. An inducer works in two different ways, namely:
- by neutralizing repressors. An inducer binds to the repressor, which causes the gene to be expressed. The repressor cannot bind to the operator because of the inducer's binding to the repressor. The transcription of operon genes can then start by RNA polymerase.
- by tying up with activators. Absent an inducer, activators often bind activator DNA sequences weakly. The complex formed by the binding of the activator and inducer to the activation sequence activates the target gene. Transcription is halted by removing the inducer. Lactose operon is the best example of inducible system.
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