Question

In an E. coli strain the i gene gets mutated and its product can not bind the inducer molecule. If the growth medium is provided with lactose, what will be the outcome?

• A. Only z gene will get transcribed
• B. z, y, a genes will be transcribed
• C. z, y, a genes will not be translated
• D. RNA polymerase will bind the promoter region

Answer 1

Answer: (C)

z, y, a genes will not be translated

Answer 2

The i gene codes for the repressor of the lac operon. An inducer (allolactose or an analog) binds to the repressor and prevents its binding to the operator, thereby releasing the repression and allowing transcription of the lac operon.
If the i gene is mutated in such a way that the repressor molecule is unable to bind with the inducer, the repressor which is now free from the inducer will now bind to the operator sequence. This will prevent the binding of RNA polymerase on the promoter sequence. Lactose in the medium cannot bind to the repressor as effectively as allolactose does. Therefore, lactose also cannot deactivate the repressor and prevent it from binding to the operator.
Thus if the operator is blocked by the repressor, there will be no transcription and the structural genes z, y and a will not get transcribed and translated.