NEETZoologyMolecular Basis of Inheritance
In a mutant strain of E. coli, a mutation occurs in the i gene of the lac operon, producing a modified repressor protein that has completely lost its ability to bind to the inducer (allolactose). What will be the transcription status of the structural genes ( z , y , and a ) in this mutant strain?
Options
- AThey will be transcribed constitutively, regardless of the presence or absence of lactose.
- BThey will remain permanently switched off in both the presence and absence of lactose.
- CThey will be transcribed normally only when lactose is present in the medium.
- DThey will be transcribed only in the absence of lactose in the medium.
Correct answer
B. They will remain permanently switched off in both the presence and absence of lactose.
Step-by-step solution
The i gene synthesises the repressor protein constitutively. In a normal lac operon, the repressor binds to the operator region and prevents RNA polymerase from transcribing the operon. When an inducer (lactose or allolactose) is present, it binds to the repressor, altering its conformation and inactivating it, which allows transcription to proceed. If a mutation causes the repressor to lose its inducer-binding site, the inducer cannot bind to it. As a result, the mutant repressor will remain permanently active and