Question

Recognise the figure and mark the correct match:

• A. 1—inducer, 2—repressor, 4—in absence of inducer, 3—in presence of inducer
• B. 4—inducer, 2—repressor, 3—in absence of inducer, 1—in presence of inducer
• C. 2—inducer, 1—repressor, 4—in absence of inducer, 3—in presence of inducer
• D. 2—inducer, 1—repressor, 3—in absence of inducer, 4—in presence of inducer

Answer 1

The correct option is D 2—inducer, 1—repressor, 3—in absence of inducer, 4—in presence of inducer

The lac operon consists of one regulatory gene (the i gene – here the term i does not refer to inducer, rather it is derived from the word inhibitor) and three structural genes (z, y, and a). The i gene codes for the repressor of the lac operon. The z gene codes for beta-galactosidase (β-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. The y gene codes for permease, which increases permeability of the cell to β-galactosides. The a gene encodes a transacetylase. Hence, all the three gene products in lac operon are required for metabolism of lactose.

Here, lactose acts as an inducer. If lactose is in the medium for the bacteria, the regulatory gene is activated. The inducer will bind to the repressor protein and render it inactive which allows transcription of the operon. Thus, the lac operon is negatively regulated in this case.

In absence of inducer, the operator region is bound by the repressor. This prevents RNA polymerase from trasncribing the operon.