Question

Which best describes the inducible operon
A) The tryptophan gene, in which the presence of tryptophan functions as a corepressor to prevent its own transcription
B) The lac gene, in which the repressor, preventing the synthesis of the enzymes that degrade lactose, is deactivated by the presence of lactose
C) A type of gene expression mostly seen in eukaryotes.
D) None of the above

Answer 1

Hint: In the lac operon, a polycistronic structural gene is regulated by a common promoter and regulatory genes. Such an arrangement is very common in bacteria and referred to as operon. For example, lac operon, trp operon, ara operon, his operon, val operon, etc.

Complete answer:
The lac operon consists of one regulatory gene (the i gene term ‘i’ does not refer to the inducer, rather it is derived from the word inhibitor) and 3 structural genes (z, y, and a) ‘i’ gene codes for the repressor of the lac operon. The z gene codes for beta-galactosidase, 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 the permeability of the cell to beta-galactosidase. The ‘a’ gene encodes a transacetylase. Hence, all three gene products in lac operon are required for the metabolism of lactose. Lactose is a substrate photo enzyme beta-galactosidase and it regulates switching on and off the operon. Hence, it is termed as an inducer.in absence of a carbon source such as glucose, if lactose is provided in the growth medium of the bacteria then the lactose is transported into the cell through the action of permease.
Thus, option B is the correct answer.

Note: The glucose or galactose cannot act as inducers for lac operon. Regulation of lac operon by a repressor is referred to as negative regulation. Lac operon is under control of positive regulation as well, but it is beyond the scope of discussion at this level.