Question

Which of the following is not produced by E. coli in the lac operon?
(A) beta-galactosidase
(B) Thiogalactoside transacetylase
(C) Lactose dehydrogenase
(D) Lactose permease

Answer 1

Hint: Concept of lac operon is only valid for prokaryotes. Lac operon has many cistrons and each cistron codes for different types of proteins. All the enzymes or proteins synthesised by lac operon are used in lactose metabolism which acts as a substrate.
Step by step explaination
Lac operon is present in prokaryotes. It is a transcriptionally regulated system. Lac operon is a polycistronic gene that is it has many cistrons (eukaryotes have only monocistronic genes). Lac means lactose.
It is regulated by common promoter genes as well as regulatory genes. This kind of arrangement is very common in prokaryotes, especially bacteria. Other examples of the operon are trp operon, ara operon, his operon, Val operon, etc.
Lac operon is made up of one regulatory gene and three structural genes. The regulatory gene is called the I gene ( here I mean inducer and not inhibitor). The three structural genes are Z, Y, and A. In lac operon I gene codes for repressor.
The Z gene codes for beta-galactosidase referred to as beta gal in short. Beta-galactosidase is mainly responsible for the hydrolysis of lactose which is a disaccharide (glucose + galactose gives lactose).
The Y gene encodes for permease, this increases the permeability of the cell for lactose.
The Lace A gene synthesizes Galactoside acetyltransferase (thiogalactoside transacetylase). Thiogalactoside transacetylase is an enzyme transferring an acetyl group from acetyl-CoA to galactosides, lactosides, and glucosides.
therefore, option (C) is the correct answer.
Note: All the three products discussed above play role in lactose metabolism whereas option (C) which is lactose dehydrogenase does not play any role in lactose metabolism. Also, note that all the genes present on an operon are needed to function in the same or related pathway.