Question

Select the incorrect statement about lac operon when lactose is present in the medium.
(A) Gene- a gets transcribed into mRNA which produces beta galactoside permease
(B) Lactose inactivates repressor protein
(C) RNA polymerase transcribes z-gene, y-gene and a-gene
(D) Allolactose is the inducer of Lac operon

Answer 1

Hint: Allolactose is the inducer of lac operon. Usually, Lac operon is negatively regulated. It is functional only in prokaryotes and it has many cistrons and each cistron codes for specific proteins. All the proteins synthesised by the operon are used in n lactose metabolism, so when lactose is in abundance then all the proteins will be produced.
Step-by-step solution:
Lac operon is only present in prokaryotes, especially bacteria. In lac operon, lac stands for lactose. The operon (lac) consists of one regulatory gene also known as I gene, I gene stands for inhibitor (and not inducer). The structural gene consists of Z gene, Y gene and A gene.
The Z gene codes for beta-galactosidase which is responsible for the hydrolysis of lactose (a disaccharide of glucose and galactose), and the Y gene codes for permease which is responsible for increasing permeability of the cell for beta galactoside often known as lactose, the A-gene codes for transacetylase.
Lactose is the inducer of the lac operon, the actual inducer is allolactose. When lactose is in abundance it binds with repressor protein and inactivates the repressor. Regulation of lac operon can be seen as regulation of enzyme synthesis by its substrate. Regulation of lac operon by a repressor is known as negative regulation. Lac operon is under positive regulation as well.

So, option (A) is correct.
Note: Lactose or allolactose to be more specific acts as both substrates as well as inducer, remember that glucose or galactose cannot act as inducers for lac operon. Also, note that I gene of lac operon is a constitutive gene that always keeps on the synthesis of repressor protein.