Question

Study the schematic representation of the genes involved in the lac operon given below and answer the questions that follow :

p

i

p

o

z

y

a

(a) Identify and name the regulatory gene in this operon. Explain its role in 'switching off' the operon.

(b) Why is the lac operon's regulation referred to as negative regulation?

(c) Name the inducer molecule and the products of the genes 'z' and 'y' of the operon. Write the functions of these gene products.

Answer 1

(a) The regulatory gene in this operon is i gene. The operon is switched off in the absence of the inducer, lactose. In the absence of lactose, the repressor gene is constitutively synthesized from the repressor gene. This active repressor protein binds to the operator gene of the operon and prevents the RNA polymerase to initiate the process of transcription.

(b) The regulation of lac operon is controlled by a repressor that is responsible for switching on and off the operon. When repressor binds to the operator, the operon is switched off and transcription is stopped, the reason why it is called negative regulation.

(c) Lactose is called the inducer molecule.
Gene 'z' codes for beta-galactosidase, which is responsible for the hydrolysis of lactose into galactose and glucose.
Gene 'y' codes for permease which increases the permeability of the cell to lactose.