Give above is the schematic representation of lac operon of E.coli. Explain the functioning of this operon when lactose is provided in the growth medium of the bacteria. 5 mark
Functioning of lac operon:
(i) The lac operon consists of one regulatory gene (the 'i' gene), and three structural genes (z,y and a) and a common promotor 'p'. The i gene codes for the repressor of the lac operon. The 'z' gene codes for b galactosidase, 'y' gene for permease and 'a' gene transacetylase. b galactosidase acts as an enzyme which brings hydrolysis of lactose into galactose and glucose. Permease increases the permeability of the cell to b galactosidase. Hence all the three gene products are required for lac operon.
(ii) Lactose is the substrate for the enzyme b galactosidase and it regulates switching on and off of the operon.
(iii) Repressor is a protein coded by 'i' gene. It binds to the operator 'o' region of the operon and prevents the RNA polymerase from transcribing the structural gene.
(iv) In the absence of carbon sources such as gluctose, if lactose is added in the growth medium of the bacteria, the lactose is transported into the cells through the action of permease. Lactose acts as inducer. It binds with repressor. Repressor inducer complex is formed. It will not be able to bind with operator. Now operator is free. RNA polymerase binds with the promoter region, operon get switched on and transcription begions.
(v) In the absence of lactose, repressor molecule is free to bind with operator region, lac operon gets switched 'off'. 5 mark