Describe how the lac operon operates, both in presence and absence of an inducer in E.coli. (5)
Functioning of the lac operon:
(i) The lac operon consists of one regulatory gene (the 'i' gene), and three structural gene (z,y and a) and a common promoter 'p' . The 'i' gene codes for the repressor of the lac operon. The 'z' gene codes for galactosidase , 'y' gene for permease and 'a' gene for transacetylase. galactosidase acts as an enzyme which brings hydrolysis of lactose into galactose and glucose. Permease increases the permeability of the cell to galactosidase. Hence all the three gene products are required for lac operon.
(ii) Lactose is the substrate for the enzyme galactosidase and it regulates swtiching 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 glucose, if lactose is added in the growth medium of the bacteria, the lactose is transported into cells through the action of permease. Lactose acts as an 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 the transcription begins.
(v) In the absence of lactose, repressor molecule is free to bind with operator region, lac operon gets switched 'off'.
The lac operon (5)