Molecular Basis of Inheritance Question 135
135. E.coli cells with a mutated $z$ gene of the lac operon cannot grow in medium containing only lactose as the source of energy because:
[2005]
(a) the lac operon is constitutively active in these cells
(b) they cannot synthesize functional betagalactosidase
(c) in the presence of glucose, E.coli cells do not utilize lactose
(d) they cannot transport lactose from the medium into the cell
Show Answer
Answer : b
Hints & Solution
(b) Operons are segments of genetic material which function as regulated unit or units that can be switched on and switched off. An operon consists of one to several structural genes. (Three in lac operon) These are genes which produce mRNAs for forming polypeptides / proteins / enzymes. $Z$ (produces enzyme $\beta$ galactosidase for splitting lactose into glucose and galactose). Y (produces enzyme galactoside permease required for entry of lactose) and $A$ (produces enzyme thiogalactoside trans- acetylase). The three structural genes of the operon produce a single polycistronic mRNA.
