In which of the following are monocistronic genes found?

Match Column-$I$ with Column-$II$:

Column-$I$	Column-$II$
$(i)$ Operator site	$(a)$ Codes for enzyme (protein)
$(ii)$ Promoter site	$(b)$ Codes for repressor
$(iii)$ Structural gene	$(c)$ Binding site for $RNA$ polymerase
$(iv)$ Regulator gene	$(d)$ Binding site for repressor

During lactose metabolism in $E. coli$,the repressor protein binds to the ...........

At what level does gene expression not occur?

An organism is able to survive on a culture medium containing nutrient $A$ through an enzyme-catalyzed process: $A \xrightarrow{X} B \xrightarrow{Y} C$. $A$ mutant organism fails to survive on this medium. However,it grows when nutrient $B$ is added to the medium. Which gene of this mutant is defective?

$E. coli$ cells with a mutated $Z$ gene of the $lac$ operon cannot grow in a medium containing only lactose as the source of energy because: