Which of the following enzymes increases the permeability of the bacterial cell to lactose?

The differentiation of organs and tissues in a developing organism is associated with which of the following?

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?

The one gene-one enzyme hypothesis was postulated by

In $lac$ operon,the switching on or switching off of the operator is achieved by . . . . . . .

In the lactose operon of $Escherichia \text{ } coli$, what is the function of the promoter?