Question

Cells of which bacterial genotype would be able to utilize lactose as an energy source?

A)I⁺O⁺Z^-Y⁺

B)I⁺O⁺Z⁺Y^- /F’ (I^SO⁺Z^-Y⁺)

C)I⁺O^CZ⁺Y^- /F’ (I⁺O⁺Z^-Y⁺)

D)I^SO⁺Z⁺Y⁺

^{Option C is right but I don't know why, if it can be explained to me.}

Answer 1

The option B and D has I^s . The I^s mutation is a dominant mutation. It encodes the super repressor in which it can not bind to the inducer. It remains bind to O+. RNA polymerase procees further to the structural gene and there is not found any expresion of structural gene. (Z). That's why lactose is used bu the cell.

A has Z-. That's why there is not fojnd any expression of lac Z gene.

In option C, The presence of O^c makes the continuous expression of lac Z gene. Because operator is constitutive mutant. The repressor can not bind to it. That's why whether glucose is absent or present, there is not found binding of repressor to operator. RNA polymerase binds to the promoter it will proceed further because there is not found any hinderance to the RNA polymerase because of absence of repression the operator. That's why it will proceed further to the structural gene and leads to the expression of lac Z gene due to which it can utilise lactose as an energy source. Lac Z encodes beta galactosidase and breaks down lactose into glucose and galactose and produces energy.