Question

Investigating the negative regulation of the lac operon by allolactose, consider the following merozygote experiment: I – O + Z + / F' I S O C Z –. Which statement correctly describes the regulation of ß-galactosidase synthesis in this merodiploid cell? Note that ß-galactosidase expression can come from either the chromosome or the plasmid.

Without lactose ß-galactosidase synthesis is ON; with lactose it is ON

Without lactose ß-galactosidase synthesis is ON; with lactose it is OFF

Without lactose ß-galactosidase synthesis is OFF; with lactose it is ON

Without lactose ß-galactosidase synthesis is OFF; with lactose it is OFF

Answer 1

Answer: Without lactose ß-galactosidase synthesis is OFF; with lactose it is OFF

merozygote : I- O+ Z+/ F' Is Oc Z-

I- = Repressor is mutated

O+= Normal operator

Z+ =Normal structural gene for ß-galactosidase

Is = Super repressor

Oc = Operator mutated

Z-= Mutated structural gene for ß-galactosidase.

All DNA elements are cis acting in lac operon, except repressor.

Repressors are trans acting. Means it can acts on both. In the first case structural gene is normal and repressor is mutated. So there is a possibility for constitutive production of ß-galactosidase, but in the second it is super repressor. So this super repressor is trans acting and prevents ß-galactosidase synthesis, both in the presence and absence of lactose.

So the Answer is Without lactose ß-galactosidase synthesis is OFF; with lactose it is OFF

For additional information:

Glucose is the major energy releasing compound in bacteria. But in the absence of glucose, they use lactose for their energy needs. This lactose can be used with the help of Lac operon.

In the presence of glucose, the lac operon is turned off. A repressor protein (LacI) binds the operator (control) region upstream of the operon preventing transcription.

When glucose is not present in the medium where lactose is present, Lac operon starts functioning. First lactose acts as an inducer of the operon. Lactose bind to LacI and removes the repessor.

Mainly 3 genes are transcribing with the help of a single promoter. These structural genes includes Z,Y, A. The transcribed mRNA provides 3 protein products after translation.