A second copy of the lac operon is introduced into the above strain (this second

Question

A second copy of the lac operon is introduced into the above strain (this second copy is on an

F’lac plasmid). This second copy has the genotype I+ P+ O+ Z- Y- (the original chromosomal copy

is still I+ P+ O- Z+ Y+). In which of the following environments will functional copies of the LacZ

and LacY proteins be expressed?

A. Glucose is absent, regardless of the presence of lactose.

B. Lactose is present, regardless of the presence of glucose.

C. Glucose is present, regardless of the presence of lactose.

D. Lactose is present and glucose is absent.

E. It won’t be expressed in any environment.

Can you explain what O- is? How do you approach this question from beginning to end?

Explanation / Answer

Lac operon works only when the glucose is absent. Lac operon doesn't operates in the presence of glucose. Given the original chromosomal copy is I+ P+ O- Z+ Y+. In this O- means the operator is mutated in such a way that repressor protein cannot bind to the operator. Due to this expression of the lacZ and lacY genes occur whether the lactose is present or asence.

                          The second copy has the genotype I+ P+ O+ Z- Y- . In this condition lacZ and lacY genes are mutated. Therefore lacZ and lacY genes doesn't express even if the lactose is present. There fore enzymes cannot be produced.

(A) When glucose is absent, regardless of the presence of lactose. Lac operon operated and lacZ and lacY genes are expressed in the original copy. In the second copy these genes are not expressed even in the absence of the glucose as these are mutated.

(B) When lactose is present, regardless of the presence of glucose. In this condition lac operon operates in the absence of the glucose. Lac operon operated and lacZ and lacY genes are expressed in the original copy. In the second copy these genes are not expressed even in the presence of lactose as these genes are mutated. Lac operon doesnt operates in the present of the glucose.

(C) When glucose is present, regardless of the presence of lactose. Lac operon doesn't operates as the activator protein is not bound to the DNA.

(D) When lactose is present and glucose is absent. In this condition lac operon operates. Lac operon operated and lacZ and lacY genes are expressed in the original copy. In the second copy these genes are not expressed even in the presence of lactose as these genes are mutated.

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