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For the following given genotypes and conditions of the lac operon, write whether the correct protein corresponding to the Z gene is being made or not made. A "+" represents wild type and deviations from the "+" examples are mutated gene elements. i) I^+ P^+ O^+ Z^+, lactose absent not made ii) I^+P^- O^+ Z^+, lactose present not made iii) I^- P^+ O^C Z^+, lactose absent made iv) I^+ P^+ O^C Z^+/I^- P^+ O^+ Z^+ lactose absent made v) I^+ P^+ O^C Z^-/I^+ P^+ O^+ Z^- lactose present not made vi) I^- P^+ O^+ Z^+/I^+ P^- O^+ Z^- lactose present made vii) I^- P^+ O^+ Z^+/I^+ P^+ O^+ Z^+ lactose absent not made viii) I^+ P^+ O^+ Z^-/I^+ P^+ O^+ Z^+ lactose and glucose present not made

Explanation / Answer

I is the repressor, P is the promoter, O is the operator, and Z is the gene for beta-galactosidase enzyme. In presence of lactose, lactose binds to the repressor and the promoter region is now free for the RNA polymerase enzyme to bind and transcribe the lac Z gene. In the absence of lactose, repressor binds to the operator region and blocks RNA polymerase binding to the promoter region. Hence, transcription of lac Z gene does not occur and beta-galactosidase is not made.

2. I+P-O+Z+, lactose present, not made

Even though lactose is present and repressor is not bound to the promoter region beta-galactosidase is not made because the promoter region is mutated. As a result RNA polymerase enzyme cannot bind to the promoter region and hence transcription of lac Z gene does not occur.

3. I-P+OcZ+, lactose absent, made

Since repressor gene is mutated no repressor protein is formed and so the promoter region is free for RNA polymerase to bind and transcribe lac Z gene. Even in the absence of lactose lac Z gene is transcribed as the operator region is constitutive.

4. I+P+OcZ+/ I-P+O+Z+, lactose absent, made

In this case the first genotype produces the repressor protein but since the operator region is constitutive, repressor protein cannot bind to the operator region. The promoter region is now free for binding of RNA polymerase resulting in the transcription of lac Z gene even in the absence of lactose.

In the second genotype, repressor protein is not present. So the promoter region is free for the RNA polymerase enzyme to bind and hence transcription of lac Z gene occurs even in the absence of lactose.

5. I+P+OcZ-/ I+P+O+Z-, lactose present, not made

In this case the first genotype produces the repressor protein but since the operator is constitutive, it cannot bind to the operator region. Promoter is free for RNA polymerase to bind and begin transcription. But since lac Z gene is mutated beta-galactosidase enzyme is not produced even in presence of lactose.

The second genotype produces the repressor protein which binds to the operator region in the absence of lactose thereby blocking RNA polymerase to bind to the promoter region. In presence of lactose, repressor protein binds to lactose and operator region is now free. RNA polymerase can bind to the promoter region. But since lac Z gene is mutated, beta-galactosidase is not formed even in presence of lactose.

6. I-P+O+Z+/ I+P-O+Z-, lactose present, made

In this case the first genotype does not produce repressor protein. So, the operator region is free and RNA polymerase can bind to the promoter region and transcribe lac Z gene even in the absence of lactose. However, the repressor protein produced from the second genotype is trans-acting ie., it binds to the operator region of the first genotype, thereby preventing the transcription of lac Z gene in the absence of lactose. Hence, only in the presence of lactose transcription of lac Z gene takes place in the first genotype to produce beta-galactosidase enzyme.

The second genotype produces repressor protein which binds to the operator region in the absence of lactose and hence transcription does not occur. But in the presence of lactose, repressor protein binds to lactose. Operator region is now free and RNA polymerase can bind to the promoter region. But since the promoter region is mutated, RNA polymerase binding does not occur. And also lac Z gene is mutated in this genotype. So, beta-galactosidase is not produced from this genotype even in the presence of lactose.

7. I-P+O+Z+/ I+P+O+Z+, lactose absent, not made

In this case the first genotype does not produce repressor protein as lac I gene is mutated. In such a case the operator region should be free and RNA polymerase should bind to the promoter and transcribe lac Z gene even in the absence of lactose. But since the second genotype produces repressor protein, it shows trans-action on the first genotype ie., the repressor protein from the second genotype binds to the operator region of the first genotype. As a result RNA polymerase cannot bind to the promoter region and hence transcription of lac Z gene does not occur from the first genotype in the absence of lactose.

In the second genotype, in the absence of lactose, repressor binds to the operator region and blocks RNA polymerase binding to the promoter region. Hence, transcription of lac Z gene does not occur and beta-galactosidase is not made.

8. I+P+O+Z-/ I+P+O+Z+, lactose and glucose present, not made

In this case the first genotype has mutated lac Z gene. So, lac Z gene will not be transcribed at all even in the presence or absence of lactose.

In the second genotype lac Z gene is not mutated and in presence of lactose the repressor should bind to lactose and operator region should be free so that RNA polymerase can bind to the promoter region and can transcribe lac Z gene. But lac Z gene is not transcribed in this case due to the presence of glucose. When glucose is present lac operon is not activated as in presence of glucose no cAMP is made. cAMP binds to CAP (catabolite activator protein). cAMP-CAP complex binds to the lac operon DNA and help RNA polymerase bind to the promoter region resulting in transcription of lac Z gene. So, lac Z gene is not transcribed from the second genotype even in presence of lactose because of glucose availability.

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