Chapter 16.1. Animation Activity: The lac Operon

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Correct. Although the genes that metabolize lactose are found on an operon, and operons often consist of genes with related function, an operon is simply a group of genes transcribed from a common promoter on a single mRNA transcript.

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Incorrect. Although the genes that metabolize lactose are found on an operon, and operons often consist of genes with related function, an operon is simply a group of genes transcribed from a common promoter on a single mRNA transcript.

2

Correct. The structural genes for the lac operon are lacZ, which encodes β-galactosidase; lacY, which encodes the lactose permease; and lacA, which encodes the transacetylase. The lacI gene encodes the regulatory protein which is not a part of the operon itself because the lacI gene is transcribed from an independent promoter.

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Incorrect. The structural genes for the lac operon are lacZ, which encodes β-galactosidase; lacY, which encodes the lactose permease; and lacA, which encodes the transacetylase. The lacI gene encodes the regulatory protein which is not a part of the operon itself because the lacI gene is transcribed from an independent promoter.

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Correct. The lacZ gene encodes β-galactosidase. β-galactosidase both converts lactose to allolactose and breaks down lactose, which is a disaccharide, into glucose and galactose. The entry of lactose into the bacterial cell is controlled by the lactose permease, which is encoded by the lacY gene.

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Incorrect. The lacZ gene encodes β-galactosidase. β-galactosidase both converts lactose to allolactose and breaks down lactose, which is a disaccharide, into glucose and galactose. The entry of lactose into the bacterial cell is controlled by the lactose permease, which is encoded by the lacY gene.

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Correct. The operator is simply a DNA sequence that overlaps the promoter region of the lac operon. The operator sequence is bound by the lacI repressor protein. Since the operator overlaps the promoter, RNA polymerase can’t transcribe the operon when the lacI repressor protein is bound to the operator.

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Incorrect. The operator is simply a DNA sequence that overlaps the promoter region of the lac operon. The operator sequence is bound by the lacI repressor protein. Since the operator overlaps the promoter, RNA polymerase can’t transcribe the operon when the lacI repressor protein is bound to the operator.

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Correct. Although the lac operon is controlled by a repressor protein, this does not automatically make it an inducible operon. Repressible operons are also sometimes controlled by repressor proteins and, like the lac operon, are not always transcriptionally active. All operons encode more than one protein. The correct answer is that the lac operon is transcriptionally active only in the presence of lactose.

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Incorrect. Although the lac operon is controlled by a repressor protein, this does not automatically make it an inducible operon. Repressible operons are also sometimes controlled by repressor proteins and, like the lac operon, are not always transcriptionally active. All operons encode more than one protein. The correct answer is that the lac operon is transcriptionally active only in the presence of lactose.

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Correct. Allolactose is the inducer of the lac operon because it directly binds to the repressor protein and creates the conformational change in the repressor protein that releases the repressor from the operator, allowing transcription to occur.

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Incorrect. Allolactose is the inducer of the lac operon because it directly binds to the repressor protein and creates the conformational change in the repressor protein that releases the repressor from the operator, allowing transcription to occur.

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Correct. Bacteria are normally haploid, having only one set of genes on a single chromosome. However if a region of the chromosome is duplicated on a plasmid, the bacterium is effectively diploid for this duplicated region. Partial diploids are very useful for understanding how operons work and for establishing dominance relationships for various mutations.

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Incorrect. Bacteria are normally haploid, having only one set of genes on a single chromosome. However if a region of the chromosome is duplicated on a plasmid, the bacterium is effectively diploid for this duplicated region. Partial diploids are very useful for understanding how operons work and for establishing dominance relationships for various mutations.

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Correct. The lacO^C mutation prevents the binding of any form of lac repressor protein to the operator, including the super repressor mutant. Therefore, in the presence of the lacO^C mutation, the operon will be transcriptionally active whether or not lactose is present.

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Incorrect. The lacO^C mutation prevents the binding of any form of lac repressor protein to the operator, including the super repressor mutant. Therefore, in the presence of the lacO^C mutation, the operon will be transcriptionally active whether or not lactose is present.

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Correct. The lacI repressor protein is a trans-regulatory element because it is able to travel to distant sites to regulate gene expression. For example, in a partial diploid containing the lac operon, only one copy of the lacI gene is sufficient to regulate both operons. In contrast, a lac operon promoter mutation would only affect the operon on the DNA molecule (chromosome or plasmid) containing the mutation. Lactose, although diffusible, is not a trans-regulatory element because it does not directly regulate gene expression. It is converted into the inducer, allolactose, which interacts with the lac repressor protein that does regulate gene expression.

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Incorrect. The lacI repressor protein is a trans-regulatory element because it is able to travel to distant sites to regulate gene expression. For example, in a partial diploid containing the lac operon, only one copy of the lacI gene is sufficient to regulate both operons. In contrast, a lac operon promoter mutation would only affect the operon on the DNA molecule (chromosome or plasmid) containing the mutation. Lactose, although diffusible, is not a trans-regulatory element because it does not directly regulate gene expression. It is converted into the inducer, allolactose, which interacts with the lac repressor protein that does regulate gene expression.

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Correct. If the lac operon were never transcribed in the absence of lactose, there would be no molecules of permease to permit the entry of lactose into the cell. Likewise, without basal transcription of this operon, there would be no β-galactosidase to convert any lactose that was present in the cell to allolactose to induce the operon.

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Incorrect. If the lac operon were never transcribed in the absence of lactose, there would be no molecules of permease to permit the entry of lactose into the cell. Likewise, without basal transcription of this operon, there would be no β-galactosidase to convert any lactose that was present in the cell to allolactose to induce the operon.

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Correct. Although the super repressor protein can diffuse and sit on any wild- type operator, it can’t bind to an operator with a lacO^C mutation. Therefore, the super repressor protein will shut down transcription on the lac operon on the bacterial chromosome, but will be unable to prevent transcription of thelac operon from the plasmid. However, the lac operon on the plasmid also has a promoter mutation that prevents binding of RNA polymerase. So, in this case, neither the chromosomal nor plasmid copies of the lac operon will be transcriptionally active whether or not lactose is present.

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Incorrect. Although the super repressor protein can diffuse and sit on any wild- type operator, it can’t bind to an operator with a lacO^C mutation. Therefore, the super repressor protein will shut down transcription on the lac operon on the bacterial chromosome, but will be unable to prevent transcription of thelac operon from the plasmid. However, the lac operon on the plasmid also has a promoter mutation that prevents binding of RNA polymerase. So, in this case, neither the chromosomal nor plasmid copies of the lac operon will be transcriptionally active whether or not lactose is present.