lac_operon

Question

What best defines an operon?

a group of bacterial genes that metabolizes lactose

a group of bacterial genes that metabolizes any substrate

a group of bacterial genes that performs a related function

a group of bacterial genes that are transcribed together on the same mRNA

All of the above.

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.

Question

Which are the structural genes of the lac operon?

lacZ

lacY

lacI

lacZ, lacY, and lacA

lacZ, lacY, lacA, and lacI

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.

Question

What the role of the protein encoded by the lacZ gene?

The lacZ gene encodes an ezyme that permits lactose to enter the bacterial cell.

The lacZ gene encodes an enzyme that converts lactose to allolactose.

The lacZ gene encodes an enzyme that converts lactose to glucose and galactose.

All of the above.

Both B and C.

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.

Question

What is the operator?

The operator is a DNA sequence that binds to the lacI repressor protein.

The operator is promoter of the lacI gene.

The operator is a protein that sits on the lac operon promoter.

The operator is a domain of the lacI repressor that binds to DNA.

The operator is an enzyme that converts lactose to allolactose.

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.

Question

The lac operon is inducible. What does this mean?

The operon is controlled by a repressor protein.

The operon is not always transcriptionally active.

The operon is transcriptionally active only in the presence of lactose.

The operon encodes more than one protein.

None of the above.

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.

Question

What is the inducer of the lac operon?

β-galactosidase

lactose

allolactose

the lacI protein

the operator

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.

Question

What is a partial diploid?

a bacterium that has two complete circular chromosomes

a bacterium that has one chromosome from its mother and one from its father

an operon in combination with its regulatory protein

a bacterium that has at least some common genes on both the chromosome and a plasmid

a mammalian cell that contains a bacterial artificial chromosome that shares some genes with mammalian chromosomal DNA

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.

Question

Which of the following is the most dominant?

lacO^C

lacO⁺

lacI⁺

lacI^-

lacI^S

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.

Question

When speaking of gene regulation, regulatory elements are often described as cis- or trans-regulatory elements. A cis-regulatory element is a nucleic acid sequence that regulates the expression of genes located on the same molecule of DNA. In contrast, a trans-regulatory element is a diffusible factor (usually a protein) that may modify gene expression at distant sites on the same or different molecule of DNA. In the lac operon, which of these is a trans-regulatory element?

the lacI promoter

the lac operon promoter

lactose

the lac operator

the lacI repressor

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.

Question

Is it possible to get any transcription of the lac operon in the absence of lactose? Why or why not?

No, without lactose the repressor protein will always prevent transcription.

Yes, some transcription must take place in order to produce the permease to allow lactose to enter the cell.

Yes, some transcription must take place in order to produce β-galactosidase to make allolactose.

Yes, because although the repressor protein keeps transcription at a very low basal level, some transcripts will be produced and translated.

B, C and D are correct.

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.

Question

What would be the result of a partial diploid with a bacterial chromosome containing a super repressor (lacI^S) mutation but otherwise wild type, and a plasmid containing a wild- type lacI gene and lacI promoter but also containing both a nonfunctional lac operon promoter mutation and a lacO^C mutation?

P⁺lacI^S….P⁺ lacO⁺lacZ⁺ lacY⁺lacA⁺/ P⁺lacI⁺….P^-lacO^C lacZ⁺ lacY⁺lacA⁺

β-galactosidase would not be produced whether or not lactose was present.

β-galactosidase would always be produced whether or not lactose was present.

β-galactosidase would only be produced in the presence of lactose.

β-galactosidase would only be produced in the absence of lactose.

It is impossible to predict whether or not β-galactosidase will be produced in any condition.

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.

You have one more chance to answer the question correctly. Try again.

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.

Chapter 16.1. Animation Activity: The lac Operon

Animation Assessment

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