Question 1

The lac operon in E. coli is regulated by allolactose, which _______ a(n) _______ of transcription.

Accepted Answer

D

Question 2

The lac operator consists of approximately 20 base pairs of DNA located

Accepted Answer

The lac operator is located in a position overlapping the transcription initiation site and extends downstream, allowing it to regulate the binding of RNA polymerase to the promoter.

Question 3

If E. coli is in an environment that is rich in lactose, a metabolite of lactose binds to

Accepted Answer

In the presence of lactose, a metabolite of lactose binds to the repressor protein, causing it to change shape and release from the operator site. This allows RNA polymerase to bind to the promoter and transcribe the genes involved in lactose breakdown. The binding of metabolite to the repressor is an example of an inducer, which activates the transcription of the lac operon.

Question 4

A mutation in the i gene encoding the lac operon repressor protein causes this protein to lose its ability to bind to the operator site. Which would you expect to observe in an E. coli population having this mutation?

Accepted Answer

The lac operon is only expressed when lactose is present and glucose is absent. However, in a population of E. coli with a mutation in the i gene causing the lac repressor protein to lose its ability to bind to the operator site, the lac operon would be constitutively expressed, meaning the cells would express lac genes even in the absence of lactose. Therefore, the best choice is D. The cells would express lac genes when lactose is absent.

Question 5

E. coli regulates the biosynthesis of the amino acid tryptophan through the tryptophan (trp) operon. When tryptophan levels are high inside the cell, tryptophan binds to a regulatory protein, which then binds to the operator site of the operon. This shuts down transcription of the genes encoding enzymes needed for tryptophan biosynthesis, and tryptophan biosynthesis is halted. The trp operon is an example of

Accepted Answer

The trp operon is an example of negative control because binding of a repressor protein (activated by high levels of tryptophan) blocks transcription of genes required for tryptophan biosynthesis.

Question 6

Which information would be helpful in determining whether a particular bacterial gene is under positive or negative transcriptional control?

Accepted Answer

The information that the gene is not transcribed when a repressor protein binds to DNA indicates negative transcriptional control. Positive transcriptional control would require information about the gene being transcribed when an activator protein binds to DNA or when a specific signaling pathway is activated. Choices A, B, and C do not give specific information about the type of transcriptional control.

Question 7

A researcher hypothesizes that expression of a particular gene is under positive transcriptional control. What evidence would support his hypothesis?

Accepted Answer

The fact that the gene is only expressed when a specific activator protein binds to an upstream DNA site supports the hypothesis that it is under positive transcriptional control. This suggests that certain factors are required to actively promote the transcription of the gene, rather than it being repressed by default or regulated by mutations within the gene itself.

Question 8

Regulation of the lac operon by lactose involves a(n) _______ that inhibits transcription, while regulation by glucose involves a(n) _______ that stimulates transcription.

Accepted Answer

Lactose acts as an inducer, binding to the repressor protein and preventing it from binding to the operator region, thus allowing transcription to occur. Glucose, on the other hand, inhibits transcription by preventing the production of the cAMP activator protein, which is necessary for RNA polymerase to bind to the promoter region. Therefore, regulation by lactose involves a repressor, while regulation by glucose involves an activator.

Question 9

How do repressors differ from activators of transcription in bacteria?

Accepted Answer

Repressors inhibit transcription by binding to the operator region of DNA, blocking the binding of RNA polymerase and preventing transcription from occurring. In contrast, activators stimulate transcription by binding to specific regulatory regions on DNA, promoting the binding of RNA polymerase and enhancing transcription. Activators and repressors both function on DNA, and can act on one or multiple chromosomes. Some activators and repressors are also proteins. However, their primary difference lies in their effect on transcription – activators stimulate, while repressors inhibit.

Question 10

Two bacterial populations were prepared in a laboratory. Population A expresses the wild-type regulator protein that controls the transcription of gene X. Population B contains a mutation in the gene for this regulator protein and does not express any form of the protein. Which statement provides a possible hypothesis to explain the gene X expression data shown in the table below? $$\begin{array} { | c | c | c | } 
\hline \text { Bacterial population } & \begin{array} { c } 
\text { Regulator protein } \
\text { expression }
\end{array} & \text { Gene } X \text { expression } \
\hline \text { A } & \text { wild-type } & + + + \
\hline \text { B } & \text { none } & 0 \
\hline
\end{array}$$

Accepted Answer

The regulator protein must be an activator because gene X is expressed in population A (where the regulator protein is present) and not expressed in population B (where the regulator protein is absent), indicating that the presence of the regulator protein stimulates gene X transcription.

Question 11

One way that a promoter differs from an enhancer is that it

Accepted Answer

Promoters are located at the beginning of the gene and recruit RNA polymerase to initiate transcription, while enhancers can be located far away from the gene and can increase the transcriptional activity of the gene by binding specific transcription factors. Therefore, the main difference between a promoter and an enhancer is their location relative to the gene they regulate.

Question 12

TATA boxes are found

Accepted Answer

TATA boxes are specific DNA sequences found in the promoters of eukaryotic genes, not in enhancers.

Question 13

An investigation was carried out to identify different sections of mammalian DNA as either promoters or enhancers of gene Y transcription. Wild-type mammalian cells functioned normally, while mutant cells lacked the ability to produce functional cohesion protein. Based on the data in the table below, which conclusion is valid? $$\begin{array} { | c | c | c | } 
\hline \text { Cells } & \text { Regulatory sequence } & \text { Expression of gene } \boldsymbol { Y } \
\hline { \text { Wild-type } } & \mathrm { A } & + + + \
& \mathrm { B } & + + + \
\hline { \text { Mutant } } & \mathrm { A } & + + + \
& \mathrm { B } & 0 \
\hline
\end{array}$$

Accepted Answer

The fact that gene Y is expressed in both wild-type and mutant cells when regulatory sequence A is present, but not expressed in mutant cells when only regulatory sequence B is present, suggests that A must be a promoter (necessary for basic gene expression) and B an enhancer (boosts expression but is not essential, and its function can be affected by the lack of cohesion protein in mutant cells).

Question 14

Looping of DNA has been observed by investigators using electron microscopy techniques. The presence of which feature in DNA might be indicated by such observations?

Accepted Answer

Enhancers are DNA elements that can increase the transcription of genes. They often function by looping the DNA to bring the enhancer in close proximity to the promoter of the gene it regulates, facilitating transcriptional activation. This looping can be observed through electron microscopy, indicating the presence of enhancers.

Question 15

A reporter gene is one that

Accepted Answer

A reporter gene is a gene that can be used to study the activity of a regulatory sequence. It is typically ligated and cloned downstream of the regulatory sequence of interest. When the regulatory sequence is activated, it causes the reporter gene to be expressed, resulting in the production of an easily detectable product (such as a fluorescent protein or enzyme) that can be used to measure the activity of the regulatory sequence. Therefore, choice B is the best answer.

Question 16

A researcher wants to isolate regulatory sequences of DNA bound to their transcription factors. She then wants to sequence the DNA fragments that form the binding sites that the transcription factors recognize and bind. Which technique should the researcher use for her first step?

Accepted Answer

Chromatin immunoprecipitation (ChIP) is a technique used to identify DNA sequences bound by specific transcription factors. It involves cross-linking proteins to DNA in cells, followed by sonication to fragment the DNA. Antibodies specific to the transcription factor of interest are then used to immunoprecipitate the protein-DNA complexes, allowing the researcher to isolate the specific DNA sequences bound by the transcription factor. These sequences can then be further analyzed, including by sequencing.

Question 17

An electrophoresis assay used to identify the sequences of DNA to which specific regulatory proteins bind is called

Accepted Answer

An electrophoretic-mobility shift assay is specifically designed to identify DNA sequences to which regulatory proteins bind. This assay involves running a gel electrophoresis on a sample containing the regulatory protein and the DNA of interest, and then observing changes in the mobility of the DNA bands on the gel as a result of the binding interaction. DNA affinity chromatography is a similar technique, but is more commonly used to isolate specific DNA-binding proteins from a complex mixture. Western blotting and Southern blotting are not used for identifying DNA regulatory sequences.

Question 18

Kadonaga and Tjian were able to isolate the transcription factor Sp1 by exploiting its ability to bind

Accepted Answer

The question mentions that Kadonaga and Tjian were able to isolate the transcription factor Sp1, which means that they were specifically targeting Sp1. The answer choices A, B, and D are not related to Sp1, but the sequence GGGCGG is a known binding site for Sp1.

Question 19

Eukaryotic gene repressor proteins are thought to act by binding to

Accepted Answer

Eukaryotic gene repressor proteins can act by binding to DNA sites in competition with activating proteins, specific activating proteins preventing their binding to DNA, or basal transcription factors, inhibiting transcription. Therefore, all of the above options are possible mechanisms by which eukaryotic gene repressor proteins act.

Question 20

Many transcriptional activators are composed of two or more independent _______, each with its own separate function.

Accepted Answer

Transcriptional activators are composed of two or more independent domains, which each have their own separate function. These domains may include a DNA-binding domain, a transcription activation domain, and other regulatory domains.

Quiz 9: Transcriptional Regulation and Epigenetics

The lac operon in E. coli is regulated by allolactose, which _ a(n) _ of transcription.