Question 1

In the first step of their experiments,Jacob and Monod treated E.coli cells with UV light or X-rays in order to _____.

Accepted Answer

Jacob and Monod used UV light or X-rays to increase the frequency of mutations in all genes, which helped them study gene regulation and the effects of mutations.

Question 2

Imagine that you've added the same indicator used by Monod to E.coli colonies growing on a plate with glucose and no lactose.One colony is yellow and the remainders are white.This result suggests that cells of the yellow colony _____.

Accepted Answer

The yellow colony indicates that the cells are able to produce beta-galactosidase, which is an enzyme encoded by the lacZ gene. Therefore, the cells must have constitutively active lacZ and lacY genes, which are necessary for lactose metabolism. The white colonies are unable to metabolize lactose, indicating that they either lack the lacZ and lacY genes or are unable to activate them under the given conditions.

Question 3

Full induction of the lac operon occurs when _____.

Accepted Answer

Full induction of the lac operon occurs when lactose levels are high and glucose levels are low. The lac operon is regulated by the presence or absence of lactose and glucose. When lactose is present and glucose is low, the lac operon is induced, allowing the production of enzymes needed to metabolize lactose. When lactose is absent and glucose is high, the lac operon is repressed, because there is no need to produce enzymes to metabolize lactose when there is plenty of glucose available as an energy source.

Question 4

According to the lac operon model proposed by Jacob and Monod,what is predicted to occur if the operator is removed from the operon?

Accepted Answer

In the lac operon model proposed by Jacob and Monod, the operator is the binding site for the repressor protein, which prevents RNA polymerase from transcribing the lac genes. If the operator is removed, there would be no repressor binding and the lac operon would be transcribed constitutively, regardless of the presence of lactose. This means that all the genes in the lac operon, including lacZ and lacY, would be transcribed.

Question 5

Imagine that you've set up a genetic screen to identify E.coli mutants that cannot metabolize the amino acid tryptophan.Beginning with a master plate containing many colonies,you prepare replica plates on medium with glucose or tryptophan as the only energy source.You would look for colonies that _____.

Accepted Answer

Colonies that can grow only on the plates with glucose indicate mutants that cannot metabolize tryptophan, as they fail to grow when tryptophan is the sole energy source.

Question 6

Imagine that you discover a bacterial operon involved in the synthesis of vitamin B₁₂.This operon is regulated by a repressor protein that binds to an operator sequence.Vitamin B₁₂ is the allosteric effector of the repressor-the molecule that binds to the repressor to affect its activity.Predict how vitamin B₁₂ will influence repressor activity.

Accepted Answer

Vitamin B₁₂ acts as an allosteric effector that enables the repressor to bind to the operator, thereby inhibiting the operon and preventing the synthesis of vitamin B₁₂ when it is abundant.

Question 7

The trp repressor blocks transcription of the trp operon when the repressor _____.

Accepted Answer

The trp repressor blocks transcription of the trp operon when it binds to tryptophan, which allows it to attach to the operator and inhibit transcription.

Question 8

Bacterial and eukaryotic cells primarily control gene expression at the level of transcription.If instead cells exerted control of gene expression primarily at the post-translational level,what would be different?

Accepted Answer

Controlling gene expression at the post-translational level means that the cell would first transcribe and translate proteins before regulating their activity or stability. This process is more energy-intensive because it involves synthesizing proteins that may not be needed immediately or in large quantities, leading to wasted resources.

Question 9

Jacob and Monod were intellectually primed to draw the conclusions they did concerning regulation of the lac operon.In part,this was due to their fascination with mechanisms of enzyme regulation.They knew that the activity of some enzymes is regulated when their reaction product binds to the enzyme,changing its shape and therefore its activity.This knowledge allowed them to easily make the intellectual leap to propose _____.

Accepted Answer

The answer of Jacob and Monod were intellectually primed to...

Question 10

Suppose several genes are grouped together in a genome and the group results in a single enzyme.If one of these genes is mutated and the enzyme is no longer produced,this gene must be part of _____.

Accepted Answer

A group of genes that are regulated together and produce a single enzyme is known as an operon. In operons, the genes are transcribed together in a single mRNA molecule, and mutations in any of the genes can affect the production of the final enzyme product. Therefore, if one of these genes is mutated and the enzyme is no longer produced, it must be part of an operon.

Question 11

Which of the following levels of gene expression allows the most rapid response to environmental change?

Accepted Answer

Post-translational control allows for the most rapid response to environmental change because it involves modifying existing proteins rather than waiting for new proteins to be synthesized through transcription and translation.

Question 12

An E.coli cell without a functional lacI gene is expected to _____.

Accepted Answer

A functional lacI gene codes for the lac repressor protein, which inhibits the expression of the lac operon. Without a functional lacI gene, the lac operon will constantly be expressed, leading to the production of β-galactosidase.

Question 13

In negative control of transcription,a gene is activated when _____.

Accepted Answer

In negative control of transcription, a regulatory protein (such as a repressor) binds to DNA to prevent or block transcription of a gene. When the regulatory protein is removed from DNA, transcription can occur. This is the case in the absence of a repressor in the lac operon of E. coli, which allows transcription of the lac genes and the metabolism of lactose. A kinase adding a phosphate to DNA or lactose transport into the cell are not typically involved in negative control of transcription. A regulatory protein binding to DNA initiates negative control, rather than activation.

Question 14

The product of the lacI gene is _____.

Accepted Answer

The lacI gene codes for the lac repressor, which is a protein that binds to the operator DNA sequence and prevents RNA polymerase from transcribing the lac genes when lactose is absent.

Question 15

Catabolite activator protein (CAP)activity is controlled by cAMP at the _____ level.

Accepted Answer

The answer of Catabolite activator protein (CAP)activity is controlled by...

Question 16

A compound known as X-Gal is widely used in molecular genetic research.When wild-type (normal)E.coli is grown on medium containing X-Gal,the bacterial colonies turn blue.In contrast,when lacZ mutants are grown on medium containing X-Gal,the bacterial colonies remain their normal white color.What is X-Gal likely to be?

Accepted Answer

X-Gal is a substrate for the enzyme β-galactosidase, which is encoded by the lacZ gene. When β-galactosidase cleaves X-Gal, it produces a blue product. LacZ mutants lack functional β-galactosidase, so they cannot cleave X-Gal, resulting in white colonies. This indicates that X-Gal is chemically similar to lactose, which is also a substrate for β-galactosidase.

Question 17

Recall from Chapter 11 that cyclin-dependent kinases are activated when they bind to a cyclin protein.This form of regulation is _____.

Accepted Answer

Cyclin-dependent kinases are activated by binding to cyclins, which is a post-translational modification. This means that the regulation occurs after the protein has been translated, making D the correct answer. Constitutive control refers to a protein that is constantly being produced, transcriptional control involves regulation of gene expression at the transcription level, and translational control involves regulation of gene expression at the level of mRNA translation.

Question 18

The product of the lacI gene is most like a car's _____.

Accepted Answer

The lacI gene encodes for the lac repressor protein, which binds to the operator site of the lac operon to prevent transcription of the genes required for lactose metabolism. This function is similar to a car's parking brake, which prevents the car from moving.

Question 19

Although the expression of most genes is tightly regulated,some genes are expressed at roughly constant rates (i.e. ,constitutively).Which of the following genes would you predict to be constitutively expressed?

Accepted Answer

Genes that code for ribosomal RNAs are essential for protein synthesis, which is necessary for cell growth and division. Therefore, it is advantageous for these genes to be constitutively expressed to ensure that the cell has a constant supply of ribosomes. Genes involved in biosynthesis or degradation of specific molecules may be regulated depending on the cell's needs, and genes involved in transport may also be regulated depending on the availability of the transported molecule. Therefore, choices A, B, and E are less likely to be constitutively expressed. Choice C may or may not be constitutively expressed depending on the organism's preferred energy source.

Question 20

E)coli and many other bacteria of the human gut need to have fine-tuned regulation of gene expression in order to _____.

Accepted Answer

The human gut is a competitive environment, with many different species of bacteria competing for space and nutrients. In order to effectively compete, these bacteria need to fine-tune their regulation of gene expression to metabolize nutrients and grow efficiently. This allows them to outcompete other bacteria and survive in the gut environment. Replicating DNA and evolving may be important for bacteria survival and adaptation, but neither requires fine-tuned regulation of gene expression.

Quiz 17: Control of Gene Expression in Bacteria