Genetics Ch. 16
To translate the three enzymes of the lac operon, ________ mRNA molecule(s) is/are transcribed.
1
1. In _____________________________, the repressor that normally interacts with the operator to inhibit transcription is inactivated by an inducer, thus permitting transcription. 2. In _____________________________, a normally inactive repressor is activated by a corepressor, thus enabling it (the activated repressor) to bind to the operator to inhibit transcription.
1. In an inducible system , the repressor that normally interacts with the operator to inhibit transcription is inactivated by an inducer, thus permitting transcription. 2. In a repressible system, a normally inactive repressor is activated by a corepressor, thus enabling it (the activated repressor) to bind to the operator to inhibit transcription.
Which of the following mutations could lead to constitutive expression of the genes of the lac operon? A) A mutation in the operator sequence B) A super repressor mutation C) A mutation in the lac-Y gene D) A mutation in the lac-Z gene
A) A mutation in the operator sequence
What term refers to clusters of bacterial genes that are under the control of a single regulatory region? A) operon B) autotroph C) attenuation D) allosteric E) prototroph
A) operon
Monod discovered that if tryptophan is present in relatively high quantities in the growth medium, the enzymes necessary for its synthesis are repressed. How does this occur?
Attenuation occurs in which transcription of the operon is terminated.
Which of the following best describes the biological role of the lac operon? A) It prevents other sugars from being metabolized until all available lactose has been used. B) It ensures that a cell produces enzymes involved in lactose metabolism in a constitutive manner. C) It ensures that a cell dedicates resources to the production of enzymes involved in lactose metabolism only when lactose is available in the environment. D) It ensures that bacterial cells produce lactose only when no other food sources are available.
C) It ensures that a cell dedicates resources to the production of enzymes involved in lactose metabolism only when lactose is available in the environment.
Which term is used to describe enzymes that are produced only when specific substrates are present in the environment? A) negatively controlled B) repressible C) inducible D) constitutive
C) inducible
If lactose is accumulating outside of E. coli and is unable to enter, then which structural gene of the lac operon is most likely mutated? A) lacI B) lacX C) lacZ D) lacY E) lacA
D) lacY
What is the role of glucose in catabolite repression?
It decreases the levels of cAMP in the cell, repressing transcription from the lac operon.
What is an allosteric molecule?
One that can change in shape and chemical activity while interacting with other molecules.
What is the function of the hairpin loop that results from pairing of regions 3 and 4 in the mRNA transcript produced by the trp operon?
The hairpin causes transcription to stop.
What would be the effect of a mutation in the lacI gene that prevented the repressor from binding to lactose?
The lac Z, Y, and A genes would not be expressed.
What is the function of the lacY gene in the lac operon?
The lacY gene codes for permease, a membrane-bound protein that transports lactose into the bacterial cell.
The regulator gene produces a _____________ protein that interacts with the ______________ to ______________ transcription. This action is prevented in the presence of lactose.
The regulator gene produces a repressor protein that interacts with the operator to shut off transcription. This action is prevented in the presence of lactose.
There has been a mutation in the gene that produces the ___________ so that it ___________ bind to the ___________ to _____________ transcription.
There has been mutation in the gene that produces the Repressor so that it can not bind to the operator to turn off Transcription.
When lactose is present, this sugar binds to the _______________ and causes an allosteric (conformational) change, rendering it ______________ of interacting with ______________ DNA. Thus, RNA polymerase ____________ the structural genes, and the enzymes necessary for lactose metabolism ______________
When lactose is present, this sugar binds to the repressor and causes an allosteric (conformational) change rendering it incapable of interacting to the operator DNA. Thus RNA polymerase transcribes the structural genes, and the enzymes necessary for lactose metabolism are produced.
Genetic regulation in bacteria can involve alterations in RNA secondary structure. What phenomenon occurs in the trp operon that involves such alterations?
attenuation
What is the name of the molecule that binds to CAP to activate transcription of the lac operon?
cAMP
Enzymes that are continuously produced are said to have a(n) ________ expression.
constitutive
Name two types of constitutive mutations.
lacI− and lacOc
lacI− is unable to bind to the ___________ DNA, and the lacOc removes this DNA sequence entirely. In both cases, the operon cannot be ___________________ .
lacI− is unable to bind to the operator DNA, and the lacOc removes this DNA sequence entirely. In both cases, the operon cannot be turned off
In the accompanying diagram, what type of control, positive or negative, is operating? The oval structure is RNA polymerase.
positive
In negative control of gene expression...
removal of the regulatory molecule from DNA stimulates transcription.
lacI codes for a(n) ________, and when lacI is mutated, the lac operon is ________.
repressor; constitutively expressed
In the lac operon, the product of structural gene lacZ is capable of ________.
splitting the linkage of lactose to produce glucose and galactose
In positive control of gene expression...
the regulatory molecule stimulates transcription.
Usually, bacteria only make tryptophan when tryptophan is absent or available in low concentration. However, a particular bacterial mutation makes tryptophan all the time whether or not tryptophan is present. What could explain this phenotype?
the terminator hairpin is unable to form
State the function of β-galactosidase in the lac system.
β-Galactosidase cleaves the lactose sugar into its components, glucose and galactose
T/F: Lactose is an inducer of the lac operon.
True- Lactose indirectly induces or stimulates the transcription of genes involved in its metabolism.
T/F The placement of the operator sequence between the promotor and the structural genes is critical to the proper function of the lac operon.
True- When the repressor binds to the operator, RNA polymerase cannot transcribe the structural genes.
Which of the following statements about the mechanism of attenuation is true? A) When tryptophan levels are low, transcription is initiated and continues through the leader region into the structural genes, resulting in the production of a polycistronic mRNA and, ultimately, the enzymes required for the biosynthesis of tryptophan. B) The levels of tryptophan in the environment affect only repression of the trp operon and do not affect attenuation. C) When tryptophan levels are low, transcription of the leader sequence proceeds, but mRNA synthesis is terminated approximately 140 nucleotides along the transcript. D) When tryptophan is present at high concentrations, the repressor binds to the operator and transcription is initiated and not terminated. As a result, the enzymes required for the biosynthesis of tryptophan are translated. Submit
A) When tryptophan levels are low, transcription is initiated and continues through the leader region into the structural genes, resulting in the production of a polycistronic mRNA and, ultimately, the enzymes required for the biosynthesis of tryptophan.
Which of the following occurs as a result of an abundance of tryptophan in E. coli? A) The leader sequence is not transcribed. B) The 5 trp genes (TrpA - TrpE) are not transcribed. C) Stalling of the ribosome at trp codons in the leader sequence D) The 5 trp genes (TrpA - TrpE) are transcribed, but not translated.
B) The 5 trp genes (TrpA - TrpE) are not transcribed.
Which binds to the operator of the lac operon? A) permease B) repressor C) activator D) beta galactosidase E) lactose
B) repressor
Some lac operon mutations allow for beta galactosidase to be expressed constitutively even in the absence of lactose. Which of the following lac genotypes would allow for this constitutive expression? A) I+ P+ O+ Z+ Y+ A+ B) I+ P+ O+ Z- Y+ A+ C) I+ P- O+ Z+ Y+ A+ D) Is P+ O+ Z+ Y+ A+ E) I+ P+ Oc Z+ Y+ A+
E) I+ P+ Oc Z+ Y+ A+
T/F Attenuator systems such as the one described for regulation of tryptophan synthesis would be just as likely to occur in eukaryotes as in prokaryotes.
False- Regulation by attenuation requires that translation of a given transcript can begin before transcription is completed. This is not possible in eukaryotes, as the two processes are spatially separated by the nuclear membrane.
Inducible system: The repressor is normally _________, but the inducer makes it _____________. Repressible system: The repressor is __________, but the corepressor makes it ______________
Inducible system: The repressor is normally active, but the inducer makes it inactive. Repressible system: The repressor is inactive, but the corepressor makes it active.
Identify whether the following statement is true or false. The terminating "hairpin" loop occurs in the trp operon when sufficient tryptophan is present.
True; the hairpin loop terminates transcription because when tryptophan is present, it is energetically favorable to prevent more of its synthesis
When lactose is first introduced into the medium, the lac operon is not induced and thus permease isn't induced. How is it possible for lactose to enter the cell before the lac operon is induced?
While the lac operon is not induced yet, there is low level expression most of the time which allows a small amount of permease to be expressed. This allows lactose to enter the cell at low levels.
A protein, like the lac repressor, is said to be ________ because when it binds lactose, it structurally changes and its chemical activity also changes.
allosteric
Attenuation functions to ___________ the synthesis of tryptophan when it _______________________ supply. It does so by ___________________ transcription of the tryptophan operon. The same phenomenon is observed when tryptophan __________________ the repressor to shut off transcription of the tryptophan operon.
Attenuation functions to reduce the synthesis of tryptophan when it is in full supply. It does so by reducing transcription of the tryptophan operon. The same phenomenon is observed when tryptophan activates the repressor to shut off transcription of the tryptophan operon.
Which of the following features of the trp operon is likely least essential to the process of attenuation? A) Trp codons near the beginning of the leader sequence B) The ability of sequences within the leader mRNA to pair with one another C) Transcription and translation of the leader sequence occur simultaneously. D) The order of the structural genes, E, D, C, B, A
D) The order of the structural genes, E, D, C, B, A
Choose the explanation why certain mutations in the regulator gene (I) of the lac system result in maximal synthesis of β-galactosidase, permease, and transacetylase even in the absence of the inducer (lactose). A) Such mutations provide modified proteins that associate with the promoter activating transcription. B) Such mutations provide modified proteins that associate with the operator activating transcription. C) Such mutations provide modified proteins that are unable to associate with the operator to shut off transcription. D) Such mutations provide modified proteins that associate with the promoter not allowing transcription to occur.
C) Such mutations provide modified proteins that are unable to associate with the operator to shut off transcription.
