Genetics Chapter 16 Review
Match the description of transcriptional control to corresponding transcriptional regulator.
- Active repressor: negative control in an inducible operon - Inactive repressor: negative control in a repressible operon - Active activator: positive control in a repressible operon - Inactive activator: positive control in an inducible operon
An operon is a group of genes under the control of a single promoter. Match each type of operon with the descriptions (inducible, constitutive, repressible).
- Inducible: transcription is turned on when a specific molecule is present - Constitutive: the amount of gene product is constant - Repressible: activating the repressor gene product inhibits transcription; sufficient amounts of the gene product inhibit further transcription
Why is transcription a particularly important level of gene regulation in both bacteria and eukaryotes?
- Transcription is the first step in the process of information transfer from DNA to protein. - For cellular efficiency, gene expression is often regulated early in the process of protein production.
Classify the given examples of prokaryotic gene expression as positive or negative gene regulation. negative
- in the presence of iron, the dtxR repressor protein binds to DNA, and the gene that encodes for the diphtheria toxin is not expressed - in the absence of lactose, the lacR repressor protein binds the lac operon - in the presence of excess tryptophan, a repressor protein binds the operator of the trp operon and prevents the operon from being transcribed
Classify the given examples of prokaryotic gene expression as positive or negative gene regulation. positive
- in the presence of lactose and low glucose, the lac operon is expressed 20-fold higher than in the absence of lactose - in the presence of the sugar arabinose, an activator protein binds the promoter of the genes responsible for processing arabinose and induces their transcription
What are the five structural genes of the negative repressible operon?
- trpE, trpD, trpC, trpB, and trpA—five enzymes together convert chorismate to tryptophan
Attenuation results when which regions of the 5′UTR region pair?
3 and 4
What is a regulator gene?
DNA sequence-encoding products that affect the operon function but are not part of the operon
Use the information gathered in the attenuation animation to answer the question. Prokaryotes commonly use attenuation as a mechanism to control gene expression, but eukaryotes do not. Why do you think that attenuation is more common in prokaryotes than eukaryotes?
In eukaryotes, transcription and translation usually take place in separate physical locations
In the trp operon, what happens to the trp repressor in the absence of tryptophan?
It binds to the regulator gene and represses transcription.
Suppose an operon has the following characteristics: (1) The operon codes for structural proteins that convert compound Q to compound B. (2) The operon is controlled by a constitutively expressed regulatory gene called reg. (3) In wild‑type individuals, the operon is transcribed in the absence of compound B but not in the presence of compound B. (4) In reg− mutants, the operon is constitutively transcribed. Is this operon inducible or repressible? Why?
It is repressible because wild-type transcription is repressed in the presence of compound B
What is the effect of high levels of glucose on the lac operon?
Little transcription takes place.
What is positive and negative control?
Positive stimulates gene expression and negative inhibits gene expression
Review the Chapter 16 module quiz!!!!!!!!!
Review the Chapter 16 module quiz!!!!!!!!!
What is the difference between a structural gene and a regulator gene?
Structural genes encode proteins; regulator genes control the transcription of structural genes.
What are negative repressible operons?
The control at the operator site is negative. But such transcription is usually on and needs to be turned off, so the transcription is repressible.
What are negative inducible operons?
The control at the operator site is negative. Molecule binding is to the operator, inhibiting transcription. Such operons are usually off and need to be turned on, so the transcription is inducible.
What are repressible operons?
Transcription is normally on and needs to be turned off.
What are inducible operons?
Transcription is usually off and needs to be turned on.
What happens when tryptophan is high?
When tryptophan is high, region 1 binds to region 2, which leads to the binding of region 3 and region 4, terminating transcription prematurely.
What happens when tryptophan is low?
When tryptophan is low, region 2 binds to region 3, which prevents the binding of region 3 and region 4, and transcription continues.
What determines if attenuation takes place?
Whether the premature termination of transcription (attenuation) takes place in the trp operon depends on the cellular level of tryptophan.
Use the information gathered in the lac operon animation to answer the question. Is it possible to get any transcription of the lac operon in the absence of lactose? Why or why not?
Yes, in spite of the presence of the repressor, a low level of basal transcription and translation must take place in order to produce the permease to allow lactose into the cell and in order to produce galactosidase to make allolactose
Use the information gathered in the lac operon animation to answer the question. What is partial diploid?
a bacterium that has at least some common genes on both the chromosome and a plasmid
Use the information gathered in the lac operon animation to answer the question. What best defines an operon?
a group of bacterial genes that are transcribed together on the same mRNA
To inhibit the transcription of a bacterial gene with antisense RNA, what sequence might the antisense RNA contain?
a sequence complementary to the promoter of the operon
What did the binding of the cAMP-CAP complex to DNA produce
a sharp bend in DNA that activates transcription.
What is a corepressor?
a small molecule that binds to the repressor and makes it capable of binding to the operator to turn off transcription
What is cAMP?
adenosine-3′, 5′-cyclic monophosphate - The concentration of cAMP is inversely proportional to the level of available glucose.
What are regulatory elements?
affect the expression of DNA sequences to which they are physically linked
What is attenuation?
affects the continuation of transcription, not its initiation. This action terminates the transcription before it reaches the structural genes.
Use the information gathered in the lac operon animation to answer the question. What is the inducer of the lac operon?
allolactose
In the presence of allolactose, the lac repressor
cannot bind to the operator.
What is antisense RNA?
complementary to targeted partial sequence of mRNA; antisense RNA can regulate translation
What is constitutive expression?
continuously expressed under normal cellular conditions (housekeeping genes)
What are regulatory genes?
either RNA or proteins, interact with other DNA sequences and affect transcription or translation of those sequences
What are structural genes?
encode proteins that are used in metabolism or biosynthesis or that play a structural role in the cell
What is a constitutive gene?
gene that is not regulated and is expressed continually
In a negative repressible operon, the regulator protein is synthesized as an
inactive repressor.
What are bacterial enhancers?
increase the rate of transcription at genes that are distant from the enhancer
Use the information gathered in the lac operon animation to answer the question. Which of the choices is the most dominant with respect to expression of the lac operon structural genes?
lacO^c
Use the information gathered in the lac operon animation to answer the question. What are the structural genes of the lac operons?
lacZ, lacY, lacA
Describe the location, cis/trans, and effect of the given lac operon mutation: Regulator-gene mutations
location: LacI Cis/trans: Trans Effect: Affect transcription of structural genes
Describe the location, cis/trans, and effect of the given lac operon mutation: Operator mutations
location: LacO Cis/trans: Cis Effect: Affect transcription of structural genes
Describe the location, cis/trans, and effect of the given lac operon mutation: Promoter mutations
location: LacP Cis/trans: Cis Effect: Affect transcription of structural gene
Describe the location, cis/trans, and effect of the given lac operon mutation: Structural-gene mutations
location: lacZ and lacY Cis/trans: Affect only lacZ or lacY Effect: Alter amino acid sequence of protein encoded by gene in which mutation occurs
What are ribozymes?
mRNA molecules with catalytic activity; ribozymes, when bound by small regulatory molecules, can induce the cleavage and degradation of mRNA. (RNA catalyst)
What are riboswitches?
molecules influence the formation of secondary structures in mRNA; riboswitches are RNA sequences in mRNA that affect gene expression.
What type of regulation does the trp operon exhibit?
negative regulation
What is an operon?
promoter + additional sequences that control transcription (operator) + structural genes; a single transcriptional unit that includes a series of structural genes, a promoter, and an operator; a gene cluster controlled by a single promoter that transcribes to a single mRNA strand
What is an inducer?
small molecule that turns on the transcription
Use the information gathered in the attenuation animation to answer the question. What effect on the expression of the trp operon might you expect if there were a deletion of the shine delgarno sequence prior to the trpD structural gene?
the entire trp operon will always be transcribed unless the cell is able to obtain tryptophan from the external environment
Use the information gathered in the attenuation animation to answer the question. What effect on the expression of the trp operon might you expect if the stretch of T-A base pairs following region 4 were deleted?
the entire trp operon will be transcribed no matter what the level of tryptophan in the cell
Use the information gathered in the attenuation animation to answer the question. What effect on the expression of the trp operon might you predict if there was a large deletion in region 3 of the 5' UTR of the trp operon?
the entire trp operon will be transcribed no matter what the level of tryptophan in the cell
Use the information gathered in the attenuation animation to answer the question. If the trp operon from E. coli was cloned between a ubiquitous eukaryotic promoter and a strong eukaryotic transcriptional termination sequence, how would this operon function in a eukaryotic cell?
the entire trp operon will be transcribed, but none of the structural genes of the trp operon will be translated no matter what the level of tryptophan is in the cell
Use the information gathered in the attenuation animation to answer the question. What effect on the expression of the trp operon might you predict if the shine delgarno sequence prior to region 1 of the trp 5' UTR was deleted?
the entire trp operon would be transcribed no matter what the level of tryptophan in the cell
Use the information gathered in the lac operon animation to answer the question. In the lac operon, what is a trans-regulatory element?
the lacI repressor
Use the information gathered in the lac operon animation to answer the question. What is the role of the protein encoded by the lacZ gene?
the lacZ gene encodes an enzyme that converts lactose to allolactose and that also breaks down lactose to glucose and galactose
Use the information gathered in the lac operon animation to answer the question. What is the operator?
the operator is a DNA sequence in the lac operon to which the lacI repressor protein binds
Use the information gathered in the lac operon animation to answer the question. The lac operon is inducible, what does this mean?
the operon is transcriptionally active only in the presence of lactose
A mutation at the operator site of an operon prevents the repressor from binding. What effect will this mutation have on transcription in a repressible operon?
the operon will always be transcriptionally active
A mutation at the operator site of an operon prevents the repressor from binding. What effect will this mutation have on transcription in an inducible operon?
the operon will always be transcriptionally active
What is the function of the repressor in the E. coli lac operon?
the repressor is a protein that prevents the transcription of the lactose genes by binding to the beginning of the lac operon
Use the information gathered in the attenuation animation to answer the question. What effect on the expression of the trp operon might you expect if a deletion occurred in the 5' UTR leader sequence that removed the trp codons in the trp operon?
the structural genes for the trp operon will not be transcribed no matter what the intracellular levels of tryptophan are
Use the information gathered in the attenuation animation to answer the question. What effect on the expression of the trp operon might you expect if the trp codons in the 5' UTR leader sequence of the trp operon were replaced by codons for isoleucine?
the structural genes for the trp operon will only be transcribed when isoleucine levels are low
At which level of gene regulation does attenuation occur?
transcription
Define catabolite repression
using glucose when available and repressing the metabolite of other sugars - This is a positive control mechanism. The positive effect is activated by the catabolite activator protein(CAP). cAMP binds to CAP; together CAP-cAMP complex binds to a site slightly upstream from the lac gene promoter.
The lac Operon of E. coli structural genes
‒ lacZ: encoding β-galactosidases ‒ lacY: encoding permease ‒ lacA: encoding transacetylase **The repression of the lac operon never completely shuts down transcription.
The lac Operon of E. coli
• A negative inducible operon • Lactose metabolism • Regulation of the lac operon • Inducer: allolactose - lacI: repressor encoding gene - lacP: operon promoter - lacO: operon operator
How does gene regulation work in both bacteria and multicellular eukaryotes?
• In bacteria, gene regulation maintains internal flexibility, turning genes on and off in response to environmental changes. • In multicellular eukaryotic organisms, gene regulation also brings about cell differentiation.
What is positive control?
• In positive transcriptional control, the regulatory protein involved is an activator. - It binds to DNA (usually at a site other than the operator) and stimulates transcription.
