chapter 18
Explain why regulation of gene expression is important and summarize the different levels for regulating gene expression in bacteria. Identify the advantages of regulation at each level, and explain why transcription initiation is an especially common point of regulation.
-All cells control or regulate the synthesis of proteins from information encoded in their DNA -The process of turning on a gene to produce RNA and protein is called gene expression -each cell controls when and how its genes are expressed -For this to occur, there must be a mechanism to control when a gene is expressed to make RNA and protein, how much of the protein is made, and when it is time to stop making that protein because it is no longer needed -The regulation of gene expression conserves energy and space -It would require a significant amount of energy for an organism to express every gene at all times, so it is more energy efficient to turn on the genes only when they are required. -only expressing a subset of genes in each cell saves space because DNA must be unwound from its tightly coiled structure to transcribe and translate the DNA -Cells would have to be enormous if every protein were expressed in every cell all the time. Gene regulation is how a cell controls which genes, out of the many genes in its genome, are "turned on" (expressed). Thanks to gene regulation, each cell type in your body has a different set of active genes—despite the fact that almost all the cells of your body contain the exact same DNA. These different patterns of gene expression cause your various cell types to have different sets of proteins, making each cell type uniquely specialized to do its job. For example, one of the jobs of the liver is to remove toxic substances like alcohol from the bloodstream. To do this, liver cells express genes encoding subunits (pieces) of an enzyme called alcohol dehydrogenase. This enzyme breaks alcohol down into a non-toxic molecule. The neurons in a person's brain don't remove toxins from the body, so they keep these genes unexpressed, or "turned off." Transcription initiation involves the interaction of DNA-dependent RNA polymerase with promoters. In bacteria, this is a highly regulated process. ... Regulation of transcription initiation occurs in the context of folding and compaction of bacterial chromosomes.
Compare and contrast regulation of gene expression in eukaryotes with that in prokaryotes. Identify additional reasons for regulation in eukaryotes (especially multicellular eukaryotes) and additional points of regulation.
-Prokaryotic gene expression is primarily controlled at the level of transcription. -Eukaryotic gene expression is controlled at the levels of epigenetics, transcription, post-transcription, translation, and post-translation. -Prokaryotic gene expression (both transcription and translation) occurs within the cytoplasm of a cell due to the lack of a defined nucleus; thus, the DNA is freely located within the cytoplasm. -Eukaryotic gene expression occurs in both the nucleus (transcription) and cytoplasm (translation). -To synthesize a protein, the processes of transcription (DNA to RNA) and translation (RNA to protein) occur almost simultaneously -the regulation of transcription is the primary method to control what type of protein and how much of each protein is expressed in a prokaryotic cell -in prokaryotic cells, the control of gene expression is mostly at the transcriptional level In eukaryotic cells, the DNA is contained inside the cell's nucleus where it is transcribed into RNA. The newly-synthesized RNA is then transported out of the nucleus into the cytoplasm where ribosomes translate the RNA into protein. The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation occurs only outside the nucleus within the cytoplasm. The regulation of gene expression can occur at all stages of the process. Regulation may occur when the DNA is uncoiled and loosened from nucleosomes to bind transcription factors (epigenetics), when the RNA is transcribed (transcriptional level), when the RNA is processed and exported to the cytoplasm after it is transcribed (post-transcriptional level), when the RNA is translated into protein (translational level), or after the protein has been made (post-translational level)
Describe the regulation of lactose metabolism genes and the biological context for this regulation. Contrast the roles of: the proteins galactoside permease, beta-galactosidase, lac I protein and CAP; the DNA sequences lac Z, lac Y, promoter, operator; the small molecules lactose, glucose and cAMP. Explain the logic of the regulation, including the advantage of an operon. Describe the two different ways in which glucose affects the system. Identify which regulatory events involve allostery. Distinguish an inducer from an activator.
-The lac operon of E. coli contains genes involved in lactose metabolism. It's expressed only when lactose is present and glucose is absent. -Two regulators turn the operon "on" and "off" in response to lactose and glucose levels: the lac repressor and catabolite activator protein (CAP). -The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon, but stops acting as a repressor when lactose is present. The lac repressor senses lactose indirectly, through its isomer allolactose. -Catabolite activator protein (CAP) acts as a glucose sensor. It activates transcription of the operon, but only when glucose levels are low. CAP senses glucose indirectly, through the "hunger signal" molecule cAMP. -lacZ gene encodes an enzyme called β-galactosidase, which is responsible for splitting lactose (a disaccharide) into readily usable glucose and galactose (monosaccharides) -lacY gene encodes a membrane protein called lactose permease, which is a transmembrane "pump" that allows the cell to import lactose. The promoter is the binding site for RNA polymerase, the enzyme that performs transcription. The operator is a negative regulatory site bound by the lac repressor protein. The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to the promoter and start transcription. The CAP binding site is a positive regulatory site that is bound by catabolite activator protein (CAP). When CAP is bound to this site, it promotes transcription by helping RNA polymerase bind to the promoter. The lac repressor is a protein that represses (inhibits) transcription of the lac operon. It does this by binding to the operator, which partially overlaps with the promoter. When bound, the lac repressor gets in RNA polymerase's way and keeps it from transcribing the operon. The gene that encodes the lac repressor is named lacI, and is under control of its own promoter. The lacI gene happens to be found near the lac operon, but it is not a part of the operon and is expressed separately. lacI is continually transcribed, so its protein product - the lac repressor - is always present. When lactose is available, some molecules will be converted to allolactose inside the cell. Allolactose binds to the lac repressor and makes it change shape so it can no longer bind DNA. Allolactose is an example of an inducer, a small molecule that triggers expression of a gene or operon. The lac operon is considered an inducible operon because it is usually turned off (repressed), but can be turned on in the presence of the inducer allolactose. The lac operon will be expressed at high levels if two conditions are met: Glucose must be unavailable: When glucose is unavailable, cAMP binds to CAP, making CAP able to bind DNA. Bound CAP helps RNA polymerase attach to the lac operon promoter. Lactose must be available: If lactose is available, the lac repressor will be released from the operator (by binding of allolactose). This allows RNA polymerase to move forward on the DNA and transcribe the operon.
Which of the following best describes regulons?
A set of separate genes or operons that contain the same regulatory sequences and are controlled by a single type of regulatory protein Global gene regulation is the coordinated regulation of many genes. There are other means of global gene regulation, such as grouping genes into a regulon.
Gene expression can be controlled at which of these levels?
All of these are correct (Translation, Transcription, and Post-Translation)
Define mutation, and explain why mutations are not necessarily good or bad. Describe the relationship between DNA damage or mistakes, and point mutations.
Based on their effects on fitness, mutations can be divided into three broad categories: the 'good' or advantageous that increase fitness, the 'bad' or deleterious that decrease it and the 'indifferent' or neutral that are not affected by selection because their effects are too small. n addition to genetic insults caused by the environment, the very process of DNA replication during cell division is prone to error. The rate at which DNA polymerase adds incorrect nucleotides during DNA replication is a major factor in determining the spontaneous mutation rate in an organism. A Mutation occurs when a DNA gene is damaged or changed in such a way as to alter the genetic message carried by that gene. A Mutagen is an agent of substance that can bring about a permanent alteration to the physical composition of a DNA gene such that the genetic message is changed.
In bacteria, several enzymes are needed for glycolysis to breakdown glucose and produce energy for the cell. These enzymes are mostly under what type of gene regulation?
Constitutive control Certain enzymes in glycolysis may occasionally be subject to positive or negative control to adjust the flow of energy or metabolites to other parts of metabolism. But mostly they are transcribed constitutively.
A hypothetical bacterium isolated from a Martian sea uses silicose, an abundant silicon-based sugar, as its main energy source. Which of the following would be the most efficient type of control for the production of silicase, the enzyme used to metabolize silicose?
Constitutive transcription of the silicase gene Constitutive transcription of the silicase gene would be the most efficient type of control for the production of silicase, the enzyme used to metabolize silicose. Constitutive expression occurs in genes whose products are required at all times. Because silicose is the main energy source for this bacterium, having the enzymes present all the time would be most efficient. Negative control occurs when a regulatory protein prevents transcription. Thus, negative control occurs when something must be taken away for transcription to occur and is not the most efficient means for control of silicase production. Post-translational control of silicase activity would involve the activation of silicase by chemical modification, such as the addition of a phosphate group. This requires one additional step beyond that of constitutive production and, thus, would not be the most efficient type of control for silicase production. An operon is a set of coordinately regulated bacterial genes that are transcribed together into one mRNA. The coordinate regulation of an operon would require the production of activating or repressor proteins and, thus, would not be as efficient as the constitutive expression of silicase.
What is the primary benefit of organizing a group of genes into an operon?
Coordinated and simplified regulation of a group of genes with a common function By organizing genes into an operon, bacteria can have coordinated and simplified regulation of a group of genes with a common function. For example, if several enzymes are needed for a biochemical pathway, using one promoter and one RNA polymerase will ensure that all of the enzymes are transcribed and translated at the same time. The co-location of genes near each other does not infer any particular advantages without the additional regulatory parts of the operon. RNA polymerase molecules can be used over and over again and do not represent a significant cost to the cell. Genes can be turned "on" and "off" without being organized into operons.
The SOS response regulon allows bacterial cells to survive and repair extensive damage to DNA. It is an example of global gene regulation, the coordinated regulation of many genes. A gene, called lexA, codes for the LexA protein that represses transcription of SOS regulon genes while the cell is healthy. If significant DNA damage occurs in a bacterial cell, what triggers the SOS response regulon to become active?
DNA damage sets off a signal for LexA to be cleaved and, therefore, inactivated. If significant DNA damage occurs in a bacterial cell, DNA damage sets off a signal for LexA to be cleaved and, therefore, inactivated. Inactivation of LexA releases the brake on the transcription of all SOS genes, more than 40 genes that code for enzymes needed for DNA repair, recombination, and specialized DNA polymerases that can use damaged DNA as a template. These proteins work to allow the cell first to survive the DNA damage and ultimately to repair it. Without the SOS response, bacteria whose DNA is extensively damaged face almost certain death. DNA damage triggers the cleavage of LexA and not its synthesis. DNA damage triggers an SOS signal that affects LexA. The DNA itself does not affect LexA. The SOS response regulon is not triggered by damage to the regulon gene promoters.
Describe the causes and general consequences of chromosome mutations. Distinguish among aneuploidy, polyploidy and changes in chromosome structure, as well as among the four different types of changes in chromosome structure.
In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division can result in cells with too few or too many copies of a chromosome. Errors can also occur when the chromosomes are being duplicated. Other factors that can increase the risk of chromosome abnormalities are: Maternal Age: Women are born with all the eggs they will ever have. Some researchers believe that errors can crop up in the eggs' genetic material as they age. Older women are at higher risk of giving birth to babies with chromosome abnormalities than younger women. Because men produce new sperm throughout their lives, paternal age does not increase risk of chromosome abnormalities. Environment: Although there is no conclusive evidence that specific environmental factors cause chromosome abnormalities, it is still possible that the environment may play a role in the occurrence of genetic errors. Chromosome structure mutations are alterations that affect whole chromosomes and whole genes rather than just individual nucleotides. These mutations result from errors in cell division that cause a section of a chromosome to break off, be duplicated or move onto another chromosome. consequences of aneuploidy: -aneuploidy causes several defects in cells from individuals with Down syndrome -increased gene and protein expression, lower viability, and increased dependency on serine to proliferate -characterised by having abnormal numbers of chromosomes in a haploid set polyploidy: -the heritable condition of possessing more than two complete sets of chromosomes. -Polyploids are common among plants, as well as among certain groups of fish and amphibians. -Ex: some salamanders, frogs, and leeches are polyploids. 4 types of structural changes in chromosome structures: 1. deletion: removes a chromosomal segment 2. duplication: repeats segment 3. inversion: reverses a segment within a chromosome 4. translocation: There are two main types of translocation. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere.
Compare and contrast actions of regulatory transcription factors in eukaryotes vs. prokaryotes. Explain why there are more of these proteins associated with a single gene in eukaryotes, how they can bind at a distance from the gene start site and why that is useful. Explain the idea of differential gene expression in different cell types and the connections between transcriptional regulation and cell signaling.
In eukaryotic cells, helix-turn-helix proteins include the homeodomain proteins, which play critical roles in the regulation of gene expression during embryonic development. The genes encoding these proteins were first discovered as developmental mutants in Drosophila. Some of the earliest recognized Drosophila mutants (termed homeotic mutants in 1894) resulted in the development of flies in which one body part was transformed into another. For example, in the homeotic mutant called Antennapedia, legs rather than antennae grow out of the head of the fly. Genetic analysis of these mutants, pioneered by Ed Lewis in the 1940s, has shown that Drosophila contains nine homeotic genes, each of which specifies the identity of a different body segment. Differential gene expression, commonly abbreviated as DG or DGE analysis refers to the analysis and interpretation of differences in abundance of gene transcripts within a transcriptome Through the process of differential gene expression, the activation of different genes within a cell that define its purpose, each cell expresses only those genes which it needs. However, the extra genes are not destroyed, but continue to be stored within the nucleus of the cell. Transcription is the first step in gene expression. It involves copying a gene's DNA sequence to make an RNA molecule. Transcription is performed by enzymes called RNA polymerases, which link nucleotides to form an RNA strand (using a DNA strand as a template).
Which molecule acts as an inducer of lac operon transcription?
Lactose Lactose acts as an inducer by causing the repressor to release from DNA and ending negative control. RNA polymerase is required for the transcription of the lac operon; however, in the absence of lactose, the RNA polymerase is unable to access the lac DNA for transcription. The lacI gene encodes the repressor protein lacI that blocks the RNA polymerase from transcribing the lac operon. β-galactosidase is a product of transcription and translation from the lac operon. The function of β-galactosidase is to cleave lactose; it does not play a role in regulation of the lac operon transcription.
In a typical bacterial cell, why is the lactose operon usually turned "off"?
Lactose is not a common carbohydrate in the environment. It is a disaccharide that makes up about 2%-8% of milk. Most bacteria will never come in contact with lactose during their lifetimes; therefore, it is inefficient to build and maintain the enzymes needed to digest it. However, because lactose is an excellent fuel molecule, it is advantageous to have the ability to have the enzymes to digest lactose if needed. This is the value of inducible operons. They can be kept inactive ("off") until they are needed. When the appropriate starting material becomes available, the operon can be induced to turn "on." Glucose inhibits lactose digestion by preventing the lactose from being turned "on." Lactose-digesting enzymes are transcribed and translated as easily as any other genes. There is no reason for most operons to be turned "off."
Which of the following conditions would result in the highest levels of transcription of the lac operon?
Low levels of glucose and high levels of lactose in the cell The lac operon is not transcribed when glucose is available because glucose prevents lactose transport into the cell. Glucose is preferred.
Which of the following experiments would help determine whether the β-galactosidase gene is regulated by lactose or glucose?
Measure the amount of β-galactosidase produced by Escherichia coli grown on a glucose plate, a lactose plate, and a glucose + lactose plate. To determine whether the β-galactosidase gene is regulated by lactose or glucose, it is necessary to compare the amount of β-galactosidase produced when E. coli are grown on glucose alone, on glucose and lactose, and on lactose alone.
Contrast each of the following types of point mutations in protein-coding genes: missense, silent, frameshift, nonsense. For each, explain the type of change and how it leads to the type of consequence. Explain how point mutations outside of protein-coding genes can also affect gene expression, and why they cannot be categorized in the same way. Infer amino acid sequences from mutated DNA sequences and vice versa (to the extent possible).
Missense mutation: changes an amino acid to another amino acid. This may or may not affect protein function, depending on whether the change is "conservative" or "nonconservative," and what the amino acid actually does. "Silent" mutation: does not change an amino acid, but in some cases can still have a phenotypic effect, e.g., by speeding up or slowing down protein synthesis, or by affecting splicing. Frameshift mutation: Deletion or insertion of a number of bases that is not a multiple of 3. Usually introduces premature STOP codons in addition to lots of amino acid changes. Nonsense mutation: changes an amino acid to a STOP codon, resulting in premature termination of translation.
Which of the following statements best summarizes how the lac operon is regulated?
Positive and negative control elements regulate the lac operon. The lac operon is regulated by the negative control of the repressor protein LacI and by the positive control in the presence of lactose, which releases the repressor protein LacI from the operator to allow transcription to occur. Lactose is an inducer of the lac operon. Under negative control, regulation proteins shut down transcription. Under positive control, regulatory proteins trigger transcription. The lac operon is under negative control by the repressor protein LacI; however, there is also positive control of the lac operon by the presence of lactose, which binds to the repressor protein LacI, releasing it from the lac operator sequence to facilitate transcription.
Which of the following describes post-translational control?
Post-translational control provides the most rapid response because only one step is needed to activate or inactivate an existing protein. Transcriptional control is particularly important because of its efficiency; it saves the most energy for the cell because it controls gene expression before the cell expends many resources. Translational control allows more rapid changes than transcriptional control in the amounts of different proteins because the mRNA has already been made and is available for translation. Positive control occurs when a regulatory protein called an activator binds to DNA and triggers transcription.
Compare and contrast types of transcriptional regulatory sequences in eukaryotes vs. prokaryotes. Distinguish among promoters, promoter-proximal elements, enhancers and silencers.
Prokaryotic cells can only regulate gene expression by controlling the amount of transcription. As eukaryotic cells evolved, the complexity of the control of gene expression increased. For example, with the evolution of eukaryotic cells came compartmentalization of important cellular components and cellular processes. A nuclear region that contains the DNA was formed. Transcription and translation were physically separated into two different cellular compartments. It therefore became possible to control gene expression by regulating transcription in the nucleus, and also by controlling the RNA levels and protein translation present outside the nucleus. Some cellular processes arose from the need of the organism to defend itself. Cellular processes such as gene silencing developed to protect the cell from viral or parasitic infections. If the cell could quickly shut off gene expression for a short period of time, it would be able to survive an infection when other organisms could not. Therefore, the organism evolved a new process that helped it survive, and it was able to pass this new development to offspring. An enhancer is a sequence of DNA that functions to enhance transcription. A promoter is a sequence of DNA that initiates the process of transcription. A promoter has to be close to the gene that is being transcribed while an enhancer does not need to be close to the gene of interest. Enhancers are located considerable distances from the promoter; proximal control elements are close to the promoter.
For an operon under negative control to be transcribed, which of the following must occur?
RNA polymerase must bind to the promoter, and the repressor must be inactive. For an operon under negative control to be transcribed, RNA polymerase must bind to the promoter, and the repressor must be inactive. Transcription of an operon under negative control involves the ability of the RNA polymerase to bind to the promoter and have no impediment in transcribing the DNA into mRNA. The repressor protein, which normally impedes the ability of RNA polymerase to transcribe the DNA, would also need to be inactivated for transcription to occur. If an RNA polymerase is present with an active repressor protein, no transcription would occur. If the RNA polymerase does not occupy the promoter, no transcription will occur. The repressor must be inactive and the RNA polymerase must be present for transcription to occur.
What effects will growth in glucose and lactose have on Escherichia coli sugar metabolism and gene expression?
The bacteria will use glucose as their primary sugar source until it is used up and then switch to lactose. E. coli does not produce β-galactosidase when glucose is present, even if lactose is present. Glucose is the preferred food source. Glucose prevents the expression of the gene for β-galactosidase. The presence of lactose without glucose stimulates expression of that gene. Bacteria will only produce β-galactosidase when lactose is present in the absence of glucose. Bacteria will use glucose as their primary sugar source until it is depleted, then switch to lactose if present, and only then will bacteria produce β-galactosidase.
Which of the following is true when mutant cells containing a mutation that eliminates lacI transcription are grown in high lactose and high glucose?
The lac operon will be transcribed at low levels despite the presence of glucose. Mutant cells containing a mutation that eliminates lacI transcription will have no repression of the lac operon. This would result in the constitutive production of low levels of β-galactosidase and permease proteins, allowing constitutive use of lactose and glucose as energy sources. Because glucose is present, and glucose inhibits the ability of lactose permease to import lactose into cells, there will only be low levels of lactose available as an energy source. The lac repressor will not be made in mutant cells in which lacI transcription is eliminated. Thus, there will be no lac repressor bound to the lac operator. Levels of transcription of the lac operon will remain constant because of the presence of glucose. Glucose inhibits the lactose permease from transporting lactose into cells. Expression from the lac operon will not be eliminated in mutant cells lacking lacI transcription. Constitutive expression of the lac operon occurs in the absence of the repressor.
If a bacterial cell came in contact with milk with fructose dissolved in it, what would happen to the lac operon?
The lac operon would be induced because fructose does not stop activation of the lac operon. A bacterial cell that comes in contact with milk containing fructose will be able to digest the lactose and fructose. The lac operon would be induced because fructose does not stop activation of the lac operon like glucose does because fructose has no effect on cellular levels of cAMP. Only lactose can bind to the lac operon repressor. Fructose would not affect the repressor. Only cAMP can bind to a catabolite activator protein (CAP) and induce the lac operon.
Why is the lacI mutant a constitutive mutant?
The mutant cells do not need an inducer to express β-galactosidase. In the absence of the repressor, the gene for β-galactosidase is expressed all the time (constitutive). Cells that are abnormal because they produce a product at all times are called constitutive mutants. The gene that was mutated to produce constitutive β-galactosidase and galactoside permease expression was named lacI. The letter "I" signifies that these mutants did not need an inducer—lactose—to express β-galactosidase or galactoside permease. The lacI mutant cells do not require lactose, glucose, or lactose and glucose to induce transcription of lacZ, lacY, and lacA.
Describe how studying mutations revealed information about regulation of transcription in the lac operon. Be able to predict the results of mutations in each component, or to work backwards from phenotypes to infer possible mutations. Distinguish between constitutive expression, inducible expression, repressible expression and no expression.
The regulator gene codes for the repressor protein, so a mutation could cause the repressor protein to be nonfunctional, or to not be made at all. This would mean that the expression of the structural genes could not be turned off, and the enzymes to break down lactose would be made all the time, even when not needed. A mutation in the regulator gene would affect the repressor
What is the mutant phenotype of bacteria that lack a functional lacY gene?
They do not accumulate lactose inside the cell. The inability to cleave lactose into glucose and galactose would be caused by a defect in the β-galactosidase gene. Cells that are able to cleave lactose because there is an abundance in the cell could not have a mutant lacY because lacY encodes the lactose permease that facilitates transport of lactose into cells. Cells that rapidly cleave lactose in the presence of glucose would suggest that the repressor protein was no longer able to bind and inhibit transcription of β-galactosidase. This could result from a defect in the ability of the repressor to be made or to bind lactose.
Contrast the use of a repressor vs. an activator to regulate transcription, including what occurs when either protein binds DNA. Be able to sketch models of transcriptional regulation and infer what type of regulation is being used.
Translation and Transcription. Transcription can be regulated by controlling the amount of mRNA that is produced. Translation can be regulated by controlling a number of mechanisms. The regulation of protein is faster because it only takes seconds to start or stop the activity. Where as synthesis takes a few minutes. repressors along with small molecules called co-repressors bind to specific repressor proteins and change their confirmation. upon binding to the corepressor the repressor gets active confirmation and binds to specific DNA upstream of the operator region. This is near the promoter region where RNA polymearse binds to start transcription. But since the operator is downstream of the promoter, the RNA polymerase cant bind and transcription is blocked. They both bind to specific DNA sequences and bring about regulation of gene activity. The Activator protein activates the expression of gene by stimulating the RNA polymerase to bind to the promoter region brings about positive control. The repressor protein inhibits gene activity by binding to the operator region upstream of translation start site. the operator is now downstream and RNA polymerase cant bind to transcribe.
What is the co-repressor in the trp operon?
Tryptophan The trp operon controls the synthesis of the amino acid tryptophan. It contains an operator that overlaps the promoter and control of transcription is mediated by a repressor protein. When tryptophan is present, it acts as a co-repressor, binding to the repressor protein and causing it bind to the operator. When tryptophan is absent, the repressor protein cannot bind to the operator. cAMP, glucose, and lactose are all involved with regulation of the lac operon.
All of the following organisms are capable of gene regulation at the transcriptional and translational level EXCEPT which one? a) hippo b) human c) bacterial cell d) fern
c) bacterial cell a bacterial cell is a prokaryotic organism and therefore its gene expression is regulated primarily at the transcriptional level only
Transcription of the lac operon is subject to positive control by a catabolite activator protein (CAP). CAP must be bound to which of the following to bind to DNA?
cAMP Transcription of the lac operon is subject to positive control by a catabolite activator protein (CAP). CAP must be bound to the molecule cyclic AMP (cAMP) to bind to DNA. When glucose levels outside the cell are high, cAMP synthesis is inhibited, and CAP does not bind DNA to activate transcription. An inducer is a small molecule that triggers the transcription of a specific gene. A repressor exerts negative control. An activator is a regulatory protein.
Control of gene expression in eukaryotic cells occurs at which level(s)?
epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels
In the lac operon, the repressor protein is encoded by which of the following?
lacI lacY encodes for a lactose permease. lacZ encodes for β-galactosidase. lacR is an invented gene.
When lactose levels are low, which statement is true about the levels of transcription from the lacI gene?
lacI transcription is unaffected. The lacI repressor protein is tightly bound, and transcription of lacZ, lacY, and lacA is unaffected. When lactose levels are low, the transcription of lacI is still occurring. The lacI repressor remains bound and inhibits the transcription of lacZ, lacY, and lacA. The lac operon is regulated by the lacI repressor protein. The answer does not provide enough detail about the levels of transcription of lacI gene. When lactose levels are low, the transcription levels of lacI are unaffected.
The lactose operon is likely to be transcribed when __________.
lactose levels are high within the cell The presence of lactose in the cell increases as the lac operon is turned on because of the increased production of the lactose permease. The presence of high levels of lactose would result in the release of the repressor protein (the product of the lacI gene) from the operator DNA sequence and transcription of the lacZ, lacY, and lacA genes. When lactose levels are low, the lac operon is not likely to be transcribed because of the binding of the repressor protein to the operator sequence. The presence of glucose and no lactose in the cell would result in tight binding of the repressor to the operator sequence and inhibition of transcription of the lac operon. Glucose inhibits the lactose transport activity of galactoside permease. When both glucose and lactose are present in the environment, the transport of lactose into the cell is inhibited. Because lactose does not accumulate in the cytoplasm, the repressor remains bound to the operator.
In the lac operon, the inducer ________ binds to the repressor. When it does, the repressor changes shape. The shape change causes the repressor to come off the DNA. This type control over protein function is ______________.
lactose; allosteric regulation In the lac operon, the inducer lactose binds to the repressor. When it does, the repressor changes shape. The shape change causes the repressor to come off the DNA. This type of control over protein function is allosteric regulation. In allosteric regulation, a small molecule binds to a protein and causes it to change its shape and activity. When the inducer binds to the repressor (which is encoded by lacI), the repressor can no longer bind to DNA, and transcription can proceed. In the lac operon, the lacI gene is expressed constitutively; that is, the repressor encoded by the gene is expressed continuously. Glucose does not interact directly with the repressor. It affects the catabolite activator protein (CAP) by affecting cAMP synthesis and binding to the CAP binding site.
Post-translational control refers to the:
regulation of gene expression after translation
Negative control of transcription occurs when a(n) __________, a regulatory protein, binds to DNA and shuts down transcription.
repressor The operator is the region that the repressor binds to. The repressor bound to the operator prevents the RNA polymerase holoenzyme from binding to the lac operon promoter. Operon is a term for a set of coordinately regulated bacterial genes that are transcribed together into one. An activator is a regulatory protein that binds to DNA, resulting in positive control.
Regulons can work through negative control using __________ and positive control using __________.
repressors; activators Regulons can work through negative control using repressors and positive control using activators. Regulons coordinate the expression of different genes by using a shared regulator that acts on a regulatory sequence found in all genes and operons of the regulon. An inducer is a small molecule that triggers transcription of a specific gene. An activator is a regulatory protein.
Genes that are transcribed constitutively are said to be __________.
transcribed all the time Genes that are transcribed constitutively are said to be transcribed all the time. Some genes—such as those that code for the enzymes required for glycolysis—are transcribed all the time, or constitutively.
The trp operon regulates the synthesis of the amino acid tryptophan. When tryptophan concentrations are high, __________ binds to the repressor protein and activates it. The repressor attaches to the operator and blocks transcription of the trp genes. This regulation is a type of ______________, a form of control in which the final product of a pathway inhibits the production of the product.
tryptophan; negative feedback The trp operon regulates the synthesis of the amino acid tryptophan. When tryptophan concentrations are high, tryptophan binds to the repressor protein and activates it. The repressor attaches to the operator and blocks transcription of the trp genes. This regulation is a type of negative feedback, a form of control in which the final product of a pathway inhibits the production of the product. Positive feedback stimulates the pathway being regulated and not repressing it. Glucose is not involved in the regulation of the trp operon.
