Category: Which of the following statements is true about gene regulation in prokaryotes

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To sign up you must be 13 or older. Terms of Use and Privacy Policy. Already have an account? Log in. Get started today! Edit a Copy. Study these flashcards. Which one of the following statements about gene regulation is incorrect? Gene regulation occurs only in prokaryotes.

Which of the following is not a reason why gene regulation in prokaryotes is simpler than in eukaryotes? Processing of mRNA does not occur in prokaryotes. True or false: Primary RNA transcripts are always spliced in the same fashion in order to maintain continuity.

which of the following statements is true about gene regulation in prokaryotes

They often bind to recently transported mRNAs and inhibit their translation. In female humans and other mammals, dosage compensation is achieved by the inactivation of one of the two X chromosomes in each cell. Which one of the following does not occur in the X -inactivation mechanism of female mammals?

Which one of the following can lead to changes in chromatin structure and is often associated with activation of transcription?

Addition of methyl or acetyl groups to lysines located in the histone tail.The DNA of prokaryotes is organized into a circular chromosome supercoiled in the nucleoid region of the cell cytoplasm. Proteins that are needed for a specific function are encoded together in blocks called operons. For example, all of the genes needed to use lactose as an energy source are coded next to each other in the lactose or lac operon.

In prokaryotic cells, there are three types of regulatory molecules that can affect the expression of operons: repressors, activators, and inducers. Repressors are proteins that suppress transcription of a gene in response to an external stimulus, whereas activators are proteins that increase the transcription of a gene in response to an external stimulus.

Regulation of gene expression

Finally, inducers are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate.

In bacteria and archaea, structural proteins with related functions—such as the genes that encode the enzymes that catalyze the many steps in a single biochemical pathway—are usually encoded together within the genome in a block called an operon and are transcribed together under the control of a single promoter.

The promoter then has simultaneous control over the regulation of the transcription of these structural genes because they will either all be needed at the same time, or none will be needed.

Figure 1. In prokaryotes, structural genes of related function are often organized together on the genome and transcribed together under the control of a single promoter. If a repressor binds to the operator, then the structural genes will not be transcribed. Alternatively, activators may bind to the regulatory region, enhancing transcription.

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They found that in E. For this work, they won the Nobel Prize in Physiology or Medicine in Although eukaryotic genes are not organized into operons, prokaryotic operons are excellent models for learning about gene regulation generally. There are some gene clusters in eukaryotes that function similar to operons.

Many of the principles can be applied to eukaryotic systems and contribute to our understanding of changes in gene expression in eukaryotes that can result pathological changes such as cancer. Each operon includes DNA sequences that influence its own transcription; these are located in a region called the regulatory region.

The regulatory region includes the promoter and the region surrounding the promoter, to which transcription factorsproteins encoded by regulatory genes, can bind.

Transcription factors influence the binding of RNA polymerase to the promoter and allow its progression to transcribe structural genes. A repressor is a transcription factor that suppresses transcription of a gene in response to an external stimulus by binding to a DNA sequence within the regulatory region called the operatorwhich is located between the RNA polymerase binding site of the promoter and the transcriptional start site of the first structural gene.

Repressor binding physically blocks RNA polymerase from transcribing structural genes. Conversely, an activator is a transcription factor that increases the transcription of a gene in response to an external stimulus by facilitating RNA polymerase binding to the promoter. An inducera third type of regulatory molecule, is a small molecule that either activates or represses transcription by interacting with a repressor or an activator.

In prokaryotes, there are examples of operons whose gene products are required rather consistently and whose expression, therefore, is unregulated. Such operons are constitutively expressedmeaning they are transcribed and translated continuously to provide the cell with constant intermediate levels of the protein products. Such genes encode enzymes involved in housekeeping functions required for cellular maintenance, including DNA replication, repair, and expression, as well as enzymes involved in core metabolism.

In contrast, there are other prokaryotic operons that are expressed only when needed and are regulated by repressors, activators, and inducers. Bacteria such as E.The trp operon is a repressor operon that is either activated or repressed based on the levels of tryptophan in the environment. Explain the relationship between structure and function of an operon and the ways in which repressors regulate gene expression. Bacteria such as E. Tryptophan is one such amino acid that E. These five genes are next to each other in what is called the tryptophan trp operon.

If tryptophan is present in the environment, then E. However, when tryptophan availability is low, the switch controlling the operon is turned on, transcription is initiated, the genes are expressed, and tryptophan is synthesized. The trp operon : The five genes that are needed to synthesize tryptophan in E. When tryptophan is plentiful, two tryptophan molecules bind the repressor protein at the operator sequence.

This physically blocks the RNA polymerase from transcribing the tryptophan genes. When tryptophan is absent, the repressor protein does not bind to the operator and the genes are transcribed. A DNA sequence that codes for proteins is referred to as the coding region.

The five coding regions for the tryptophan biosynthesis enzymes are arranged sequentially on the chromosome in the operon. Just before the coding region is the transcriptional start site. The promoter sequence is upstream of the transcriptional start site. Each operon has a sequence within or near the promoter to which proteins activators or repressors can bind and regulate transcription. A DNA sequence called the operator sequence is encoded between the promoter region and the first trp-coding gene.

This operator contains the DNA code to which the repressor protein can bind. When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor, which changes shape to bind to the trp operator. Binding of the tryptophan—repressor complex at the operator physically prevents the RNA polymerase from binding and transcribing the downstream genes. When tryptophan is not present in the cell, the repressor by itself does not bind to the operator; therefore, the operon is active and tryptophan is synthesized.

Because the repressor protein actively binds to the operator to keep the genes turned off, the trp operon is negatively regulated and the proteins that bind to the operator to silence trp expression are negative regulators. When glucose levels decline in E. Just as the trp operon is negatively regulated by tryptophan molecules, there are proteins that bind to the operator sequences that act as a positive regulator to turn genes on and activate them.

For example, when glucose is scarce, E.

Gene Regulation

To do this, new genes to process these alternate genes must be transcribed. This type of process can be seen in the lac operon which is turned on in the presence of lactose and absence of glucose.

which of the following statements is true about gene regulation in prokaryotes

The cAMP molecule is a signaling molecule that is involved in glucose and energy metabolism in E. When glucose levels decline in the cell, accumulating cAMP binds to the positive regulator catabolite activator protein CAPa protein that binds to the promoters of operons that control the processing of alternative sugars, such as the lac operon.

The CAP assists in production in the absence of glucose. CAP is a transcriptional activator that exists as a homodimer in solution, with each subunit comprising a ligand-binding domain at the N-terminus, which is also responsible for the dimerization of the protein and a DNA-binding domain at the C-terminus. CAP has a characteristic helix-turn-helix structure that allows it to bind to successive major grooves on DNA.

This opens up the DNA molecule, allowing RNA polymerase to bind and transcribe the genes involved in lactose catabolism. When cAMP binds to CAP, the complex binds to the promoter region of the genes that are needed to use the alternate sugar sources.

This increases the binding ability of RNA polymerase to the promoter region and the transcription of the genes. As cAMP-CAP is required for transcription of the lac operon, this requirement reflects the greater simplicity with which glucose may be metabolized in comparison to lactose.

As glucose supplies become limited, cAMP levels increase. This cAMP binds to the CAP protein, a positive regulator that binds to an operator region upstream of the genes required to use other sugar sources.

The lac operon is an inducible operon that utilizes lactose as an energy source and is activated when glucose is low and lactose is present. A major type of gene regulation that occurs in prokaryotic cells utilizes and occurs through inducible operons.There are many differences between prokaryotic and eukaryotic cells. Some of these differences are structural whereas others are procedural.

Two of the processes that are substantially different between prokaryotes and eukaryotes are gene expression and the regulation of it. Both types of cells transcribe DNA into mRNA, which is then translated into polypeptides, but the specifics of these processes differ. Prokaryotes lack nuclei and other organelles, which are specialized, membrane-bound compartments, whereas eukaryotes do have them.

In fact, the word "eukaryote" means "true nucleus. Transcription thus occurs in the nucleus, and the mRNA transcript is subsequently exported through nuclear pores pores in the nuclear envelope to the cytoplasm for translation.

By contrast, prokaryotic transcription and translation are not spatially or temporally segregated. Promoter elements are short sequences of DNA that bind to a cell's transcriptional initiation factors. Prokaryotes have three promoter elements: one that is upstream of the gene being transcribed, one that is 10 nucleotides downstream of it and one that is 35 nucleotides downstream.

Eukaryotes have a much larger set of promoter elements, the primary one being the TATA box. Eukaryotic transcription initiation factors assemble an initiation complex, which dissociates at the end of initiation. Prokaryotic transcription initiation factors do not assemble an initiation complex. Prokaryotes have 70S ribosomes whereas eukaryotes have 80S ribosomes.

The "S" refers to the sedimentation coefficient, a measure of a particle's size, mass and shape. An 80S ribosome is composed of a 40S subunit and a 60S subunit while a 70S ribosome consists of a 30S subunit and a 50S subunit.

In addition to having different transcription and translation machinery, prokaryotes and eukaryotes differ in their gene regulation. Eukaryotic regulation is much more complex and often relies on various feedback mechanisms, developmental processes and environmental factors.

By contrast, prokaryotes regulate entire metabolic pathways rather than regulating each enzyme separately. When a cell needs more or less of a pathway's enzymes, it simply transcribes more or less of that pathway's mRNA.

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Regulation of Gene Expression in Prokaryotes and Eukaryotes

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Log in. Get started today! Ch Edit a Copy. Study these flashcards. Madison E. Bacteria rarely regulate gene products through. Genes that encode proteins that are always needed are called:. The operator of the lactose operon in E.

How does the lactose repressor block transcription of the lactose operon?

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The gene that codes for the repressor protein of the E. Lactose induces the transcription of the lactose operon by:. The inducer for the lactose operon in E. Inducible genes are usually actively transcribed when:. Repressible genes are usually actively transcribed:. An inducible operon is usually controlled by:.

which of the following statements is true about gene regulation in prokaryotes

In the tryptophan operon, the repressor actively binds to the operator when:. Catabolite activator protein:.

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In the lac operon, cAMP binds to:. The role of CAP in the lac operon is:. Translational controls regulate:. Which of the following statements about regulation of gene expression in bacteria is false? CAP active means. Which of the following statements concerning gene regulation in eukaryotes is false? Temporal gene regulation refers to which of the following circumstances? Densely staining regions of highly compacted chromatin that are generally not transcribed are:.Regulation of gene expressionor gene regulation[1] includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products protein or RNA.

Sophisticated programs of gene expression are widely observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources. Virtually any step of gene expression can be modulated, from transcriptional initiationto RNA processingand to the post-translational modification of a protein.

Often, one gene regulator controls another, and so on, in a gene regulatory network. Gene regulation is essential for virusesprokaryotes and eukaryotes as it increases the versatility and adaptability of an organism by allowing the cell to express protein when needed. In multicellular organisms, gene regulation drives cellular differentiation and morphogenesis in the embryo, leading to the creation of different cell types that possess different gene expression profiles from the same genome sequence.

Although this does not explain how gene regulation originated, evolutionary biologists include it as a partial explanation of how evolution works at a molecular leveland it is central to the science of evolutionary developmental biology "evo-devo". Any step of gene expression may be modulated, from the DNA-RNA transcription step to post-translational modification of a protein. The following is a list of stages where gene expression is regulated, the most extensively utilised point is Transcription Initiation:.

Hence these modifications may up or down regulate the expression of a gene. Some of these modifications that regulate gene expression are inheritable and are referred to as epigenetic regulation. Transcription of DNA is dictated by its structure. In general, the density of its packing is indicative of the frequency of transcription. Octameric protein complexes called nucleosomes are responsible for the amount of supercoiling of DNA, and these complexes can be temporarily modified by processes such as phosphorylation or more permanently modified by processes such as methylation.

Such modifications are considered to be responsible for more or less permanent changes in gene expression levels. Methylation of DNA is a common method of gene silencing. DNA is typically methylated by methyltransferase enzymes on cytosine nucleotides in a CpG dinucleotide sequence also called " CpG islands " when densely clustered. Analysis of the pattern of methylation in a given region of DNA which can be a promoter can be achieved through a method called bisulfite mapping.

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Methylated cytosine residues are unchanged by the treatment, whereas unmethylated ones are changed to uracil. Abnormal methylation patterns are thought to be involved in oncogenesis. Histone acetylation is also an important process in transcription. Often, DNA methylation and histone deacetylation work together in gene silencing.

The combination of the two seems to be a signal for DNA to be packed more densely, lowering gene expression. Regulation of transcription thus controls when transcription occurs and how much RNA is created. Transcription of a gene by RNA polymerase can be regulated by several mechanisms. Specificity factors alter the specificity of RNA polymerase for a given promoter or set of promoters, making it more or less likely to bind to them i.

Repressors bind to the Operatorcoding sequences on the DNA strand that are close to or overlapping the promoter region, impeding RNA polymerase's progress along the strand, thus impeding the expression of the gene. The image to the right demonstrates regulation by a repressor in the lac operon. General transcription factors position RNA polymerase at the start of a protein-coding sequence and then release the polymerase to transcribe the mRNA.

Activators enhance the interaction between RNA polymerase and a particular promoterencouraging the expression of the gene. Activators do this by increasing the attraction of RNA polymerase for the promoter, through interactions with subunits of the RNA polymerase or indirectly by changing the structure of the DNA.

Enhancers are sites on the DNA helix that are bound by activators in order to loop the DNA bringing a specific promoter to the initiation complex.Let us make an in-depth study of the gene expression regulation.

After reading this article you will learn about 1. Regulation of Gene Expression in Prokaryotes and 2. Regulation of Gene Expression in Eukaryotes. Some proteins are required at some time and yet other proteins are required at another time.

Moreover these proteins are required in lesser quantities at one time, yet at other times they may be required in higher quantities. There are yet another class of proteins which are constantly always present in the cell, like the enzymes of the TCA cycle. Those genes whose products are constantly present in the cell are called constitutive genes or housekeeping genes.

The regulation is primarily at the level of transcription. The gene or a set of related genes are switched on or off as per the need of the cell.

These changes are brought about by some proteins or modulator. As soon as the hormone is destroyed the gene expression diminishes. The mechanism of regulation, though similar in the prokaryotes and eukaryotes, it differs in some aspects. Hence regulation of gene expression in prokaryotes and eukaryotes will be taken separately.

Many prokaryotic genes are regulated in units called operons. Operon is unit of genetic expression consisting of one or more related genes and sequences gene controlling them, which includes the operator and promoter sequences that regulate their transcription. This complete set of sequences i.

When glucose is present in the media where the cell is growing, then the lac operon is switched off and when the medium is devoid of glucose, and instead lactose is present as the sole source of carbon, then the Lac operon becomes operational.

The transcription by RNA polymerase begins at the promoter site i. Under all circumstances i. This protein binds to the operator site in the DNA and thus prevents the movement of the RNA polymerase beyond this point sitewhich results in the inhibition of the synthesis of the structural genes Z, Y and A.

Thus, when the cell is utilizing glucose as the only carbon source, the lac operon is switched off.


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