How do prokaryotic cells turn genes on and off

Second, lactose must be present. Only when glucose is absent and lactose is present will the lac operon be transcribed. This makes sense for the cell, because it would be energetically wasteful to create the proteins to process lactose if glucose was plentiful or lactose was not available. Transcription of the lac operon is carefully regulated so that its expression only occurs when glucose is limited and lactose is present to serve as an alternative fuel source.

Why do you think this is the case? If glucose is absent, then CAP can bind to the operator sequence to activate transcription. If lactose is absent, then the repressor binds to the operator to prevent transcription.

If either of these requirements is met, then transcription remains off. Only when both conditions are satisfied is the lac operon transcribed Table 1. Describe how transcription in prokaryotic cells can be altered by external stimulation such as excess lactose in the environment.

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Privacy Policy. Skip to main content. Module Gene Expression. Search for:. Prokaryotic Gene Regulation Discuss different components of prokaryotic gene regulation The DNA of prokaryotes is organized into a circular chromosome supercoiled in the nucleoid region of the cell cytoplasm.

Learning Objectives Understand the basic steps in gene regulation in prokaryotic cells Explain the roles of repressors in negative gene regulation Explain the role of activators and inducers in positive gene regulation. Show Answer An operon is composed of a promoter, an operator, and the structural genes. They must occur in that order. Show Answer There are three types of regulatory molecules: repressors, activators, and inducers.

Watch this video to learn more about the trp operon. Practice Question Transcription of the lac operon is carefully regulated so that its expression only occurs when glucose is limited and lactose is present to serve as an alternative fuel source.

Show Answer Tryptophan is an amino acid essential for making proteins, so the cell always needs to have some on hand. Proteins that are needed for a specific function, or that are involved in the same biochemical pathway, 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. Finally, inducers are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate. 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 [link]. 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.

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. Watch this video to learn more about the trp operon.

For eukaryotes, cell-cell differences are determined by expression of different sets of genes. For instance, an undifferentiated fertilized egg looks and acts quite different from a skin cell, a neuron, or a muscle cell because of differences in the genes each cell expresses. A cancer cell acts different from a normal cell for the same reason: It expresses different genes. Using microarray analysis , scientists can use such differences to assist in diagnosis and selection of appropriate cancer treatment.

Interestingly, in eukaryotes, the default state of gene expression is "off" rather than "on," as in prokaryotes. Why is this the case? The secret lies in chromatin, or the complex of DNA and histone proteins found within the cellular nucleus. The histones are among the most evolutionarily conserved proteins known; they are vital for the well-being of eukaryotes and brook little change. When a specific gene is tightly bound with histone, that gene is "off. This is where the histone code comes into play.

This code includes modifications of the histones' positively charged amino acids to create some domains in which DNA is more open and others in which it is very tightly bound up. DNA methylation is one mechanism that appears to be coordinated with histone modifications, particularly those that lead to silencing of gene expression. On the other hand, when the tails of histone molecules are acetylated at specific locations, these molecules have less interaction with DNA, thereby leaving it more open.

The regulation of the opening of such domains is a hot topic in research. For instance, researchers now know that complexes of proteins called chromatin remodeling complexes use ATP to repackage DNA in more open configurations. Scientists have also determined that it is possible for cells to maintain the same histone code and DNA methylation patterns through many cell divisions.

This persistence without reliance on base pairing is called epigenetics, and there is abundant evidence that epigenetic changes cause many human diseases. For transcription to occur, the area around a prospective transcription zone needs to be unwound. This is a complex process requiring the coordination of histone modifications, transcription factor binding and other chromatin remodeling activities.

Many of these proteins are activators, while others are repressors; in eukaryotes, all such proteins are often called transcription factors TFs.

In the test tube, scientists can find a footprint of a TF if that protein binds to its matching motif in a piece of DNA. Some activating TFs even turn on multiple genes at once. All TFs bind at the promoters just upstream of eukaryotic genes, similar to bacterial regulatory proteins.

However, they also bind at regions called enhancers, which can be oriented forward or backwards and located upstream or downstream or even in the introns of a gene, and still activate gene expression.

The trp operon includes three important regions: the coding region, the trp operator and the trp promoter. The coding region includes the genes for the five tryptophan biosynthesis enzymes. Just before the coding region is the transcriptional start site. Between the promoter and the transcriptional start site is the operator region.

The trp operator contains the DNA code to which the trp repressor protein can bind. However, the repressor alone cannot bind to the operator. When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor, which changes the shape of the repressor protein to a form that can bind to the trp operator.

Binding of the tryptophan—repressor complex at the operator physically prevents the RNA polymerase from binding to the promoter and transcribing the downstream genes. When tryptophan is not present in the cell, the repressor by itself does not bind to the operator, the polymerase can transcribe the enzyme genes, and tryptophan is synthesized.

Because the repressor protein actively binds to the operator to keep the genes turned off, the trp operon is said to be negatively regulated and the proteins that bind to the operator to silence trp expression are negative regulators. Watch this video to learn more about the trp operon. Just as the trp operon is negatively regulated by tryptophan molecules, there are proteins that bind to the promoter sequences that act as positive regulators to turn genes on and activate them.

For example, when glucose is scarce, E. To do this, new genes to process these alternate sugars must be transcribed. The cAMP molecule is a signaling molecule that is involved in glucose and energy metabolism in E.

Accumulating cAMP binds to the positive regulator catabolite activator protein CAP , a protein that binds to the promoters of operons which control the processing of alternative sugars. When cAMP binds to CAP, the complex then binds to the promoter region of the genes that are needed to use the alternate sugar sources Figure. CAP binding stabilizes the binding of RNA polymerase to the promoter region and increases transcription of the associated protein-coding genes.

The lac Operon: An Inducible Operon The third type of gene regulation in prokaryotic cells occurs through inducible operons , which have proteins that bind to activate or repress transcription depending on the local environment and the needs of the cell.

The lac operon is a typical inducible operon. As mentioned previously, E. One such sugar source is lactose. The lac operon encodes the genes necessary to acquire and process the lactose from the local environment.

The Z gene of the lac operon encodes beta-galactosidase, which breaks lactose down to glucose and galactose. However, for the lac operon to be activated, two conditions must be met.