Regulation of Gene Expression | Biology for the Elderly I (2023)

Define the term regulation in the context of genes.

For a cell to function properly, the necessary proteins must be synthesized at the right time. All cells control or regulate protein synthesis using information encoded in their DNA. The process of turning on a gene to produce RNA and protein is calledgene expression. Whether in a simple unicellular organism or a complex multicellular organism, each cell controls when and how its genes are expressed. For this to happen, there must be some mechanism that controls when a gene is expressed to make RNA and protein, how much protein is made, and when it's time to stop making that protein because it's no longer needed.

Regulation of gene expression saves energy and space. An organism would require a significant amount of energy to express each gene at all times, so it is more energy efficient to activate genes only when they are needed. Furthermore, expressing only a subset of genes saves space in each cell, since DNA must be uncoiled from its tightly coiled structure in order to transcribe and translate the DNA. Cells would have to be huge if every protein was constantly expressed in every cell.

Control of gene expression is extremely complex. Failures in this process damage the cell and can lead to the development of many diseases, including cancer.

gene expression

For a cell to function properly, the necessary proteins must be synthesized at the right time. All cells control or regulate protein synthesis using information encoded in their DNA. The process of turning on a gene to produce RNA and protein is calledgene expression. Whether in a simple unicellular organism or a complex multicellular organism, each cell controls when and how its genes are expressed. For this to happen, there must be some mechanism that controls when a gene is expressed to make RNA and protein, how much protein is made, and when it's time to stop making that protein because it's no longer needed.

Regulation of gene expression saves energy and space. An organism would require a significant amount of energy to express each gene at all times, so it is more energy efficient to activate genes only when they are needed. Furthermore, expressing only a subset of genes saves space in each cell, since DNA must be uncoiled from its tightly coiled structure in order to transcribe and translate the DNA. Cells would have to be huge if every protein was constantly expressed in every cell.

Control of gene expression is extremely complex. Failures in this process damage the cell and can lead to the development of many diseases, including cancer.

Gene regulation makes cells different

Generegulationcontrols a cell which of the many genes in its genome is "on" (expressed). Thanks to genetic regulation, each type of cell in your body has a different set of active genes, even though almost every cell in your body contains the exact same DNA. These different patterns of gene expression result in your different cell types having 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 toxins, such as alcohol, from the bloodstream. To do this, liver cells express genes that code for subunits (pieces) of an enzyme called alcohol dehydrogenase. This enzyme breaks down the alcohol into a non-toxic molecule. The neurons in a person's brain do not remove toxins from the body, so they keep those genes unexpressed or "turned off." Similarly, liver cells do not signal with neurotransmitters, so they keep neurotransmitter genes turned off (Figure 1).

There are many other genes that are differentially expressed between liver cells and neurons (or any two cell types in a multicellular organism like yours).

How do cells “decide” which genes to activate?

Now there is a difficult question! Many factors that can affect which genes a cell expresses. As we saw above, different cell types express different sets of genes. However, two different cells of the same type can also have different patterns of gene expression depending on their environment and internal state.

In general terms, we can say that the pattern of gene expression in a cell is determined by information both inside and outside the cell.

• examples of informationinsidethe cell: what proteins it has inherited from its parent cell, whether its DNA is damaged, and how much ATP it has.
• examples of informationOutsideof the cell: chemical signals from other cells, mechanical signals from the extracellular matrix, and nutrient levels.

How do these signals help a cell "decide" which genes to express? Cells don't make decisions the way you or I would. Instead, they have molecular pathways that convert information, such as the binding of a chemical signal to its receptor, into a change in gene expression.

As an example, let's look at how cells respond to growth factors. A growth factor is a chemical signal from a neighboring cell that directs a target cell to grow and divide. You could say that the cell "feels" the growth factor and "decides" to divide, but how do these processes actually work?

• The cell recognizes the growth factor by physically binding the growth factor to a receptor protein on the cell surface.
• Growth factor binding causes the receptor to change shape and triggers a series of chemical events in the cell that activate proteins called transcription factors.
• Transcription factors bind to specific DNA sequences in the cell nucleus and cause the transcription of genes related to cell division.
• The products of these genes are different types of proteins that cause cell division (drive cell growth and/or move the cell through the cell cycle).

This is just one example of how a cell can convert a source of information into a change in gene expression. There are many others, and understanding the logic of gene regulation is now an area of ​​ongoing research in biology.

Growth factor signaling is complex and involves activation of a variety of targets, including transcription factor proteins and non-transcription factor proteins.

Regulation of prokaryotic and eukaryotic genes

To understand how gene expression is regulated, we must first understand how a gene encodes a functional protein in a cell. The process takes place in both prokaryotic and eukaryotic cells, just in slightly different ways.

Prokaryotes are single-celled organisms that lack a nucleus, so their DNA floats freely in the cell's cytoplasm. To synthesize a protein, the processes of transcription and translation take place almost simultaneously. When the resulting protein is no longer needed, transcription stops. Consequently, the main method of controlling what type of protein and how much of each protein is expressed in a prokaryotic cell is through the regulation of DNA transcription. All subsequent steps are done automatically. When more protein is needed, more transcription is produced. Therefore, the control of gene expression in prokaryotic cells is mainly at the transcriptional level.

By contrast, eukaryotic cells have intracellular organelles that add to their complexity. In eukaryotic cells, DNA is contained in the cell nucleus, where it is transcribed into RNA. The newly synthesized RNA is then transported from the nucleus to the cytoplasm, where ribosomes translate the RNA into protein. The transcription and translation processes are physically separated by the nuclear membrane; Transcription occurs only within the nucleus, and translation occurs only outside the nucleus in the cytoplasm. Regulation of gene expression can occur at all stages of the process (Figure 1). Regulation can occur when DNA unwinds and unwinds from nucleosomes to bind transcription factors.epigeneticlevel) when RNA is being transcribed (transcription level), when RNA is processed and exported to the cytoplasm after transcription (posttranscriptionallevel) when the RNA is being translated into protein (translation level) or after the protein has been produced (post-translationaleben).

Differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in Table 1. The regulation of gene expression is discussed in detail in later modules.

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