Regulation of Gene Expression | Specialized Biology I (2023)

Define the term regulation as it applies to genes.

For cells to function properly, the necessary proteins must be synthesized at the right time. All cells control or regulate protein synthesis based on the information encoded in their DNA. The process of activating genes to produce RNA and proteins is calledgene expressionWhether they are simple single-celled organisms or complex multicellular organisms, each cell controls when and how its genes are expressed. To do this, there must be a mechanism to control when genes are expressed to make RNA and protein, how much protein is made, and when to stop making that protein because it is no longer needed.

Regulation of gene expression saves energy and space. Organisms require a lot of energy to express each gene at any given time, so it is more energy efficient to activate genes only when needed. Also, only a portion of the genes in each cell saves space because the DNA must be uncoiled from its tightly coiled structure in order for the DNA to be transcribed and translated. If every protein is expressed in every cell all the time, the cells must be huge.

The regulation of gene expression is extremely complex. Errors in this process are harmful to cells and can lead to the development of many diseases, including cancer.

gene expression

For cells to function properly, the necessary proteins must be synthesized at the right time. All cells control or regulate protein synthesis based on the information encoded in their DNA. The process of activating genes to produce RNA and proteins is calledgene expressionWhether they are simple single-celled organisms or complex multicellular organisms, each cell controls when and how its genes are expressed. To do this, there must be a mechanism to control when genes are expressed to make RNA and protein, how much protein is made, and when to stop making that protein because it is no longer needed.

Regulation of gene expression saves energy and space. Organisms require a lot of energy to express each gene at any given time, so it is more energy efficient to activate genes only when needed. Also, only a portion of the genes in each cell saves space because the DNA must be uncoiled from its tightly coiled structure in order for the DNA to be transcribed and translated. If every protein is expressed in every cell all the time, the cells must be huge.

The regulation of gene expression is extremely complex. Errors in this process are harmful to cells and can lead to the development of many diseases, including cancer.

Gene regulation makes cells different

gene regulationit is how a cell controls which of the many genes in its genome are "on" (expressed). Due 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 cause your different cell types to have different sets of proteins, making each cell type uniquely specialized to do its job.

For example, one of the functions of the liver is to remove toxins such as alcohol from the blood. To do this, liver cells express a gene that encodes a subunit (fragment) of an enzyme called alcohol dehydrogenase. This enzyme breaks down alcohol into non-toxic molecules. Neurons in the human brain do not remove toxins from the body, so they keep these genes unexpressed or "turned off." Also, liver cells do not use neurotransmitters to send signals, so they keep the neurotransmitter genes turned off (Figure 1).

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

How do cells "decide" which genes to activate?

Now a tricky problem! Many factors can influence which genes a cell expresses. As mentioned 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 a broad sense, we can say that the pattern of gene expression in a cell is jointly determined by information inside and outside the cell.

• Example of information fromiCell: what proteins it inherits from its parent cell, whether its DNA is damaged, and how much ATP it has.
• Example of information fromexteriorCells: chemical signals from other cells, mechanical signals from the extracellular matrix, and nutrient levels.

How do these signals help cells "decide" which genes to express? Cells don't make decisions like you or me. Instead, they have molecular pathways that translate information, such as the binding of chemical signals to their receptors, into changes in gene expression.

For example, let's consider how cells respond to growth factors. Growth factors are chemical signals from neighboring cells that instruct target cells to grow and divide. We can say that the cells "notice" the growth factor and "decide" to divide, but how do these processes actually take place?

• Cells detect growth factors by physically binding them to receptor proteins on the cell surface.
• Growth factor binding causes the receptor to change shape, triggering a series of chemical events in the cell that activate proteins called transcription factors.
• Transcription factors bind to certain DNA sequences in the nucleus and cause the transcription of genes involved in cell division.
• The products of these genes are different types of proteins that cause cells to divide (drive cell growth and/or push cells through the cell cycle).

This is just one example of how cells translate sources of information into changes in gene expression. There are many others, and understanding the logic of gene regulation is an ongoing field of research in biology today.

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

Regulation of prokaryotic and eukaryotic genes

To understand how gene expression is regulated, we must first understand how genes code for functional proteins in cells. This process occurs in prokaryotic and eukaryotic cells, but in slightly different ways.

Prokaryotes are single-celled organisms without a nucleus, so their DNA can flow freely in the cytoplasm. To synthesize proteins, the processes of transcription and translation occur almost simultaneously. Transcription stops when the protein produced is no longer needed. Therefore, the most important way to control which types of proteins are expressed in prokaryotic cells, and how much of each protein is expressed, is to regulate DNA transcription. All subsequent steps are done automatically. When more protein is needed, more transcription is produced. Thus, in prokaryotic cells, the control of gene expression is mainly at the level of transcription.

By contrast, eukaryotic cells have intracellular organelles that add to their complexity. In eukaryotic cells, DNA is contained in the 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 in the nucleus, and translation occurs only in the cytoplasm outside the nucleus. Regulation of gene expression can occur at all stages of this process (Figure 1). Regulation can occur when DNA unwinds from the nucleosome and becomes loose to bind to transcription factors.epigeneticlevel) when RNA is transcribed (transcriptional level), when RNA is processed post-transcriptionally and exported to the cytoplasm (after transcriptionlevel) when RNA is translated into protein (translation level) or after protein is produced (after translationcharacter).

Table 1 summarizes the differences in the regulation of gene expression between prokaryotes and eukaryotes. The regulation of gene expression will be discussed in detail in later modules.

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