## What regulates gene expression?

Cells contain a large number of genes, not all of which are used all the time. From this point of view, and at the same time to save energy, the process of gene expression regulation is very important. However, not all genes can be safely turned off or on. The products of some genes are so necessary that their absence would be fatal for the cells. They are called housekeeping genes and are expressed under all conditions. Other genes can be regulated depending on whether their products are needed by the cell at a given time. We refer to the process of turning on gene expression as induction, and repression refers to when gene expression is turned off. By turning off the expression of genes, the organism saves energy, which it can use in the synthesis of other products. Various cascades of enzymatic reactions or signalling pathways are part of the regulation of gene expression. For example, prokaryotic cells of bacteria respond to the presence of nutrients by triggering the expression of genes that allow them to efficiently process these nutrients. Their specific feature is that by reacting to one stimulus, they can trigger the expression of all necessary genes at once. This is ensured by the formation of polycistronic transcripts, which are long mRNA transcripts encoding the entire set of necessary proteins.

In eukaryotic cells, gene activity is also influenced by chromatin structure. Regions of packed chromatin, called heterochromatin, generally contain inactive genes. A slight loosening of the structure makes genes accessible to regulatory factors that trigger their transcription (this unfolded form of chromatin is called euchromatin). Gene expression can thus be regulated by chromatin remodelling, which you will read more about in Chapter 7 - From epigenetics to human diseases.

In the human body, there is a large number of different types of cells that contain the same genetic information. It is the regulation of gene expression that ensures the differences between cells with a specialized function. This means, for example, that although the same DNA is found in the nucleus of a neuron and a muscle cell, the specific switching on and off of certain genes can differentiate them sufficiently and adapt them to a specific function and thus be either a neuron or a muscle cell. The regulation of the expression of genetic information also includes various signalling pathways that lead to activation, or suppression of the transcription of certain genes. Similarly to the case of bacteria, a certain impulse triggers the cascade reaction. The trigger can come from the surrounding environment, or it can be a molecule located directly in the cell. It is often various ions, hormones, enzymes or other signalling proteins that are recognized by receptors on the surface or inside the cell. Their capture triggers the transmission of a signal between several molecules, which leads to the activation of transcription factors. These proteins then trigger the expression of a wide variety of genes. Cell signalling plays an important role especially in multicellular organisms, as it allows specialised (and often distant from each other) cells to communicate with each other, which leads to a complex response of the organism to a certain stimulus.