The different cell types in the human body look different and do very different things: Compare, for example, liver cells and brain cells. How do they manage to be so different given that they have the same DNA? The answer is that cells regulate the expression of their genes – that is, they control when and where their genes are used to make protein. As a result, different cell types make a different complement of proteins.
In fact, the expression of a gene can be regulated by other genes. Biologists represent this using a gene regulatory network, a diagram that shows how genes interact. Figure 13.1 shows an example of such a network for some genes in the bacterium Bacillus subtilis. In the diagram, each gene is represented by a circular node. To show that one gene regulates another, we draw an edge, that is, a line with an arrow. This indicates that one gene (the one which the arrow is drawn from) regulates the transcription of the second (the one which the arrow is drawn to). The effect of this regulation might either be positive (upregulation) or negative (downregulation), but we won’t make a distinction between those two cases here.
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