## The basic principles of regulation of cellular activities apply equally to bacterial cells as to human cells

Our cells are exposed to a constantly changing environment and therefore must be able to evaluate these changes correctly and respond adequately. For example, when an interesting source of nutrients occurs in their vicinity, they must be able to recognise it, get it into the cytoplasm and process it efficiently. They do so with the help of enzymes, coded by genes, which the cell "turns on" after detecting the nutrient and "turns off" in its absence (gene expression is described in Chapter 3). How is this control carried out? How does a cell know which gene to turn off/on and when to do it?

Important answers to these questions were provided by research on cells, namely on bacterial cells. Jacques Monod (1910 – 1976), a French biologist, conducted an experiment in which the bacteria Escherichia coli were cultured in a medium that either contained a mixture of sugars glucose and maltose, or a mixture of glucose and lactose (Figure 20.3). Bacteria in the glucose+maltose medium had a smooth growth curve, while in the case of the glucose+lactose mixture, the growth stopped for a period halfway through the experiment. Only after restarting did the number of cells reach same number of cells as the bacteria growing in a glucose+maltose mixture. Monod correctly assumed that this pausing occurred as after all the glucose was consumed, the cells then adapted to a new carbon source (lactose). Genetic analysis of this phenomenon led to a model of the regulatory circuit responsible for this adaptation (Figure 20.3). It turned out that the principle of gene activity regulation identified in E. coli is also used in certain variations by more complex (e.g. human) cells. This led Monod to make a metaphorical generalisation: "What is true for E. coli is also true for an elephant."