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Every organism is formed by cells. Different cell-types serve different functions. For example the red blood-cells are supplying the body with oxigen and are disposing off carbon dioxide, white blood-cells serve the immune-response, muscle-cells allow us to move our body, liver-cells are detoxifying the body and skin-cells are protecting against effects from outside, to mention only some examples. All these different cell-types developed from one single cell, the egg, which matured within the body of the female to a new living being. During this maturing-process this first cell was dividing and propagating. We call this proliferation. Its offsprings were differentiating into several different cell-types. Some cells have only a temporary function and are dying later on again. This suicide of some cells is necessary for example for seperation of the fingers of the hand. The cells which firstly were growing between the fingers are dying. The technical term for this cell-death is apoptosis. All these processes are also present in the adult organism. Constantly, cells have to be replaced, so that other cells have to divide and propagate. Undifferentiated cells, so called stem-cells, are differentiating to specialized cells and also the programmed cell-death is existing, for example after skin-cells were damaged heavily by UV-radiation of the sun, so that they can not function normally anymore. In massive cases we speak about sunburn. All these processes are highly regulated. Cells are allowed to divide and propagate only as long as it is necessary to reach the purpose of this process. If we hurt ourselves for example, there is a signal which causes cells to start proliferation to close the wound. As soon as this happened there must be a second signal which tells the cells to stop proliferation. If they would not stop they would grow uncontrolled. Such cases of uncontrolled growth are existing. We call this disease cancer and the proliferations tumors.
In the same way as proliferation has to be regulated, also differentiation and apoptosis have to be regulated. These regulatory processes are caused by signals from outside of the cell. Like the organism also the cell is a highly complex system with different regions for varying functions. Control of these processes is done by the nucleus. That means that signals from outside the cell have to be transmitted through the cell-body to the nucleus, where they are processed and transformed into reactions. There are several possibilities for such signal-transductions. I am working on one of them, on signal-transduction by protein-cascades.

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