Every cell of the body contains the entire DNA sequence, that is, all 25,000 genes. One could believe that all cells look alike and that all have the same functions. This is not the case, however.

The reason is that different genes are read in different cells. Whether an ovum develops into a muscle cell, into a hair or into a cell of the pancreas is managed by a program that regulates exactly which gene will be switched on or off, and when this will happen.

Some genes are always active in nearly all cells, as are, for instance, the genes that are responsible for cell metabolism.

Other genes are required only in few cells and in certain situations. After all, a nerve cell has a different set of tasks than a liver cell. In order for liver cells to break down alcohol and for brain cells to carry out intellectual functions, nerve cells and liver cells – as any other cells – are individually equipped with components and tools. And in order to obtain this special equipment, the genes have to be expressed, i.e. the blueprints have to be read, in the right cells and at the right time for the building components and tools. A gene that is “on” or “expressed”, is called an active gene in the terminology of genetics.

For example, each cell of the body contains the genetic information for the pigment melanin. However, this pigment is not needed in all parts of our body. Because of that, melanin is only active in our skin, our hair and in our eyes, where it produces the protein that contributes to the color of our hair, skin and eyes.

As another example, each human cell contains the information for the assembly of the hormone insulin. But the insulin gene is only activated in the pancreas, and to be more precise, only when the body needs to break down sugar.