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5.3Cell Differentiation

The genes of zygotes are replicated and passed on to the cells that form our bodies, which therefore have all the same genetic information as the zygote. This fact has been demonstrated by an animal cloning experiment (see Chapter 11). The experiment demonstrated that even a nucleus taken from an animal somatic cell has the ability to form the structure of the entire body.
As development progresses, germ cells containing the same genetic information diverge, some becoming muscle cells and some nerve cells. This phenomenon is called "cell differentiation." Various tissues and organs are formed by such differentiation of germ cells into various kinds of cells. Comparisons of genes expressed in differentiated cells have shown that there are differences in the proteins of genes expressed in different types of cells (Fig. 5-4). For example, genes for proteins necessary for contraction are expressed in muscle cells, and genes for proteins necessary for excitation and stimulus transduction are expressed in nerve cells. Of course, the basic genes indispensable for maintenance of cell function are expressed in both kinds of cells.

Fig. 5-4. Schematic Diagram of Cell Differentiation

Differentiated cells exhibit gene expression patterns different from those of their original cells. Cells that have differentiated into different types of cells also express mutually different gene patterns. Differentiated cells show different gene expression patterns in order to perform their respective different functions.

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