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Signal transduction converts an environmental signal into a cellular response. A cascade allows rapid response, amplification and control.

transduction conversion of a signal of one type into another type enzymes proteins that control a reaction in a cell apoptosis programmed cell death phosphorylation addition of the phosphate group PO43-

exposed to estrogen circulating in the blood, but only a subset of them make estrogen receptors, and are therefore sensitive to its influence.

Chemical signals such as hormones bind to their receptors, usually at the surface of the cell (the plasma membrane), but sometimes within the cell. This causes a conformation (shape) change in the receptor. The conformation change typically alters the ability of the receptor to bind to another molecule in the cell, modifying that molecule's conformation, or triggering other actions.

This sequence of events triggered by the signal-receptor interaction is called a transduction cascade. A transduction cascade involves a network of enzymes that act on one another in specific ways to ultimately generate precise and appropriate responses. These responses may include alterations in cell motility or division, induction of the expression of specific genes, and the regulation of apoptosis. The molecular details of several such cascades are known, but many more undoubtedly remain to be discovered.

The value of this complex cascade of events is severalfold. First, the network of interactions provides many levels of control, so that the cell can tailor the magnitude and timing of its response very finely. Second, the many levels of interaction allow amplification of the original signal to quickly produce a strong response to a small stimulus. For example, there may be only a few hundred copies of a specific receptor on the surface of a typical cell. Activation of even a small percentage of them, acting through these amplifying enzyme cascades, can result in activation of millions of downstream target molecules. This explains how even very small amounts of signals such as growth factor can have such profound effects on appropriately receptive cells.

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