Signal transduction

Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation, which ultimately result in a response. Proteins responsible for detecting stimuli are generally termed receptors, although in some cases the term sensor is used. The changes elicited by ligand binding (or signal sensing) in a receptor give rise to a cascade of biochemical events along a signalling pathway. When signalling pathways interact with one another they form networks, which allow cellular responses to be coordinated. At the molecular level, such responses include changes in the transcription or translation of genes, and post-translational and conformational changes in proteins, as well as changes in their location. These molecular events are the basic mechanisms controlling cell growth, proliferation, metabolism and many other processes. In multicellular organisms, signal transduction pathways have evolved to regulate cell communication in a wide variety of ways.

Each component (or node) of a signalling pathway is classified according to the role it plays with respect to the initial stimulus. Ligands are termed first messengers, while receptors are the signal transducers, which then activate primary effectors. Such effectors are often linked to second messengers, which can activate secondary effectors, and so on. Depending on the efficiency of the nodes, a signal can be amplified (a concept known as signal gain), so that one signalling molecule can generate a response involving hundreds to millions of molecules. As with other signals, the transduction of biological signals is characterised by delay, noise and interference, which can range from negligible to pathological. With the advent of computational biology, the analysis of signalling pathways and networks has become an essential tool to understand cellular functions and disease.

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