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Signaling Cascade

After the ligand binds to the cell-surface receptor, the activation of the receptor’s intracellular components sets off a chain of events that is called a signaling pathway or a signaling cascade. In a signaling pathway, second messengers, enzymes, and activated proteins interact with specific effector proteins, which are in turn activated in a chain reaction that eventually leads to altered gene expression therefore in a change in the cell’s activity or environment (Figure 1).

From the top, the red sphere named ‘Extracellular Signal Molecule’ is attached to a green chalice-like structure named ‘Receptor Protein’, which lower part is embedded inside a grey horizontal, slightly bent line called ‘plasma membrane of the target cell’. From the bottom of the green structure, the black arrow points down to dark green square, from which another arrow points down to dark green triangle, from which another arrow points down to dark green circle. All dark green structures are named ‘second messengers’. From the dark green circle, black arrow goes through the dark blue, horizontal line separating cytosol from the nucleus and points down towards light blue half-sphere shaped structure named ‘transcription regulatory protein’. Finally, the last black arrow points down to a red box named ‘altered gene expression’ with a fragment of double helix structure under it.

Figure 1: Signal transduction cascade Following binding of the ligand to its matching receptor on the cell surface, the signal is transmitted to the inside of the cell and propagated by second messengers. These, in turn, activate effector proteins (such as transcription factors) that induce a change in gene expression of the cell.

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