Gene Switches

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Heart cells and neurons

All animals, including you and me, begin as a single egg. Once fertilized, that egg becomes many different kinds of cells. Here we see just two examples, heart cells (left) and nerve cells, or neurons. Altogether, multicellular organisms like humans have thousands of differentiated cells. Each is optimized for use in the brain, the liver, the skin, and so on. Remarkably, the DNA inside all these cells is exactly the same. What makes the cells differ from one another is that different genes in that DNA are either turned on or off in each type of cell.

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