Gene Switches

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Red blood cells

Take a typical cell, such as a red blood cell. Each gene within that cell has a coding region. This region encodes the information used to make a particular protein, such as the hemoglobin in the red blood cells seen here. (Hemoglobin shuttles oxygen to the tissues and carbon dioxide back out to the lungs—or gills, if you're a fish.) But another region of the gene, called "regulatory DNA," determines whether and when the gene will be expressed, or turned on, in a particular kind of cell. If you're a brain cell, for instance, you wouldn't want the genes encoding hemoglobin proteins to be transcribed. This precise transcribing of genes is handled by proteins known as transcription factors, which bind to the regulatory DNA, thereby generating instructions for the coding region.

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