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.