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.