Cells called spermatogonium become sperm through the process of meiosis. A mature sperm has only half the DNA required for a functional cell, so it can’t survive on its own. However, they do have flagella and mitochondria that power their journey through the reproductive tract in search of an egg. Human sperm typically die within forty-eight hours if they have not fertilized an egg. (For more about meiosis, see the chapter “Cellular Basics.”)