We all started as one cell. That's pretty hard to imagine, but that's true. And now there's about a hundred trillion cells in each of our bodies, and they each have to carry the complete DNA instruction set. So that means when cells divide, a process has to occur of copying all of that. And that is mitosis, where each chromosome has to be turned into a pair, and then they have to split apart properly so that each of the daughter cells gets a complete set.Francis S. Collins, M.D., Ph.D.