DNA possesses genes, small snippets of biological instructions, that guide everything from how tall you become to how you respond to stress. A lot of genetic material fits inside that yolk-like nucleus. Nearly six feet of the stuff are crammed into a space that is measured in microns. A micron is 1/25,000th of an inch, which means putting DNA into your nucleus is like taking thirty miles of fishing line and stuffing it into a blueberry. The nucleus is a crowded place.

One of the most unexpected findings of recent years is that this DNA, or deoxyribonucleic acid, is not randomly jammed into the nucleus, as one might stuff cotton into a teddy bear. Rather, DNA is folded into the nucleus in a complex and tightly regulated manner. The reason for this molecular origami: cellular career options. Fold the DNA one way and the cell will become a contributing member of your liver. Fold it another way and the cell will become part of your busy bloodstream. Fold it a third way and you get a nerve cell—and the ability to read this sentence.