What do a human, a rose, and a bacterium have in common? Each of these things — along with every other organism on Earth — contains the molecular instructions for life, called deoxyribonucleic acid or DNA. Encoded within this DNA are the directions for traits as diverse as the color of a person’s eyes, the scent of a rose, and the way in which bacteria infect a lung cell.
DNA is found in nearly all living cells. However, its exact location within a cell depends on whether that cell possesses a special membrane-bound organelle called a nucleus. Organisms composed of cells that contain nuclei are classified as eukaryotes, whereas organisms composed of cells that lack nuclei are classified as prokaryotes. In eukaryotes, DNA is housed within the nucleus, but in prokaryotes, DNA is located directly within the cellular cytoplasm, as there is no nucleus available.