The oldest type of cell is the prokaryotic cell. Prokaryotic cells are the simplest and most ancient type of cells and lack a nucleus. They are typically small, ranging in size from 0.1 to 5 micrometers, and have a simple cellular structure. Prokaryotic cells lack membrane-bound organelles, such as mitochondria and chloroplasts, and their DNA is typically arranged in a single circular chromosome. Prokaryotic cells are widespread in nature and include bacteria and archaea.