Prokaryotes are single-celled organisms that lack a nucleus and other membrane-bound organelles. The word prokaryote comes from the Greek words "pro" meaning "before" and "karyon" meaning "nut" or "kernel". Prokaryotes are classified into two domains: Bacteria and Archaea. They are much smaller than eukaryotic cells and are typically between 0.1-5 μm in diameter. Prokaryotic cells are not divided up on the inside by membrane walls, but consist instead of a single open space. The majority of prokaryotic DNA is found in a central region of the cell called the nucleoid, and it typically consists of a single large loop called a circular chromosome. Prokaryotes are found in nearly all environments, including extreme habitats, and often form biofilms. Bacteria and archaea are the two types of prokaryotes.