A prokaryote is a single-celled organism that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelles. The word prokaryote comes from the Greek πρό- (pro-) “before” and καρυόν (karyon) “nut or kernel”. All the intracellular water-soluble components (proteins, DNA and metabolites) are located together in the same volume enclosed by the cell membrane, rather than in separate cellular compartments. Prokaryotes, however, do possess protein-based microcompartments, which are thought to act as primitive organelles (protein-bounded and lipid-bounded organelles). Some prokaryotes also have multicellular stages in their life cycles, such as myxobacteria, or create large colonies, like cyanobacteria. Prokaryotic cells can be divided into two domains, Archaea and Bacteria, with the remainder of species, called eukaryotes, in a third domain. Molecular studies have provided insight into the evolution and interrelationships of the three domains of biological species. Eukaryotes are organisms, including humans, whose cells have a well defined membrane bound nucleus (containing chromosomal DNA) and organelles. The division between prokaryotes and eukaryotes reflects the existence of two very different levels of cellular organization. Distinctive types of prokaryotes include extremophiles and methanogens; these are common in some extreme environments.