 Prokaryote, the prokaryote is a unit cellular organism that lamps a membrane-bound nucleus, mitochondria, or any other membrane-bound organelle. Prokaryotes are divided into two domains, archaea and bacteria. In contrast, species with nuclei and organelles are placed in the third domain, eukaryotic. Prokaryotes reproduce without fusion of gametes. The first living organisms are thought to have been prokaryotes. In the prokaryotes, all the intracellular water-soluble components proteins, DNA and metabolites are located together in the cytoplasm enclosed by the cell membrane, rather than in separate cellular compartments. Bacteria, however, do possess protein-based bacterial microcompartments, which are thought to act as primitive organelles enclosed in protein shells. Some prokaryotes, such as cyanobacteria, may form large colonies. Others, such as mitzvah bacteria, have multicellular stages in their life cycles. Other 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. Prokaryotes are common in some extreme environments.