Prokaryote

A prokaryote (; from Ancient Greek πρό pró, meaning 'before'; and κάρυον káruon, meaning 'nut' or 'kernel'), less commonly spelled procaryote, is a cellular organism that lacks a distinct cell nucleus or other membrane-bound organelles. In the earlier two-empire system, prokaryotes formed the empire Prokaryota; in the current three-domain system, based upon molecular phylogenetics, prokaryotes are divided into two domains Bacteria and Archaea. A third domain, Eukaryota, which consists of single- and multi-cellular organisms with nuclei, organelles and complex cellular architecture, is sometimes considered a clade under Archaea in the two-domain system. All prokaryotes are single-celled microorganisms that reproduce asexually via binary fission, although horizontal gene transfer is also common. Some prokaryotes such as cyanobacteria form colonies held together by biofilms, and large microbial communities formed by symbiotic colonies of different prokaryote groups can create multilayered microbial mats, which can become mineralized and geologically preserved in the fossil records as stromatolites. Prokaryotes are important biodegraders and primary producers of the aquatic and terrestrial ecosystems, and play a crucial part in the Earth's carbon, nitrogen, phosphorus and oxygen cycles. Prokaryotes are among the earliest known life forms on Earth, having evolved from pre-cellular first universal common ancestor (FUCA) with a notable diversification around 3.72–4.18 Ga, and the last universal common ancestor (LUCA, who is considered "prokaryote-grade") evolved around 4.09–4.33 Ga. Molecular phylogenetics has provided insight into the interrelationships of the three domains of life. The division between prokaryotes and eukaryotes reflects two very different levels of cellular organization; only eukaryotic cells have an membrane-enclosed nucleus that contains their DNA, and other membrane-bound organelles including mitochondria (which evolved from prokaryotic endosymbionts through an evolutionary process known as symbiogenesis). More recently, the primary division has been seen as that between Archaea and Bacteria, since eukaryotes may be part of the archaean clade and...