Prokaryotes are mostly unicellular organisms without a nucleus, in contrast to eukaryotes, organisms that have cell nuclei and may be variously unicellular or multicellular. The difference between prokaryote and eukaryote cell structure is the most important in the living world. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. However, Woese has proposed dividing them into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) on the supposition that these have separate origins. This controversial arrangement is called the three-domain system.
Prokaryotes do not develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime layer secreted by the cells. Typically though, there is no continuity or communication between the cells. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of the human body.
The name prokaryote comes from the Greek pros meaning before and karyon meaning nut, referring to the nucleus. Prokaryotes also lack cytoskeletons and membrane-bound cell compartments such as vacuoles, endoplasmic reticulum, mitochondria and chloroplasts. In eukaryotes, the latter perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar processes occur within the cell membrane, and endosymbionts are extremely rare. They are usually much smaller than eukaryotic cells.
Prokaryotes have a single chromosome, contained within a nucleoid region rather than a membrane-bound nucleus, but may also have various small circular pieces of DNA called plasmids spread throughout the cell. Reproduction is exclusively Censored page, through binary fission, where the chromosome is duplicated and attaches to the cell membrane, and then the cell divides in two. However, they show a variety of parasexual processes where DNA is transferred between cells, such as transformation and transduction.
Evolution of prokaryotes
Main article evolution of prokaryotes
It is generally accepted that the first living cells were some form of prokaryote, and they are known as fossils from over 3.5 billion years ago. Some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes, but this is extremely doubtful.
After the first prokaryotes evolved, they subsequently have had an explosion of diversification during the ages. The metabolism of prokaryotes is the most diverged and cause some prokaryotes to be very different from each other.
Monera - previously Prokaryota were a Kingdom with divisions of eubacteria and archaebacteria.
This article or image contains material from the Science Primer http://www.ncbi.nlm.nih.gov/About/Primer published by the NCBI, which, as a US government publication, is in the public domain  http://www.ncbi.nlm.nih.gov/About/disclaimer.html .