ABSTRACT
Bacteria are small unicellular organisms that belong taxonomically to the domain Bacteria (or Eubacteria) in the kingdom Prokaryotae (also known as Prokaryota or Monera, which includes a second domain Archaea, or Archaebacteria for ancient bacteria). Both the domains Bacteria and Archaea appear to have evolved independently from an ancient common ancestor. With sizes ranging from 10−7 to 10−4 mm, prokaryotes are bigger than viruses (10−8 to 10−6 mm), but smaller than eukaryotes (10−5 to 10−3 mm). Indeed, eukaryotes may have arisen from ancient bacteria entering into endosymbiotic associations with the ancestors of eukaryotic cells (possibly related to the Archaea) to form either mitochondria or hydrogenosomes. A subsequent independent engulfment of cyanobacterial-like organisms by some mitochondria-containing eukaryotes may have led to the formation of chloroplasts in algae and plants.
