The Three Domain System

A domain is the highest taxonomic rank.There are three domains: Bacteria, Archaea and Eukarya.Bacteria are single-celled prokaryotes with no membrane-bound organelles, 70S ribosomes, murein (peptidoglycan) cell walls and a single loop of DNA with no histones. Their RNA polymerase contains 5 proteins.Archaea are single celled prokaryotes with their genes and proteins more similar to eukaryotes. They have 70S ribosomes, their membranes are made of branched hydrocarbon chains (not fatty acids) linked to glycerol by ether linkages, their cell walls do not contain murein and they have a more complex form of RNA polymerase than bacteria, which contains 8-10 proteins and is similar to that of eukaryotes.Eukarya are organisms with eukaryotic cells. They have 80S ribosomes, membrane bound organelles such as mitochondria and chloroplasts and cell walls, if any, do not contain murein(peptidoglycan). Their RNA polymerase contains 12 proteins.