Compare and contrast bacteria, Archaea, and eukarya. What's a domain?.com, .org, .edu are domain names. They can immediately classify a site into a general category. So canthe domains Bacteria, Archaea, and Eukayra.The DomainCarl Woese wasn't completely happy with the six-kingdom system. It didn't show that all four eukaryotekingdoms are more closely related to each other than to the two bacteria kingdoms. It also didn't showthat the two bacteria kingdoms are as different from each other as they are from the eukaryotekingdoms. To show these similarities and differences, Woese introduced a new taxon called the domain.He defined domain as a taxon higher than the kingdom.The Three-Domain SystemIn 1990, Woese and his colleagues proposed a new classification system containing three domains:Bacteria, Archaea, and Eukarya. As shown in Figure below, the Bacteria domain was formerly theEubacteria kingdom, and the Archaea domain was formerly the Archaebacteria kingdom. The Eukaryadomain includes all four eukaryote kingdoms: plants, animals, protists, and fungi. The three-domainsystem emphasizes the similarities among eukaryotes and the differences among eukaryotes, bacteria,and archaea. By using domains, Woese was able to show these relationships without replacing thepopular six-kingdom system.Relationship Between Three-Domain and Six-Domain Systems of ClassificationsDomains:BacteriaArchaeaEukaryaKingdoms:Eubacteria ArchaebacteriaPlantsAnimalsFungiProtistsThis diagram shows how the three-domain system of classification is related to the six-kingdom system.Both the Eubacteria and Archaebacteria kingdoms are raised to the level of domains (Bacteria andArchaea domains, respectively) in the three-domain system. The other four kingdoms make up the thirddomain (Eukarya domain).Archaea were first found in extreme environments. For example, they were found in the hot watergeysers in Yellowstone National park. Archaea have since been found in all of Earth's habitats. They arenow known to be present everywhere and in high numbers. They may contribute as much as 20 percentto Earth's total biomass.Woese's three-domain system was quickly adopted by many other biologists. There were some critics,however, who argued that the system put too much emphasis on the uniqueness of Archaea. Laterstudies confirmed how different Archaea are from other organisms. For example, organisms belogningto Archaea were found to differ from both Eukarya and Bacteria in the composition of their cellmembranes and the system they use for DNA replication. These differences convinced most critics thatthe three-domain system was justified. After its introduction in 1990, the three-domain system becameincreasingly popular. Within a decade of its introduction, it had largely replaced earlier classifications. How Are the Three Domains Related?Comparing ribosomal RNA base sequences, Woese and his colleagues also showed that organismsbelonging to Eukarya are more similar to Archaea than they are to Bacteria. Figure below is aphylogenetic tree based on their analysis. This tree places Archaea and Eukarya in the same clade. Itrepresents the hypothesis that Archaea and Eukarya shared a more recent common ancestor with eachother than either shared with Bacteria.Phylogenetic Tree of LifeBacteriaArchaeaEukaryaGreenFilamentousbacteriaSpirochetesEntamoebae Slime AnimalsmoldsFungiGrampositivesMethanosarcinaMethanobacteriumHalophilesProteobacteriaPlantsMethanococeusCyanobacteriaCiliatesT.celerThermoproteusPyrodicticumPlanctomyces.FlagellatesBacteroidesCytophagaTrichomonadsMicrosporidiaThermotogaDiplomonadsAquifexThis phylogenetic tree is based on comparisons of ribosomal RNA base sequences among livingorganisms. The tree divides all organisms into three domains: Bacteria, Archaea, and Eukarya. Humansand other animals belong to the Eukarya domain. From this tree, organisms that make up the domainEukarya appear to have shared a more recent common ancestor with Archaea than Bacteria.The results of a study published in 2007 seem to conflict with this hypothesis. Comparing DNA basesequences, the 2007 study suggested that the domain Archaea may be older than either Bacteria orEukarya. That would make Archaea the most ancient group of organisms on Earth. It is not yet known,which, if either, hypothesis is correct. Scientists need to learn more about Archaea and theirrelationships with other organisms to resolve these questions.The Future of ClassificationThe three-domain system is unlikely to be the final word on classification. The system is based on thecurrent state of knowledge. As knowledge increases, the three-domain system may need revision. Forexample, the number of domains may change as scientists learn more about those life forms wecurrently know little about.A recent discovery illustrates this point. In 2003, scientists identified a new virus called mimivirus. Itresembles bacteria in size and number of genes. However, the virus cannot respond to stimuli or growby cell division, both of which are traits of bacteria and other living organisms. Mimivirus' uniquecombination of traits seems to place it at the boundary between living and nonliving things. Somescientists think mimivirus might represent a new domain of life.

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Bacteria , Archaebacteria and Eukarya : These are three different domains . 
Archaebacteria : These…