<Tr> <Td> Metabolism </Td> <Td> Various, with methanogenesis unique to Archaea </Td> <Td> Various, including photosynthesis, aerobic and anaerobic respiration, fermentation, and autotrophy </Td> <Td> Photosynthesis, cellular respiration and fermentation </Td> </Tr> <Tr> <Td> Reproduction </Td> <Td> Asexual reproduction, horizontal gene transfer </Td> <Td> Asexual reproduction, horizontal gene transfer </Td> <Td> Sexual and asexual reproduction </Td> </Tr> <P> Archaea were split off as a third domain because of the large differences in their ribosomal RNA structure . The particular RNA molecule sequenced, known as 16S rRNA, is present in all organisms and always has the same vital function: the production of proteins . Because this function is so central to life, organisms with mutations of their 16S rRNA are unlikely to survive, leading to great stability in the structure of this nucleotide over many generations . 16S rRNA is also large enough to retain organism - specific information, but small enough to be sequenced in a manageable amount of time . In 1977, Carl Woese, a microbiologist studying the genetic sequencing of organisms, developed a new sequencing method that involved splitting the RNA into fragments that could be sorted and compared to other fragments from other organisms . The more similar the patterns between species were, the more closely related the organisms . </P> <P> Woese used his new rRNA comparison method to categorize and contrast different organisms . He sequenced a variety of different species and happened upon a group of methanogens that had vastly different patterns than any known prokaryotes or eukaryotes . These methanogens were much more similar to each other than they were to other organisms sequenced, leading Woese to propose the new domain of Archaea . His experiments showed that the Archaea were more similar to eukaryotes than prokaryotes, even though they were more similar to prokaryotes in structure . This led to the conclusion that Archaea and Eukarya shared a more recent common ancestor than Eukarya and Bacteria in general . The development of the nucleus occurred after the split between Bacteria and this common ancestor . Although Archaea are prokaryotic, they are more closely related to Eukarya and thus cannot be placed within either the Bacteria or Eukarya domains . </P>

Archaebacteria (archaea) and the origin of the eukaryotic nucleus