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Archaea are prokaryotes, cells without a nucleus, that form a domain of life separate from Bacteria and Eukarya. Carl Woese and George Fox recognized them in 1977 by comparing ribosomal RNA sequences, and Woese, Otto Kandler and Mark Wheelis proposed the three-domain system in 1990. Archaea resemble bacteria in cell size and organization but resemble eukaryotes in much of their machinery for transcription and translation, and they have membrane lipids found in no other domain. They include methanogens, extreme halophiles and hyperthermophiles, and they are also common in ordinary soils and oceans.
Build a three-column table comparing Bacteria, Archaea and Eukarya for nucleus, membrane lipids, cell wall, RNA polymerase, histones, the first amino acid used in translation and sensitivity to antibiotics. Then place a handful of named organisms, such as Methanobrevibacter in the human gut and Halobacterium in salt ponds, into habitats on a map of the Earth.
For most of the twentieth century, biologists divided life into prokaryotes and eukaryotes. Prokaryotes were all lumped together as bacteria. In the 1970s Carl Woese and his colleagues began comparing the sequences of small-subunit ribosomal RNA, a molecule present in every cell. In 1977 Woese and George Fox reported that certain methane-producing microbes differed from ordinary bacteria as much as either group differed from eukaryotes. In 1990 Woese, Otto Kandler and Mark Wheelis proposed three domains: Bacteria, Archaea and Eukarya.
Archaea look like bacteria under a microscope. They are small, they lack a nucleus and membrane-bound organelles, and many have a single circular chromosome. But their chemistry is distinct.
Their membranes are unique. Bacterial and eukaryotic membrane lipids have straight fatty acid chains attached to glycerol by ester bonds. Archaeal lipids have branched isoprenoid chains attached by ether bonds, and the glycerol has the opposite orientation. Some archaea have lipids long enough to span the whole membrane, forming a single layer instead of a bilayer. These membranes stay intact at high temperatures and low pH.
Their cell walls lack peptidoglycan. Many archaea have a protein surface layer (S-layer), and some methanogens have a different polymer called pseudopeptidoglycan. As a result, penicillin and lysozyme do not affect them.
Their gene expression resembles that of eukaryotes. Archaeal RNA polymerase is a complex enzyme like eukaryotic RNA polymerase II. Archaea use a TATA-binding protein to start transcription. Many package DNA with histone proteins, and they begin translation with methionine, while bacteria use formyl-methionine.
Archaea are metabolically varied. Methanogens are strict anaerobes that produce methane. They live in swamps, rice paddies, the rumen of cattle and the human gut. Extreme halophiles, such as Halobacterium, thrive in water many times saltier than the sea, and some use a light-driven proton pump called bacteriorhodopsin. Hyperthermophiles live in hot springs and deep-sea vents. One strain of Methanopyrus kandleri has grown in the laboratory at 122 °C.
Many archaea are not extreme at all. DNA surveys show they are abundant in ocean water and soils. Some, the ammonia-oxidizing archaea, are major players in the nitrogen cycle.
Archaea also matter for the history of life. In 2015, DNA sequenced from deep-sea sediment near a hydrothermal vent field called Loki's Castle revealed the first members of a group later called the Asgard archaea, whose genes include many once thought unique to eukaryotes. Many biologists now think eukaryotes arose from within or close to this archaeal lineage.
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