Bacteria & Archaea (Empire Prokaryota)
Prokaryotes are typically unicellular organisms that lacks a nuclear membrane-enclosed nucleus, lack mitochondria, or any other eukaryotic membrane-bound organelles. Some prokaryotes can form large colonies or have multicellular stages in their life cycles. Prokaryotes are asexual, reproducing without fusion of gametes, and can transfer genes horizontally. The DNA of most bacteria is found in a single, circular chromosome, and is distributed throughout the cytoplasm rather than contained within a membrane-enclosed nucleus.
Bacteria first arose on Earth approximately 4 billion years ago.
Archaea include organisms that can live in the most extreme environments on Earch. Some live at high-temperature or in extremely hypersaline, alkaline, or acid waters, or digestive systems. They are abundant in the sea plankton.
Archaea and eukaryotes lack peptidoglycan in their cell walls and have multiple RNA polymerases that contain multiple polypeptides. Bacteria contain a simple RNA polymerase consisting of four polypeptides. Archaea have ether bonds connecting fatty acids to molecules of glycerol, and no archaea have been discovered that contain ester-linked lipids produced by bacteria and eukaryotes. Various features of protein synthesis in the archaea are similar to those of eukaryotes but not of bacteria. Various types of metabolism exist in both archaea and bacteria that do not exist in eukaryotes, including nitrogen fixation, denitrification, chemolithotrophy, and hyperthermophilic growth. The production of methane as a metabolic by-product occurs only in Archaea. Classical photosynthesis using chlorophyll has not been found in any archaea.
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