Chapter 26 - Chapter 26 How Did the Living World Begin to...

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Chapter 26 How Did the Living World Begin to Diversify? All life is in one of three domains : Bacteria – prokaryotes Archaea – prokaryotes Eukarya – eukaryotes All domains share some characteristics : conduct glycolysis replicate DNA semiconservatively DNA encodes polypeptides produce polypeptides by transcription and translation use the same genetic code have cytoplasm, plasma membranes and ribosomes All three domains had a single common ancestor. The common ancestor was prokaryotic Likely had a circular chromosome and structural genes organized into operons. Three domains are products of billions of years of natural selection. Prokaryotes were the only life-forms for billions of years. Present-day Archaea share more recent common ancestor with eukaryotes than with Bacteria old 5-kingdom system that put all prokaryotes together in “Monera” has been replaced now use three domain system How Did the Living World Begin to Diversify? All three domains are the result of billions of years of evolution and are well adapted to present- day environments None is “primitive” Earliest prokaryote fossils date back at least 3.5 billion years, and even then there was considerable diversity Where Are Prokaryotes Found? Prokaryotes are important to the biosphere:
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Some perform key steps in cycling of nitrogen, sulfur, and carbon Some trap solar energy and from inorganic chemical source Some help animals digest food Prokaryotes are found in every conceivable habitat on the Earth. Extremely hot, acidic, basic, and/or salty places In the presence of oxygen, or without oxygen At the bottom of the sea. In rocks more than 2 km into Earth’s solid crust. Certain shapes are common to prokaryotes: spheres, rods, and curved or spiral forms. Cocci (singular coccus ) are spherical, and live singly or in two- or three-dimensional arrays of chains, plates, or blocks Bacilli – rod-shaped; live in chains or singularly Spirilla – curved forms and live singly Spirochetes – corkscrew-shaped and live singly Nearly all prokaryotes are unicellular Even in chains or clusters, each individual cell is fully viable and independent Associations arise when cells adhere to each other after binary fission Chains are called filaments , which may be branching, or be enclosed in a tubular sheath Prokaryotes usually live in communities of different species, including microscopic eukaryotes. Microscopic organisms are sometimes referred to as microbes . Many microbial communities perform beneficial services, (e.g., digestion of our food, breakdown of municipal wastes). Many microbial communities form biofilms formed when cells contact a solid surface and excrete a gel-like polysaccharide matrix that traps other cells Bacteria in biofilms communicate with chemical signals Biologists are investigating ways to block the signals that lead to formation of the
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