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Summary of Chapter 20 Viruses Archea and Bacteria201 Viruses Viroids and PrionsViruses are noncellular while prokaryotes are fully functioning organisms All viruses have at least two parts an outer capsid composed of protein subunits and an inner core of nucleic acid DNA or RNAnever both Some also have an outer membranous envelopeViruses are obligate intracellular parasites that can be maintained only inside living things cannot multiply outside of cell such as a chicken egg or those propagated in cell tissue culture202 The Prokaryotes The bacteria domain Bacteria and archaea domain Archaea are prokaryotes Prokaryotic cells lack a nucleus and most of the other cytoplasmic organelles found in eukaryotic cells Prokaryotes reproduce asexually by binary fission Their chief method for achieving genetic variation is mutation but genetic recombination by means of conjugation transformation and transduction has been observed Prokaryotes differ in their need and tolerance for oxygen There are obligate anaerobes facultative anaerobes and aerobic prokaryotes Some prokaryotes are autotrophic and some are heterotrophic203 The BacteriaBacteria are the more prevalent type of prokaryote The classification of bacteria is still being developed The primary importance at this time is the shape of the cell and the structure of cell wall which affects Gram staining Bacteria occur in three basic shapes spiralshaped spirillum rodshaped bacillus and round coccusSome prokaryotes are autotrophiceither photoautotrophs photosynthetic or chemoautotrophs chemosynthetic Some photosynthetic bacteria cyanobacteria give off oxygen and some purple and green sulfur bacteria do not Chemoautotrophs oxidize inorganic compounds such as hydrogen gas hydrogen sulfide and ammonia to acquire energy to make their own food Surprisingly chemoautotrophs support communities at deepsea ventsMany bacteria are chemo heterotrophs aerobic heterotrophs and are saprotrophic decomposers that are absolutely essential to
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