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18.4 – Bacteria and Archaea Prokaryotes are widespread on earth Prokaryotes – single-celled, no membrane-bound nucleus or organelles, smaller than eukaryotic cells, grouped based on their need for oxygen Obligate anaerobes – prokaryotes that cannot live in the presence of oxygen, ex: archaea prokaryotes that produce methane gas (live in marshes, bottom of lakes, and in digestive tracts of herbivores) Obligate aerobes – organisms that need oxygen in their environment, ex: microorganisms that cause tuberculosis and leprosy Facultative aerobe – prokaryote that can survive whether oxygen is present in the environment or not Bacteria & Archaea are structurally similar (single-celled prokaryotes), but have different molecular characteristics Domain Bacteria & Domain Archaea contain ALL known prokaryotes Domain Bacteria is more widespread and diverse Domain Archaea can survive in extreme environments Structural comparisons: o Both archaea and bacteria are small, single-celled organisms that have cell walls and plasma membranes o Archaea come in many shapes, while most bacteria fall into 3 shapes: Rod-shaped: bacilli Spiral-shaped: spirilla or spirochetes Spherical: cocci o DNA is in the form of a circle and is surrounded by cytoplasm o May have plasmids – small pieces of genetic material that can replicate separately from the main chromosome o Most prokaryotes can move on their own by gliding or using flagella Flagellum – long, whip-like structure outside of a cell that is used for movement o May also have pili – thinner, shorter structures that allow prokaryotes to stick to surfaces or other prokaryotes Molecular comparisons: o Bacteria and archaea are not closely related o Archaea membranes contain lipids that are not found anywhere else on earth o Bacteria cell walls contain polymers called peptidoglycan Amount of peptidoglycan in cell walls is an important characteristic of bacteria Gram stain Gram positive – thicker peptidoglycan layer, stains purple Gram negative – thinner peptidoglycan layer, stains red Bacteria have various strategies for survival Gene exchange: o Prokaryotes reproduce via asexual reproduction o Binary fission – chromosome copies itself, cell grows and splits, taking a full set of genes with it o Conjugation – prokaryotes exchange parts of their chromosomes through a hollow pili bridge Surviving harsh conditions: o Some bacteria can produce an endospore – a specialized cell with a thick, protective wall