Chapter 28 Prokaryotes The First Cells Microfossils are fossilized forms of microscopic life Oldest are 3 5 billion years old Main formations in S Africa and western australia in cratons rock layer of undisturbed continental crust Seem to resemble present day prokaryotes Stromatolites are mats of cyanobacterial cells that trap mineral deposits Oldest are 2 7 billion years old Modern forms are also known Living things are selective in the carbon isotopes used Living things incorporate carbon 12 level of carbon 12 than nonliving things Isotopic analysis of carbon 12 in fossils suggests that carbon fixation was active as much as 3 8 BYA carbon fixation occurs during photosynthesis Biomarkers o Organic molecules of biological origin o Proven difficult to find o Hydrocarbons derived from fatty acid tails of lipids were found in ancient rocks Indicates that cyanobacteria are at least 2 7 billion years old o Carbon isotope ratios in ancient rocks may push the origin of life back before 3 5 BYA Prokaryotic Diversity Oldest structurally simplest and most abundant forms of life Abundant for over a billion years before eukaryotes Less than 10 of species are known Fall into 2 domains Bacteria also called Eubacteria Archaea formerly called Archaebacteria Many archaeans are extremophiles Characteristics of Prokaryotes Unicellularity o Most are single celled o May stick together to form associations o Some can form complex biofilms Cell size Chromosome o Size varies o Most are less than 1 mm in diameter o Single circular double stranded DNA o Found in the nucleoid region of cell no nucleus o Often have plasmids Cell division o Most divide by binary fission Genetic recombination o Occurs through horizontal gene transfer o NOT a form of reproduction Internal compartmentalization o No membrane bounded organelles o No internal compartment o Ribosomes differ from eukaryotic form Flagella o Simple in structure o Different from eukaryotic flagella Metabolic diversity o Oxygenic and anoxygenic produce sulfur not oxygen photosynthesis o Chemolithotrophic use energy stored in bonds of inorgainic molecules o Heterotophic o Bacteria VS Archaea Plasma membrane o All prokaryotes have a plasma membrane o Bacterial lipids are unbranched Use ester bonds o Archaean membranes are formed on glycerol skeleton with ether linkages not ester Hydrocarbons may be branched or have rings Tetraether polymer allows extremophiles to withstand high temperatures Cell wall o All prokaryotes have cell walls o Bacteria have peptidoglycan o Archaea lack peptidoglycan DNA replication Gene Expression o Both have single replication origin o Archaeal DNA replication is more similar to that of eukaryotes o Archaeal transcription and translation are more similar to those of eukaryotes Early Classification Characteristics Relied on staining characteristics and observable phenotypes 1 Photosynthetic or nonphotosynthetic 2 Motile or nonmotile 3 Unicellular colony forming or filamentous 4 Shape and arrangement 5 Formation of spores 6 Importance as human pathogens or not 7 Nutritional requirements biochemical activities Molecular Classification 1 Amino acid sequences of key proteins 2 Percent guanine cytosine content 3 Nucleic acid hybridization a Closely related species will have more base pairing 4 Gene and RNA sequencing a Especially rRNA 5 Whole genome sequencing Based on these molecular data several prokaryotic groupings have been proposed Bergey s Manual of Systematic Bacteriology 2nd edition 2001 o 3 of 5 volumes completed Large scale sequencing of random samples indicates vast majority of bacteria have never been cultured or studied in detail Cell wall Prokaryotic Cell Structure 3 basic shapes o Bacillus Rod shaped o Coccus Spherical o Spirillum Helical shaped o Vibio comma shaped o Peptidoglycan forms a rigid network Maintains shape Withstands hypotonic environments Archaea have a similar molecule o Gram stain most important stain in bacteriology Gram positive bacteria have a thicker peptidoglycan wall and stain a purple color Gram negative bacteria contain less peptidoglycan and do not retain the purple colored dye retain counterstain and look pink Gram negative bacteria o Lipopolysaccharide LPS lipid portion called endotoxin released when bacteria die o Endotoxin causes fever and in high amounts results in endotoxic shock severe drop in blood pressure death may occur Bacterial cell wall target of many antibiotics penicillin Antibiotics target features of prokaryotic cells that are not found in eukaryotic cells therefore less toxic to patient o S layer o Rigid paracrystalline layer found in some bacteria and archaea o Outside of peptidoglycan or outer membrane layers in gram negative and gram positive bacteria Capsule o Diverse functions often involves adhesion or protection o Gelatinous layer found in some bacteria o Protects from the immune system inhibits phagocytosis by host s white blood cells o Slime layer more diffuse layer of polysaccharides on the outside makes bacteria virulence factor sticky adherence Flagella o Slender rigid helical structures o Composed of the protein flagellin o Involved in locomotion spins like propeller Fimbria not in book o Short hairlike structures o Attachment o Found in gram negative bacteria o Pili o Long tubular structures o Conjugation Endospores o Develop a thick wall around their genome and a small portion of the cytoplasm o When exposed to environmental stress o Highly resistant to environmental stress Especially heat low nutrient level o When conditions improve can germinate and return to normal cell division o Genus Bacillus and Clostridium Bacteria causing tetanus botulism and anthrax Nucleoid region o Contains the single circular chromosome o May also contain plasmids small circles of DNA may contain genes for antibiotic resistance or toxins Ribosomes o Smaller than those of eukaryotes o Differ in protein and RNA content o Target for antibiotics Prokaryotic Genetics Prokaryotes do not reproduce sexually o Inclusions or granules store excess nutrients or inorganic substances 3 types of horizontal gene transfer o Conjugation cell to cell contact o Transduction by bacteriophages o Transformation from the environment All 3 processes also observed in archaea
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