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Ch. 27 (Prokaryotes and Cell Wall (Prokaryotes and Motility (Eukaryote and…

- - - - The cells of prokaryotes are simpler than those of eukaryotes in their internal structure and physical arrangement of their DNA.
      - Prokaryotic cells lack the complex compartmentalization associated with the membrane enclosed organelles in eukaryotic cells.
      - Some prokaryotic cells have membranes that perform metabolic functions.
      - The membranes are infoldings of the plasma membrane.
      - Prokaryotes have circular chromosomes, while eukaryotes have linear chromosomes.
      - In prokaryotes the chromosome associated with fewer proteins than are the chromosomes of eukaryotes.
      - Prokaryotes lack a nucleus, the chromosome is in the nucleoid, a part of cytoplasm that is not enclosed by a membrane.
      - A typical prokaryotic cell may also have smaller rings of independently replicating DNA molecules called plasmids.
      - Reproduction and Mutation
        
        Prokaryotes can reproduce quickly in favorable environments.
        
        By binary fission, a single prokaryotic cell divides into 2 cells and then divides into 4, 8, 16 and so on.
        
        Prokaryotes can divide every 1-3 hours, while some species can produce a new generation in 20 minutes.
        
        Prokaryotic species potential for rapid population growth emphasizes key features of their biology: small, reproduce by binary fission and have short generation times.
        
        Prokaryotes do not reproduce sexually.
        
        Escherichia coli (E. coli) is a bacteria that reproduces by binary fission in a human intestine.
        
        After repeated rounds of division, most of the offspring cells are identical to the parent cell.
        
        If errors occur in DNA replication, some of the offspring cells differ genetically.
        
        New mutations can increase genetic diversity quickly in species with short generation times and large populations.
        
        Genetic Recombination
        
        Genetic recombination is the combining of DNA rom two sources.
        
        In eukaryotes, the sexual processes of meiosis and fertilization combine DNA from two individuals in a single zygote.
        
        Meiosis and fertilization does not occur in prokaryotes.
        
        Transformation, transduction and conjugation can bring prokaryotic DNA from different cells.
        
        The. moment of genes from one organism to another is horizontal gene transfer.
        
        Transformation and Transduction
        
        Transformation is the genotype and possibly phenotype of a prokaryotic cell are altered by the uptake of foreign DNA from its surroundings.
        
        Streptococcus pneumoniae can be transformed into pneumonia causing cells if the cells are exposed to DNA from a pathogenic strain.
        
        In transduction, phages carry prokaryotic genes from one host cell to another.
        
        Transduction results from accidents that occur during the phage replicative cycle.
        
        A virus that carries prokaryotic DNA may not be able to replicate because it lacks some or all of its own genetic material.
        
        The virus can attach to another prokaryotic cell and inject prokaryotic DNA acquired from the first cell.
        
        If DNA is then incorporated into the recipient cell's chromosome by crossing over, a recombinant cell is formed.
- - - - Sometimes mutation in a chromosomal gene of the pathogen can confer resistance.
      - A mutation in one gene may take it less likely that the pathogen will transport an antibiotic into its cell.
      - Mutation in another gene can alter the intracellular target protein for an antibiotic molecule, reducing its inhibitory effect.
      - Sometimes bacteria has resistance genes which code for enzymes that destroy or hinder the effectiveness of certain antibiotics.
      - The resistance genes are often carried by plasmids known as R (R for resistance) plasmids.
      - Exposing a bacterial population to a specific antibiotic will kill antibiotic sensitive bacteria but not those that happen to have R plasmids with genes that counter the antibiotic.
      - Many R plasmids have genes that encode pili that enable DNA transfer from one bacterial cell to another by conjugation.
      - Some R plasmids carry genes for resistance to as any as ten antibiotics.
      - Nutritional Modes
        
        Organisms that obtain energy from light are known as phototrophs.
        
        Those that obtain energy from chemicals are called chemotrophs.
        
        Organisms that need only CO2 or related compounds as a carbon source are called autotrophs.
        
        Heterotrophs require at least one organic nutrients, like glucose, to make other organic compounds.
        
        The Role of Oxygen in Metabolism
        
        Prokaryotic metabolism varies with respect to oxygen.
        
        Obligate aerobes must use oxygen (O2) for cellular respiration and cannot grow without it.
        
        Obligate anaerobes are poisoned by O2.
        
        Some obligate anaerobes live exclusively by fermentation; others extract chemical energy by anaerobic respiration I which substances, like nitrate ions and sulfate ions, accept electrons at the "downhill" end of electron transport chains.
        
        Facultative anaerobes use O2 if its present but can also carry out fermentation or anaerobic respiration in an anaerobic environment.
        
        Nitrogen Metabolism
        
        Nitrogen is essential for the production of amino acids and nucleic acids in all organisms.
        
        Eukaryotes can obtain nitrogen from a limited group of nitrogen compounds and prokaryotes can metabolize nitrogen in many different forms.
        
        Some cyanobacteria and some methanogens conversation atmospheric nitrogen (N2) to ammonia (NH3) by nitrogen fixation.
        
        The cells can incorporate the fixed nitrogen into amino acids and other organic molecules.
        
        Metabolic Cooperation
        
        Cooperation between prokaryotic cells allows them to use environmental resources they could not use as individual cells.
        
        Cooperation takes places between specialized cells of a filament.
        
        Cyanobacteria Anabaena has genes that encode proteins for photosynthesis and for nitrogen fixation.
        
        A single cell cannot carry both processes at the same time because photosynthesis produces O2, which inactivates the enzymes involved in nitrogen fixation.
        
        Instead of living as isolated cells, Anabaena forms filamentous chains.
        
        Most cells in a filament carry out only photosynthesis, while a few specialized cells called heterocyst carry out only nitrogen fixation.
        
        Heterocyst are surrounded by a thickened cell wall that restricts entry of O2 produced by neighboring photosynthetic cells.
        
        Metabolic cooperation between different prokaryotic species often occurs in surface-coating colonies known as biofilms.
        
        Cells in a biofilm secrete signaling molecule that recruit nearby cells, causing the colonies to grow.
        
        The cells produce polysaccharides and proteins that stick the cells to the substrate and to one another.
        
        Channels in the biofilm allow nutrients to reach cells in the interior and wastes to be expelled.
        
        Biofilms can cause problems by contaminating industrial products and medical equipment and contributing tooth decay and more serious health problems.
- - - - The prokaryotic CRISPR-Cas system helps bacteria and archaea defend against attack by viruses, has developed into a new tool for altering genes in an organism.
      - The genomes of many prokaryotes contain short DNA repeats called CRISPRs that interact with Can proteins.
      - Cas proteins act together with "guide RNA" made from the CRISPR region, can cut any DNA sequence to which they are directed.
      - Scientists have been able to exploit this system by introducing a Car protein t guide RNA into cells whose DNA they want to alter.
      - The CRISPR-Cas9 system has opened new lines of research on HIV.
      - Care must be taken to guard against the unintended consequences that could arise when applying technology.
      - Research also seek to reduce the use of petroleum and other fossil fuels by engineering bacteria that can produce ethanol.
      - Another way to harness prokaryotes is by bioremediation, the use of organisms to remove pollutants from soil, air and water.
      - One bioremediation application includes cleaning up oil spills and precipitating radioactive material out of ground water.
- - - - Ecosystems depend on the continuing recycling of chemical elements between the living and nonliving components of the environments and prokaryotes play a major role in the process.
      - Some chemoheterotrophic prokaryotes function as decomposers by breaking down dead organisms and waste products and unlocking supplies of carbon, nitrogen, and other elements.
      - Prokaryotes also convert some molecules to forms that can be taken up by other organisms.
      - Cyanobacteria and other autotrophic prokaryotes use CO@ to make organic compounds and are passed through food chains.
      - Cyanobacteria also produce atmospheric O2, and a variety of prokaryotes fix atmospheric nitrogen into forms that other organisms can use to make the building blocks of proteins and nucleic acids.
      - Prokaryotes can increase the availability of nutrients that plants require for growth.
      - Prokaryotes can also decrease the availability of key plant nutrients .
      - Ecological Interactions
        
        Symbiosis, an ecological relationship in which two species live in close contact with each other.
        
        Prokaryotes often form symbiotic associations with much larger organisms.
        
        The larger organism in a symbiotic relationship is known as the host and the smaller is the symbiont.
        
        There are cases in which a prokaryote and its host participate in mutualism, an ecological interaction between two species in which both benefit.
        
        Other interactions take the form of commensalism, an ecological relationship in which one species benefits whole the other is not harmed or helped in any significant way.
        
        Some prokaryotes engage in parasitism, en ecological relationship in which a parasite eats the cell contents, tissues or body fluids of its host.
        
        Parasites harm but do not kill their host immediately.
        
        Parasites that cause disease are called pathogens, many of which are prokaryotic.