Organismal Biology #13
PROCARYOTES
Performance Objectives:
Procaryotes are one-celled organisms whose cells lack a true (membrane-bounded) nucleus and other eucaryotic organelles. Procaryotes include the Archaebacteria, true bacteria, and Cyanobacteria.

Procaryotic cells: Cells without true nuclei, lacking many other structures found in eucaryotic cells.

Procaryotic cell walls: Contain substances like muramic acid, absent in eucaryotes.
  • Gram-negative cell walls: two membranes, separated by a thin layer of peptidoglycan.
  • Gram-positive cell walls: one membrane, surrounded by a very thick peptidoglycan layer.

    Chemical diversity: Procaryotes have greater chemical diversity than eucaryotes. They can subsist on a greater variety of foodstuffs, have a greater range of chemical substances that can be tolerated, and can subsist in a variety of atmospheres, both with and without oxygen.
  • Energy sources:   Phototrophic (sunlight) vs. Chemotrophic (chemical energy)
  • Carbon sources:   Autotrophic (inorganic carbon incl. CO2) vs.
          Heterotrophic (organic molecules, mostly from other organisms)
  • Anaerobic (oxygen-intolerant) vs. Aerobic (oxygen-dependent) vs. Facultative (OK either way)

    Cell shapes: Commonly "rod"-shaped, but many are spherical ("cocci").
          Less common shapes: bent rods, pear-like, gentle spirals, corkscrews ("spirochete")

    Procarytoic chromosomes: Generally arranged in a single circular loop containing DNA but no histone proteins. Partial recombination may occur during conjugation. Most procaryotes also have small chromosome fragments that can detach from the main chromosome and exist separately for long periods as plasmids, small, circular samples of DNA similar to certain viruses.

    Archaea (Archaebacteria): A group of strict anaerobes (killed by oxygen) that include the methane-producers (methanogens), the extreme halophiles, and the extreme thermophiles. Their RNA sequences have only minimal homology to the RNA of other procaryotic or eucaryotic organisms, and the cell walls are also unique.

    True bacteria: The majority of procaryotes, with RNA sequences homologous to those of Cyanobacteria and eucaryotes (but not Archaebacteria). Most are heterotrophs. A few autotrophs use a variety of energy sources, but none contains chlorophyll a and none can split water in the Hill reaction.

    Cyanobacteria (= Cyanophyta, blue-green bacteria, or blue-green algae): All are similar to bacteria in structure and their RNA sequences are homologous. All are oxygen-tolerant autotrophs that can use sunlight for energy and CO2 as a carbon source. They contain chlorophyll a and can split water in the Hill reaction.


    A possible classification of Bacteria:
    • Proteobacteria (includes the majority of bacteria):
      • "Alpha proteobacteria":   includes Rhizobium and other symbiotic bacteria
      • "Beta proteobacteria":   includes Nitrosomonas and other soil bacteria
      • "Gamma proteobacteria":   a great variety, incl. sulfur bacteria, Escherichia coli,
            and many pathogenic species (e.g., Salmonella, Legionella, Vibrio)
      • "Delta proteobacteria":   slime-secreting bacteria (myxobacteria) and bacteria
            that attack other bacteria (Bdellovibrio, etc.)
      • "Epsilon proteobacteria":   includes mostly pathogens (Campylobacter, Helicobacter, etc.)
    • Chlamidias: obligate energy parasites that can only survive inside animal cells
    • Spirochetes: motile, corkscrew-shaped bacteria
    • Gram-positive bacteria: a great variety of mostly pathogenic species (Bacillus, Clostridium, Streptococcus, Staphylococcus, Streptomyces, Mycoplasma)
    • Cyanobacteria: photoautotrophs possessing chlorophyll a



    Viruses are fragments of nucleic acid (DNA or RNA, never both), often surrounded by protein, that can replicate only with the help of intact cells. (A few viruses also have capsules derived from host cell membranes.)
    • All viruses have a lytic cycle, in which they invade a cell, replicate inside, then rupture the cell and release their progeny.   Some viruses also have a lysogenic cycle, in which they lie dormant and replicate as part of the host DNA.
      • Lytic cycle (in all viruses):
        1. Virus first attaches to host cells and injects nucleic acid only.
        2. Viral DNA or RNA is replicated using the host cell's enzymes.
        3. Host cell ruptures, releasing thousands of new virus particles.
      • Lysogenic cycle (in some viruses only):
        1. Viral DNA inserts into the host cell chromosome.
        2. Virus then hides (lysogenic stage), replicating as part of host DNA.
        3. Upon "activation," the virus takes over the cell's reproductive machinery and resumes the lytic cycle.
    • Viral shapes can be helical, icosahedral (20-sided), or complex (with head and tail).
    • Viruses have very few of the characteristics of life; they cannot reproduce without the gene-replicating machinery of the host cell.
    • Viruses are classified by the the type of nucleic acid (either DNA or RNA).


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