Lecture #24—Prokaryotes
Prokaryotes are the earliest fossils we have. They appear as microfossils 4.3 billion years ago and they are beautifully preserved as blue green bacteria living as part of stromatolites dated at 3.4 BYA.
Prokaryotes are the world’s smallest and simplest cells. They have only one circular chromosome and lack a nuclear membrane. There are two types of prokaryotes: Eubacteria (=bacteria) and Archaebacteria (=archaea). Both are similar looking but their biochemistry is different. E.g. The transcription process in bacteria is blocked by antibiotics but not in archaea.
Archaea include species that live in unusually harsh environments (e.g. salt lakes, hot springs) and are called extremophiles. They appear to be more closely related to eukaryotes than bacteria. The latter which appear to have arisen earlier in time in an anaerobic (lacking O2) environment.
Prokaryotes live freely and as mutualists, parasites, and commensals in and upon the human body.
Biofilm consists of bacteria that attach to a hard surface and once their numbers reach a critical size (quorum sensing) they began secreting a polysaccharide, protein and extra cellular DNA slime. Other organisms (e.g. yeasts, protozoa, algae) can join the complex. Once the biofilm is mature, individual bacteria break away from the matrix and can establish new colonies. Biofilm is resistant to antibiotics and chemicals which cannot penetrate the matrix. Dentures and mouth-guards, are a prime location for biofilm growth. Biofilms have been associated with infection in contact lenses, catheters, heart valves, pacemakers, and artificial joints.
Five stages of biofilm development: (1) Initial attachment, (2) Irreversible attachment, (3) Maturation I, (4) Maturation II, and (5) Dispersion. Each stage of development in the diagram is paired with aphotomicrographof a developingP. aeruginosabiofilm. All photomicrographs are shown to the same scale. D. Davis-From: D. Monroe. "Looking for Chinks in the Armor of Bacterial Biofilms".PLoS Biology5(11, e307).DOI:10.1371/journal.pbio.0050307.}
Prokaryotes reproduce by binary fission (the cell simply splitting in two) not by mitosis. They have a high mutation rate because their reproduction can be rapid (every 20 minutes). Prokaryotes do not go through meiosis or traditional fusion of games associated with sex. But they do exchange genetic material by several mechanisms increasing their variability.
Bacterial Transformation—a bacterium picks up naked DNA from the environment and incorporates it into its own genome. This can occur between members of the same species or different species.
Bacterial Transduction—a virus transfers DNA from one bacterium to another which incorporates that into its genome. This can occur between members of the same species or different species.
Bacterial Conjugation—two different bacteria join together by way of a thin tube (a pilus) and extrachromosomal DNA (plastid) is passed from one to another. This can occur between members of the same species or different species.
Horizontal Gene Transfer: this occurs when genes are passed between different species. Thus, evolution in prokaryotes is not only via the traditional linear transfer of genes by inheritance but by this mechanism as well. It makes the sorting of family trees difficult.
OXYGEN REVOLUTION: this is the event in geological history when Earth changed from an anaerobic world to an aerobic world (one with lots of O2) This occurred over millions of years because photosynthesis evolved. But O2 being toxic to most prokaryotes caused their extinction except in a few environments.
Some bacteria developed ways of detoxifying the O2; one of which was to use the O2. These were aerobic bacteria. Some became mutualistic with others and specialized into structures we call mitochondria.
Different Types of Energy Capture: What they suggest about evolution.
Heterotophism. This is where an organism takes organic molecules from the environment and breaks them down to get energy (ATP). This might have happened if life originated in organic soup. Many current-day bacteria rely exclusively on this method of metabolism and most organisms in the world have such enzyme steps that are anaerobic and release energy. This suggests it evolved early. Note: the end step in the process is a 3 carbon compound (pyruvic acid). Different organisms have different ways of dealing with this compound; if they are anaerobic they excrete it or convert it to another compound first. Also, note that very little energy has been produced.
Chemotrophism (chemosynthesis). Organism captures the energy from inorganic chemical reactions. Some people believe the first prokaryotes followed this route.
Simple Photosynthesis as seen in green bacteria.
Notice how similar this reaction is to the Chemosynthesis above.
Complex Photosynthesis as seen in Blue-green bacteria.
Uses two photosystems to capture light
Complex photosynthesis in plants.
Uses two photosystems & two chlorophylls, cha & chb
Terms/Concepts to Define
Oxygen Revolution
Prokaryotes
Eubacteria
Archaebacteria
Archaea
Extremophiles: Methanogens, Halophiles, Thermophiles
Anaerobic
Aerobic
Binary fission
Bacterial transformation
Bacterial transduction
Bacterial conjugation
Plasmid
Pilus (pili)
Horizontal gene transfer
Heterotrophic
Autotrophic
Chemosynthesis (Chemotrophism)
Photosynthesis (Phototrophism)
Glycolysis
Hydrothermal vent
Biofilm
Can you answer these questions?
1. Give two differences between Eubacteria and Archaebacteria.
2. Distinguish between prokaryotes and eukaryotes.
3. Draw a phylogenetic tree (a cladogram) showing the relationship between the three domains.
4. Why do we believe that prokaryotes originated in an anaerobic environment?
5. Compare how cells divide in prokaryotes vs. eukaryotes.
6. Along with mutation, prokaryotes increase their genetic variability in three other ways. Name them and explain their mechanisms.
7. Horizontal gene transfer is said to greatly speed up evolution. Why is this so?
8. What are the major ways that prokaryotes capture energy? Explain their mechanisms.
9. Why do we believe that the first organisms on Earth might have used heterotrophic nutrition?
10. Describe a plausible sequence for the evolution of photosynthesis.