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BIOL 1000
Paula Wilson

CHAPTER 21: PROKARYOTES The Full Extent of Prokaryote Diversity is Unknown 9/14/2012 6:32:00 PM - everything we know so far about prokaryotes is based on a tiny fraction of the total number of the species - we have only identified 6000 species, which is as low as 1% of the total number of species - there are entire habitats such as oceans (70% of Earths surface) for which we know almost nothing about the prokaryote inhabitants - we have only been able to identify so few species of prokaryotes because we have been able to make detailed studies only of prokaryotes that we can grow in culture vast majority of prokaryotes cannot grow on the media and conditions we can provide prokaryotes thrive in extreme conditions, and we cannot yet create favorable growth conditions in our labs Prokaryotes Make up Two of the Three Domains of Life: - two domains of living organisms, Archaea and Bacteria, consist of prokaryotes (the third domain, Eukarya, includes all eukaryotes) - bacteria are the prokaryotic organisms most familiar to us including those responsible for diseases of humans and animals - Archaea are not as well known as they were only discovered about 40 years ago - Archaea share some cellular features with eukaryotes and some with bacteria, but still have other features that are different - many Archaea live under very extreme conditions that no other organisms, including bacteria, can survive Prokaryote Structure and Function 9/14/2012 6:32:00 PM - prokaryotes are the smallest organisms in the world - few species are 1 to 2 micrometers long, from 500 to 1000 of them would fit side by side across the dot above this letter i". WHAT?!?!? THATS SO TINY! - despite their small size, they dominate life on Earth! - estimates of total prokaryote diversity are in the billions of species - estimates for their total collective mass (their biomass) on Earth exceeds that of animals and may be greater than that of all plant life - prokaryotes colonize every niche on Earth that supports life including surfaces and cavities of a healthy human body such as skin, mouth, nasal passages, etc. the bacteria in and on your body outnumber all the other cells in your body!!! - for about 3 billion years prokaryotes were the only forms of life on Earth and had time to diversity and expand into every habitat on Earth before the first eukaryotes appeared thus, it is not surprising that prokaryote diversity is greater than that of eukaryotes Prokaryotic Cells Appear Simple in Structure Compared with Eukaryotic Cells: - three cell shapes are common among prokaryotes: spiral coccoid (coccus = berry): spherical rods: cylindrical - some Archaea even have square cells (FIGURE 21.2 PAGE 467) MEMORIZE PROKARYOTE CELL DIAGRAM ON PAGE 468 FIGURE 21.3 At first glance, a typical prokaryotic cell seems much more simple than a eukaryotic cell - a cell wall and plasma membrane surrounding a cytoplasm with DNA concentrated in one region and ribosomes scattered throughout - the chromosome is packed into an area of the cell called the nucleoid it is not contained in a membrane-bound nucleus - prokaryotic cells have no organelles equivalent to the mitochondria, ER, or Golgi complex of eukaryotic cells - new microscopic techniques reveal that prokaryote cells do have a cytoskeleton not homologous to that of a eukaryote but serving some of the same functions and have a more sophisticated organization than previously thought i. Internal Structures: - genome of most prokaryotes consist of a single, circular DNA molecule, although some (very rare) have a linear chromosome Ex: causative agent of Lyme disease - many prokaryotes also contain small circles of DNA called plasmids (FIGURE 21.4 PAGE 468), which contain genes for nonessential but beneficial functions such as antibiotic resistance plasmids replicate independently of the cells chromosomes and can be transferred from one cell to another - the horizontal gene transfer allows antibiotic resistance and other traits to spread very quickly in bacterial populations - horizontal gene transfer also occurs when bacterial cells take up DNA from their environment (e.g. from other cells that have lysed) or when viruses transfer DNA from one bacterium to another Example: evidence indicates that a virus transferred toxin-encoding genes from Shigella dysenteriae (which causes bloody diarrhea) to E.coli, resulting in the deadly 0157:H7 strain. - like eukaryotic cells, prokaryotic cells contain ribosomes - Bacterial ribosomes are smaller than the eukaryotic ribosomes but carry out protein synthesis by essentially the same mechanisms as those of eukaryotes - Archaeal ribosomes resemble those of bacteria in size but differ in structure; protein synthesis in Archaea is a combination of bacterial and eukaryotic processes, with some unique archaeal features THUS, antibiotics that stop bacterial infections by targeting ribosome activity do not interfere with archaeal protein synthesis ii. Prokaryotic Cell Walls: - most prokaryotes have a cell wall that lies outside their plasma membrane - the primary component of bacterial cell walls is peptidoglycan: a polymer of sugars and amino acids, which forms linear chains - cell wall has great strength and rigidity Bacteria can be divided into two broad groups based on their reaction to the Gram stain procedure: traditionally used as the first step in identification of an unknown bacterium 1. Gram-positive cells 2. Gram-negative cells - cells are first stained with crystal violet, rinsed with ethanol, and then counterstained with safranin - some cells retain the crystal violet and appear purple when viewed under the microscope these = Gram-positive cells - in other bacteria, ethanol washes out the crustal violet from the cells, which are colorless until counterstained with safranin these = Gram- negative cells and appear pink under the microscope This is because gram-positive cells have a cell wall composed of a single thick peptidoglycan layer, whereas the Gram-negative bacteria has a thin peptidoglycan layer outside the plasma membrane AND an external outer membrane outer membrane protects the bacteria from harmful substances in the environment, thus that is why Gram-negative cells do not get stained therefore, Gram-negative cells are less sensitive to penicillin than Gram-positive cells - cell walls of some Archaea are assembled from a molecule related to peptidoglycan but with different molecular components and bonding structures - other have walls assembled from proteins or polysaccharides instead of peptidoglycan - Archaea vary in response to Gram stain, so this procedure is not useful in identification of Archaea - the cell wall of many prokaryotes is surrounded by a layer of polysaccharides known as capsule
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