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Micro Topic 2 - Topic 2 THE PROKARYOTIC CELL(chapter 4 a...

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Topic 2. THE PROKARYOTIC CELL (chapter 4) a) All living things are classified as prokaryotes (before nucleus) or eukaryotes (true nucleus). 1. OVERVIEW OF PROKARYOTES a) All prokaryotes are single-celled organisms and all are bacteria. b) The DNA is a single, circular strand with no nuclear membrane. c) There may be extrachromosomal DNA in the form of plasmids. d) There are no histones in the DNA and no nucleolus. e) There are no membrane-bound organelles. f) The cell wall is composed of peptidoglycan. g) There are no cilia, but conjugation pili may be present. h) Cell division is by binary fission. 2. SIZE, SHAPE AND ARRANGEMENT OF BACTERIAL CELLS (Figures 4.1 - 4.5 pg 76-78). a) Most bacteria are 0.2-2.0 μ m in diameter (a red blood cell is 7.5 μ m in diameter) and 2-8 μ m in length. b) They occur in 3 basic shapes: a. coccus (round), b. bacillus (rod-shaped), c. spiral c) Different arrangements of these shapes: diplococci, streptococci, tetrads, sarcinae, staphylococci, diplobacillus, streptobacillus and coccobacillus. d) Usually bacteria are monomorphic (one shape) but some are pleomorphs (have several different shapes). e) More rare shapes include stellate bacteria (star shaped), vibrios (comma shaped), square bacteria (found in the Red Sea 1981) and triangular bacteria (discovered in 1986). 3. STRUCTURES EXTERNAL TO CELL WALL A) GLYCOCALYX a) Gelatinous protective polysaccharide and /or polypeptide surrounding cell wall. a. Called a capsule if attached to cell wall. i. Capsules may protect bacteria from phagocytosis. 1
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ii. Capsules can thus enhance pathogenicity e.g. encapsulated strains of Streptococcus pneumoniae cause pneumonia, while non-encapsulated strains do not. b. Called a slime layer if not attached to cell wall. i. Usually thinner and less well attached than capsules. ii. Protects the cell against desiccation, helps traps nutrients and allows cells to adhere together. B) FLAGELLA (Figures 4.7 and 4.8 pg 80). a) These are filamentous appendages for movement (chemotaxis, phototaxis). b) There are four different arrangements of flagella: a. Monotrichous – single, polar flagellum. b. Amphitrichous – two flagella, one at each end.
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