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3rd lecture Cell Biology The ultrastructures of prokaryotic cells 1) The prokaryotic cell is simpler than the eukaryotic cell at every level, with one exception: The cell envelope is more complex. 2) The ultrastructures of prokaryotic cells include: a) The Nucleoid b) The cytoplasmic structures, which include (ribosomes, plasmids and episomes, inclusion bodies and storage granules and endospores). c) The cell envelope, which include (cytoplasmic membrane, the periplasmic space, the cell wall and the outer membrane). d) The exterior components of the cell wall, which include Extracellular polymeric substance (EPS), flagella, pili and fimbriae). The Nucleoid 1) Prokaryotes have no true nuclei; instead, they package their DNA in a structure known as the nucleoid. 2) The negatively charged DNA is at least partially neutralized by small polyamines and magnesium ions. 3) Histone-like proteins exist in bacteria and presumably play a role similar to that of histones in eukaryotic chromatin. 4) Electron micrographs of a typical prokaryotic cell reveal the absence of a nuclear membrane and a mitotic apparatus. 5) The exception to this rule is the planctomycetes, a divergent group of aquatic bacteria, which have a nucleoid surrounded by a nuclear envelope consisting of two membranes. 6) The distinction between prokaryotes and eukaryotes that still holds is that prokaryotes have no eukaryotic-type mitotic apparatus. 7) The nuclear region (Figure 1) is filled with DNA fibrils. 1 Dr. Mohanad J.K. Al-Dawah 3rd lecture Cell Biology 8) The nucleoid of most bacterial cells consists of a single continuous circular molecule ranging in size from 0.58 to almost 10 million base pairs. 9) In bacteria, the numbers of nucleoids, and therefore the number of chromosomes, depend on the growth conditions. 10) Rapidly growing bacteria have more nucleoids per cell than slowly growing ones; however, when multiple copies are present, they are all the same (prokaryotic cells are haploid). Figure 1: nuclear region of E. coli chromosome The cytoplasmic structures (matrix) 1) The cytoplasm is the aqueous solution containing all cytoplasmic components and constitutes the region within the cytoplasmic membrane, excluding the nucleoid. 2) The cytoplasmic matrix is about 70% water and lacks a cytoskeleton. 3) Despite the apparent homogeneity, the matrix is highly organized with respect to protein location. 2 Dr. Mohanad J.K. Al-Dawah 3rd lecture Cell Biology Ribosomes 1) The ribosome is the platform and active site of protein synthesis. 2) Prokaryotic ribosomes constitute up to 10% of the dry cell mass, have a sedimentation coefficient of 70S, and are composed of a large (50S) and small (30S) ribosomal subunit. 3) Their small subunit has a 16S rRNA subunit (consisting of 1540 nucleotides) bound to 21 proteins. 4) The large subunit is composed of a 5S RNA subunit (120 nucleotides) and a 23S RNA subunit (2900 nucleotides) and 31 proteins. 5) The rRNAs form stable three-dimensional structures, which serve as a structural scaffold for ribosomal protein attachment, as depicted in Figure 2. 6) In addition to their structural role, the 16S rRNA has a region complementary to the Shine-Delgarno region of mRNA, which facilitates mRNA binding to the ribosome for translation. Plasmids and episomes: 1) Plasmids are autonomous extrachromosomal replicons, which are widespread 3 amongst the bacteria and offer metabolic and Dr. Mohanad J.K. Al-Dawah 3rd lecture Cell Biology physiological flexibility of the organism’s response to environmental changes and stresses. 2) With rare exceptions, plasmids exist within the cell as highly supercoiled circular dsDNA molecules of a few kilobases, such as ColVK30 which is 2 kbp in length, to over a hundred kilobases in length, for example the CAM plasmid of Pseudomonas. 3) Plasmids are present in host cells as either single copies or multiple copies in excess of 40 per cell. 4) Plasmids may encode one or more phenotypic features, which may in turn be expressed by the host. 5) These features include antibiotic resistance, toxin production, adherence antigens, bacteriocin production and sex pilus and conjugation 6) A representation of R100 is included as a typical plasmid in Figure 3. 7) Plasmids are commonly classified according to their molecular characteristics, gene functions (particularly antibiotic resistance patterns that they confer), incompatibility groups, host range, and bacteriophage susceptibility of hosts. 8) A few types of plasmids are also capable of inserting into the host chromosome, and these integrative plasmids are sometimes referred to as episomes in prokaryotes. Figure 3: Atypically plasmid R100 4 Dr. Mohanad J.K. Al-Dawah 3rd lecture Cell Biology Inclusion bodies and storage granules 1) Bacteria often store reserve materials in the form of insoluble granules, which appear as refractile bodies in the cytoplasm when viewed by phase contrast microscopy. These so-called inclusion bodies 2) Usually function in the storage of energy or as a reservoir of structural building blocks. 3) Most cellular inclusions are bounded by a thin nonunit membrane consisting of lipid, which serves to separate the inclusion from the cytoplasm proper. 4) Poly-a-hydroxybutyric acid (PHB) is one of the most common inclusion bodies produced when the source of nitrogen, sulfur, or phosphorous is limited and there is excess carbon in the medium. 5) Another storage product formed by prokaryotes when carbon is in excess is glycogen, which is a polymer of glucose. 6) PHB and glycogen are used as carbon sources when protein and nucleic acid synthesis are resumed. Endospores: 1) Members of several bacterial genera are capable of forming endospores. 2) The two most common are gram-positive rods: the obligatory aerobic genus Bacillus and the obligatory anaerobic genus Clostridium. 3) These organisms undergo a cycle of differentiation in response to environmental conditions: 4) The process, sporulation, is triggered by near depletion of any of several nutrients (carbon, nitrogen, or phosphorous). 5) Each cell forms a single internal spore that is liberated when the mother cell undergoes autolysis. 5 Dr. Mohanad J.K. Al-Dawah 3rd lecture Cell Biology 6) The spore is a resting cell, highly resistant to desiccation, heat, and chemical agents; when returned to favorable nutritional conditions and activated, the spore germinates to produce a single vegetative cell. The cell envelope: 1) Prokaryotic cells are surrounded by complex envelope layers that differ in composition among the major groups. 2) These structures protect the organisms from hostile environments, such as extreme osmolarity, harsh chemicals, and even antibiotics. Cell membrane Structure: 1) The bacterial cell membrane, also called the cytoplasmic membrane, is visible in electron micrographs of thin sections (figure 4). 2) It is a typical “unit membrane” composed of phospholipids and upward of 200 different kinds of proteins. 3) Proteins account for approximately 70% of the mass of the membrane, which is a considerably higher proportion than that of mammalian cell membranes. 4) At least 50% of the cytoplasmic membrane must be in the semifluid state for cell growth to occur. 5) The membranes of prokaryotes are distinguished from those of eukaryotic cells by the absence of sterols. 6) The only exception being mycoplasmas that incorporate sterols, such as cholesterol, into their membranes when growing in sterolcontaining media. Differences between bacterial and Archaeal cell membrane a) Some Archaeal cell membranes contain unique lipids, isoprenoids, while bacterial cell membrane contain fatty acids b) Isoprenoids linked to glycerol by ether, while fatty acid by an ester linkage. 6 Dr. Mohanad J.K. Al-Dawah 3rd lecture Cell Biology c) Some of these lipids have no phosphate groups, and therefore, they are not phospholipids. d) In other species, the cell membrane is made up of a lipid monolayer consisting of long lipids with glycerol ethers at both ends. e) These unusual lipids contribute to the ability of many Archaea to grow under environmental conditions such as high salt, low pH, or very high temperature. Figure 4: electron micrograph of Bacillus megaterium cell membrane. poly-β-hydroxybutyric acid inclusion body, PHB; cell wall, CW; nucleoid, N; plasma membrane, PM; “mesosome,” M; and ribosomes, R. (Reproduced with permission). Function of cell membrane a) Selective permeability and transport of solutes. b) Electron transport and oxidative phosphorylation in aerobic species. c) Excretion of hydrolytic exoenzymes. d) Bearing the enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymers, and membrane lipids. e) Bearing the receptors and other proteins of the chemotactic and other sensory transduction systems. 7 Dr. Mohanad J.K. Al-Dawah