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Lecture 3

Lecture 3 - Prokaryotes.docx

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Biological Sciences
Bebhinn Treanor

Prokaryotes (21) Prokaryotes make up two of the three domains of life: Archaea and Bacteria (third domain is Eukaraya)  Thrive almost everywhere (including extreme habitats to hostile for most organisms)  Show remarkable diversity (greatest metabolic diversity of all organisms)  Appear simple in structure compared to eukaryotic cells  Classified into two domains that differ in structure, physiology and biochemistry Bacteria: most familiar to us as they are responsible for disease in humans and other animals (some essential for health and some used in food production like cheese, yogurt etc.) Archaea: Only discovered about 40 years ago so not well known  Share similar cellular features with eukaryotes and some with bacteria (some also unique features to Archaea  Many can live under extreme conditions that no other organisms (including bacteria) can survive 1. Prokaryotic morphology  Are unicellular  Smallest organisms in the world (1 to 10 µm  Three common shapes (spiral, spherical or coccoid and cylindrical or rods; rare square cells) 2. Prokaryotic Structure  DNA packed into area called nucleoid: majority of genome is consisted in a ring of DNA (single circular chromosome); genome is 1/100 in size compared to eukaryotic cells  Plasmids: smaller rings of DNA which provide resistance to antibiotics, replicate independently of chromosome and can be transferred via pili  Ribosomes: in prokaryotic cells are smaller than in eukaryotic cells but there is similar protein synthesis (archaeal ribosomes are similar to eukaryotic ribosomes as they both are sensitive to antigens whereas bacterial ribosomes are not)  Bacterial cell wall: maintains shape of the cell and prevents bursting in a hypotonic environment o Some bacterial cell walls contain outer membrane o Contains lipopolysaccharide (LPS) o Differences in cell wall structure used to classify bacteria o Most bacterial cell walls contain peptidoglycan which is a polymer of modified sugars cross- linked by short polypeptides (there is related molecule in Archaea, but different molecular components and bonding structure) o Gram stain is used to classify different structures of the bacteria wall composition (negative- pink or positive-purple depending on whether the peptidoglycan layer is in the middle or on the outside)  Cell membrane (or plasma membrane)  Capsule: sticky layer made of polysaccharides that lies outside cell wall and protects cell o Protects bacteria from external environment and is considered a virulence factor (helps to evade detection by immune cells)  Pili: a hairlike appendage found on the surface of many bacteria o Aids attachment of bacteria to host surfaces o Required for colonization during infection and to initiate formation of a biofilm o Conjugative (sex)-pili allow transfer of plasmids (DNA) between bacteria (one method for Horizontal Gene Transfer HGT)  Flagella: sensory and locomotive organelle o Composed of single cell with surface appendage and is a sensory organelle (sensitive to external environment) o Prokaryotic and eukaryotic flagella differ in protein composition, structure, and mechanism of propulsion  Lack internal membrane-bound organelles  Semblance of cytoskeleton Cell wall 3. Reproduce by binary fission  Asexual reproduction: produces exact copies of parent and can result in rapid population growth Mechanisms promoting genetic diversity in prokaryotes 1. Rapid reproduction and mutation: new mutation (even if rare) can increase genetic diversity in species with short generation times due to the speed of reproduction 2. Genetic recombination (combining DNA from two sources): conjugation (DNA transferred between prokaryotic cells via pilus), transformation (uptake of DNA from surroundings) and transduction (bacteriophages, viruses that infect bacteria, carry prokaryotic gene from one cell to another) Diversity in Prokaryotes  Chemoautotrophs (self-fe
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