Phage - Phage Bacteriophage Viruses Viruses are small...

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Unformatted text preview: Phage Bacteriophage Viruses Viruses are small packages of genes Consist of protein coat around nucleic acids (DNA or RNA) Capsomere of capsid Classification of Viruses RNA Capsomere Glycoprotein 18 ¥ 250 nm 70–90 nm 20 nm Membranous Head envelope Tail Capsid sheath RNA Tail fiber DNA 50 nm Glycoprotein 80–200 nm (a) Tobacco mosaic (b) Adenoviruses: virus: helical polyhedral RNA-virus DNA-virus DNA 80 ¥ 225 nm 50 nm (c) Influenza viruses: enveloped helical RNA-virus 50 nm (d) Bacteriophage: complex-polyhedral DNA-virus Figure 18.4. Colorized TEMs The Lytic Cycle (virulent phage) Viruses • Bacteriophage T4 Obligate intracellular parasites • Cannot grow or reproduce by itself • Have no independent metabolic pathways for energy synthesis • Reproduce (replicate) only by using host cell machinery • Non-cellular • (Is it living?) 1 Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 5 Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 0.5 mm Culture of virulent (lytic) phages • Bacteriophage • Agar plate with “bacterial lawn” (solid white field of bacteria) • Plaques: clear, bacteria-free region growing around one original viralinfected bacterium = pfu: plaque-forming unit Heyer and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell ’s DNA is hydrolyzed. Phage assembly 4 Assembly. Three separate sets of Figure 18.1: T4 Bacteriophages infecting an E. coli bacterium cell 2 Entry of phage DNA Head Tails Tail fibers Figure 18.6 proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. 3 Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. Koch’s postulates In order to prove definitively that a particular pathogen causes a disease, researchers must: 1. Find the pathogen in each individual that has the disease. 2. Isolate the pathogen from a diseased individual, and grow the pathogen in a pure culture. ÿ But cannot grow viruses independent of host cells. 3. Induce the disease in healthy in dividuals by infecting them with the pure pathogen. 4. Isolate the pathogen from the newly infected individuals. 1 ...
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