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Chapter 19: The Genetics of Viruses The Discovery of Viruses • Tobacco mosaic disease: – stunts growth, gives leaves a mosaic coloration • Late 1800s, researchers hypothesized a particle smaller than bacteria caused the disease • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) Fig. 19-2 RESULTS 1 Extracted sap from tobacco plant with tobacco mosaic disease 2 Passed sap through a porcelain filter known to trap bacteria 4 Healthy plants became infected 3 Rubbed filtered sap on healthy tobacco plants Structure of Viruses • Viruses are: – not cells – means “poison” in Latin – very small infectious particles • Viruses consist of: – Nucleic acid (DNA or RNA) – Protein coat (capsid) surrounding DNA/RNA – Sometimes a membranous envelope Viral structure Viral reproduction: Lytic Cycle • Host range: – infect a limited range of host cells – Based on receptor molecules on cell surface • The lytic cycle: 1. 2. 3. 4. 5. attachment injection hydrolyzation assembly release Simplified Viral Reproductive Cycle Animation: Phage T4 Lytic Cycle • Results in host cell death • Virulent virus: reproduce only via lytic cycle Animation: Phage Lambda Lysogenic and Lytic Cycles Fig. 19-5-5 1 Attachment 2 Entry of phage 5 Release DNA and degradation of host DNA Phage assembly 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers Viral reproduction: Lysogenic Cycle • • • • Genome replicated w/o destroying host cell Viral genetic material incorporated into host cell DNA (prophage) Temperate virus (capable of using lytic and lysogenic cycles) May give rise to lytic cycle Mutations • All viruses can mutate (reproduction) • Organisms often not immune to new strains – Epidemic: outbreak from new strain (flu) – Pandemic: global outbreak usually lethal (Spanish flu) RNA viruses • Retroviruses: transcribe DNA from RNA (RNA--->DNA) – Reverse transcriptase – Most plant viruses (TMV) • No proofing, more mutations • HIV--->AIDS Animation: HIV Reproductive Cycle Viroids and prions • Viroids: – tiny, naked, circular RNA that infect plants • Prions: – “mad cow disease” – a transmissible protein Table 19-1a Table 19-1b You should now be able to: 1. Explain how capsids and envelopes are formed 2. Distinguish between the lytic and lysogenic reproductive cycles 3. Explain why viruses are obligate intracellular parasites 4. Describe the reproductive cycle of an HIV retrovirus 5. Describe three processes that lead to the emergence of new diseases 6. Describe viroids and prions