The Genetics of Viruses 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)

RESULTS Extracted sap from tobacco plant with tobacco mosaic disease 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 3 Rubbed filtered sap on healthy tobacco plants Figure 19.2 What causes tobacco mosaic disease? 4 Healthy plants became infected

Structure of Viruses Viruses are: Viruses consist of: 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: attachment injection hydrolyzation assembly release Results in host cell death Virulent virus: reproduce only via lytic cycle Simplified Viral Reproductive Cycle Animation: Phage T4 Lytic Cycle

Animation: Phage Lambda Lysogenic and Lytic Cycles

Attachment Entry of phage DNA and degradation of host DNA Release Fig. 19-5-5 1 Attachment 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 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)

Animation: HIV Reproductive Cycle 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: Prions: tiny, naked, circular RNA that infect plants Prions: “mad cow disease” a transmissible protein

Table 19-1a Table 1

Table 19-1b Table 1

You should now be able to: Explain how capsids and envelopes are formed Distinguish between the lytic and lysogenic reproductive cycles Explain why viruses are obligate intracellular parasites Describe the reproductive cycle of an HIV retrovirus Describe three processes that lead to the emergence of new diseases Describe viroids and prions