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Chapter 15 Viruses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The flu pandemic of 1918 killed more people in the United States than World Wars I and II, the Korean War, and the Vietnam War combined. Learning Outcomes • Explain how viruses spread through a population and cause disease. • Understand why many biologists do not consider viruses to be alive. • Describe the general structure of viruses. • Identify how viruses are categorized. • Describe the major stages of viral replication in a host cell. • Contrast the difference between the lytic cycle versus lysogenic cycle of viruses. Learning Outcomes • Explain how viruses cause disease in animals • Give examples of various virally mediated human diseases • Explain how prions and viroids cause disease. • Explain how viruses cause disease in plants 15.1 Viruses Are Infectious Particles of Genetic Information and Protein A. Viruses Are Smaller and Simpler Than Cells • 80nm in diameter – 12x smaller than bacterium • All viruses have – Genetic information – DNA or RNA – Protein coat – capsid • Some may have an envelope Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a.Tobacco mosaic virus (filamentous) b.T-even bacteriophage (spaceship) RNA c. Adenovirus (icosahedral) Capsid protein 75 nm 50 nm TEM (false color) TEM (false color) 100 nm TEM (false color) e. Poxvirus (oval,enveloped) d. Herpesvirus (icosahedral, enveloped) Envelope Capsid 200 nm TEM (false color) 300 nm TEM (false color) a: © Dr. O. Bradfute/Peter Arnold/Photolibrary; b: © Eye of Science/Photo Researchers; c: © E.O.S./ Gelderblom/Photo Researchers; d: © NIAID/Peter Arnold/Photolibrary; e: © George Musil/Visuals Unlimited/Getty Images Figure 15.1 Viruses of Many Shapes and Sizes. 15.1 Viruses Are Infectious Particles of Genetic Information and Protein B. Virus’s Host Range Consists of the Organisms It Infects • Virus can enter only a cell that has a specific receptor on its surface • Virtually all species of animals, fungi, plants, protists, and bacteria get viral infections • Reservoir of a virus is the site where it exists in nature 15.1 Viruses Are Infectious Particles of Genetic Information and Protein C. Are Viruses Alive? • Most biologists do not consider a virus to be alive – Does not metabolize, respond to stimuli, or reproduce on its own • Do have genetic material – do mutate – Natural selection • Not part of a taxonomic hierarchy 15.2 Viral Replication Occurs in Five Stages 1. 2. 3. 4. 5. • Attachment Penetration Synthesis Assembly Release Amount of time varies Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein coat Viral DNA Virus 1 Receptor Host cell 1 Attachment: Virus binds cell surface receptor. Viral DNA 2 Transcription 2 Penetration: Viral nucleic acid is released inside host cell. RNA 3 Replication 3 Translation 3 Synthesis: Host cell manufactures viral nucleic acids and proteins. Coat proteins and other proteins 4 Viral DNA 4 Assembly: New viruses are assembled from newly synthesized coat proteins, enzymes, and nucleic acids. 5 Release: New viruses leave the host cell. 5 Figure 15.2 Viral Replication. 15.3 Cell Death May Be Immediate or Delayed A. Some Viruses Kill Cells Immediately • Lytic infection – Virus enters cell, immediately replicates and lyses host – “Phage therapy” for bacterial infections • Unlikely to acquire resistance • Targeted to specific bacteria Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Virus infects new host cell. Daughter cells carry Integrated viral DNA. Host DNA Lysis occurs; new viruses released from host cell. Viral DNA Viral DNA released in host cell. (A) LYTIC PATHWAY New viruses assemble in host cell. (B) LYSOGENIC PATHWAY Chromosome replicates as cell divides. Viral proteins Viral DNA Host cell produces viral proteins and viral DNA. Viral DNA incorporated in host chromosome as prophage. Figure 15.3 Lysis and Lysogeny. 15.3 Cell Death May Be Immediate or Delayed B. Viral DNA Can “Hide” in a Cell • Lysogenic infection – Genetic material of a virus is replicated along with the host cell’s chromosome • Prophage – Host not immediately destroyed – Can switch to lytic pathway 15.4 Effects of a Viral Infection May Be Mild or Severe A. Symptoms Result from Cell Death and the Immune Response • Influenza virus causes flu – Dead and damaged cells in the airway cause the respiratory symptoms of influenza, including cough and sore throat – Fever and body aches caused by cytokines released by immune system 15.4 Effects of a Viral Infection May Be Mild or Severe B. Some Animal Viruses Linger for Years • Latent infection does not produce disease symptoms – Viral genetic information is inside the cell • Herpes simplex virus type I – Cold sores on lips – Stressed cells release viruses that infect other cells – Cold cores are localized death of these cells 15.4 Effects of a Viral Infection May Be Mild or Severe B. Some Animal Viruses Linger for Years • HIV – Retrovirus – uses reverse transcriptase – RNA genome – Infects helper T cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Loss of these cells leads to AIDS Glycoprotein Reverse transcriptase RNA Protein coat Viral envelope 50 nm • Human papillomavirus causes cervical cancer by signaling host cell to divide continuously Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glycoprotein 1 Virus binds receptors on cell membrane and enters cell. Enzymes remove viral protein coat. Reverse transcriptase RNA Protein coat Viral envelope 2 Reverse transcriptase catalyzes formation of DNA complementary to viral RNA. Host cell 50 nm 3 New DNA strand serves as a template for complementary DNA strand. 1 Reverse transcriptase Viral proteins and RNA Viral RNA 2 Chromosomal DNA 5 Viral genes transcribed to RNA. Some RNA will be packaged into new viruses. 6 RNA-DNA hybrid 7 3 Viral DNA 4 6 Viral mRNA translated into HIV proteins at ribosomes in cytoplasm. 7 Protein coats surround viral RNA and enzymes. 5 DNA 8 New viruses bud from host cell. 8 Figure 15.4 Replication of HIV. 4 Double-stranded DNA is incorporated into host cell’s genome. 15.4 Effects of a Viral Infection May Be Mild or Severe C. Drugs and Vaccines Help Fight Viral Infections • Some antiviral drugs interfere with enzymes or other proteins that are unique to viruses • Few medicines inhibit viruses without killing infected host cells • Complicated by genetic variability of viruses 15.4 Effects of a Viral Infection May Be Mild or Severe C. Drugs and Vaccines Help Fight Viral Infections • Vaccination teaches immune system to recognize virus • Childhood vaccinations greatly reduced or eliminated many viral Figure 15.5 Viruses from Eggs. infections • Fertilized chicken eggs used to produce viruses 15.5 Viruses Cause Diseases In Plants • First virus ever discovered was tobacco mosaic virus • Most spread by plant-eating insects • Spread through plasmodesmata • Can use posttranscriptional gene silencing to fight off infections Figure 15.6 Sick Plants. Clicker Question • What organisms DO NOT get viral infections? A.Animals (including humans) B.Plants C.Bacteria D.Fungi E.All of the above CAN get viral infections 15.6 Viroids and Prions Are Other Noncellular Infectious Agents A. A Viroid Is an Infectious RNA Molecule • Naked RNA • Does not encode proteins • Interferes with ability to produce proteins Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Viroid (circular RNA) Theodore Diener/USDA Plant Virology Laboratory Figure 15.7 Viroids Infect Plants. 15.6 Viroids and Prions Are Other Noncellular Infectious Agents B. A Prion Is an Infectious Protein • “Proteinaceous infectious particle” • PrP – normal cellular protein Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. – Multiple shapes – One abnormal and can cause disease • Contact with abnormal forms causes normal form to switch LM 60 µm Normal Diseased (cow): © Pixtal/age fotostock (RF); (tissue): © Ralph Eagle Jr./Photo Researchers; (normal prion): © James King–Holmes/Inst. of Animal Health/Photo Researchers; (diseased prion): © BSIP/Photo Researchers Figure 15.8 Prion Disease. 15.6 Viroids and Prions Are Other Noncellular Infectious Agents B. A Prion Is an Infectious Protein • Abnormal form causes brain cells to die – Spongiform encephalopathies – “Mad cow disease” (BSE) – Human forms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Kuru – cannibalism • Creutzfeld-Jakob disease (CJD) • Fatal familial insomnia LM 60 µm (cow): © Pixtal/age fotostock (RF); (tissue): © Ralph Eagle Jr./Photo Researchers Figure 15.8 Prion Disease. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.c 15.7 Investigating Life: Scientific Detectives Follow HIV’s Trail • Ancestor of HIV may be virus called simian immunodeficiency virus (SIV) • 5 questions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. 2. 3. 4. 5. Are genomes of HIV and SIV similar? Do viral genes share sequences? Is SIV common enough to spread? Do HIV and SIV occur in same places? Is their a transmission pathway? 15.7 Investigating Life: Scientific Detectives Follow HIV’s Trail • SIVsm identified as source of HIV-2 • Origin of HIV-1 unknown – SIVcpz met many criterion – Only #3 a question • Later study found rates in wild at 18% Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SIVcpzANT Eastern chimpanzee U455 LAI HIV-1 (group M) ELI YBF30 HIV-1 (group N) SIVcpzGAB2 SIVcpzGAB1 SIVcpzUS ANT70 MVP5180 Central chimpanzee HIV-1 (group O) Figure 15.9 HIV-1 from SIV. 15.7 Mastering Concepts How could researchers use a similar strategy to study the origin of a new influenza virus?