Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 15 Viruses

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

Figure 15.1 Viruses of Many Shapes and Sizes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a.Tobacco mosaic virus (filamentous) b.T-even bacteriophage (spaceship) c. Adenovirus (icosahedral) 75 nm TEM (false color) 50 nm TEM (false color) 100 nm TEM (false color) d. Herpesvirus (icosahedral, enveloped) e. Poxvirus (oval,enveloped) TEM (false color) 200 nm TEM (false color) 300 nm Capsid Envelope Capsid protein RNA 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

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.Attachment 2.Penetration 3.Synthesis 4.Assembly 5.Release Amount of time varies

Figure 15.2 Viral Replication. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hostcell Viral DNA Replication ViralDNA Protein coat Viral DNA Virus Receptor Attachment: Virus binds cell surface receptor. Penetration: Viral nucleic acid is released inside host cell. Synthesis: Host cell manufactures viral nucleic acids and proteins. Assembly: New viruses are assembled from newly synthesized coat proteins, enzymes, and nucleic acids. Release: New viruses leave the host cell. Transcription RNA Translation Coat proteins and other proteins

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

Figure 15.3 Lysis and Lysogeny. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lysis occurs; new viruses released from host cell. New viruses assemble in host cell. Host cell produces viral proteins and viral DNA. Viral DNA incorporated in host chromosome as prophage. Chromosome replicates as cell divides. (B) LYSOGENIC PATHWAY (A) LYTIC PATHWAY Viral proteins Viral DNA Daughter cells carry Integrated viral DNA. Viral DNA released in host cell. Viral DNA Virus infects new host cell. Host DNA

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 Loss of these cells leads to AIDS 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 Reverse transcriptase RNA Protein coat Viral envelope 50 nm

Figure 15.4 Replication of HIV. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. DNA Chromosomal DNA ViralDNA ViralproteinsandRNA New viruses bud from host cell Virus binds receptors on cell membrane and enters cell. Enzymes remove viral protein coat. Reverse transcriptase catalyzes formation of DNA complementary to viral RNA. New DNA strand serves as a template for complementary DNA strand. Double-stranded DNA is incorporated into host cell’s genome. Viral genes transcribed to RNA. Some RNA will be packaged into new viruses. Viral mRNA translated into HIV proteins at ribosomes in cytoplasm. Protein coats surround viral RNA and enzymes. Glycoprotein Reverse transcriptase RNA Protein coat Viral envelope 50 nm RNA-DNA hybrid Viral RNA Host cell Reverse transcriptase

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 infections Fertilized chicken eggs used to produce viruses Figure 15.5 Viruses from Eggs.

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 Figure 15.7 Viroids Infect Plants. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Viroid (circular RNA) Theodore Diener/USDA Plant Virology Laboratory

15.6 Viroids and Prions Are Other Noncellular Infectious Agents B. A Prion Is an Infectious Protein “Proteinaceous infectious particle” PrP – normal cellular protein –Multiple shapes –One abnormal and can cause disease Contact with abnormal forms causes normal form to switch Figure 15.8 Prion Disease. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. NormalDiseased 60 µm LM (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

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 Kuru – cannibalism Creutzfeld-Jakob disease (CJD) Fatal familial insomnia Figure 15.8 Prion Disease. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 60 µm LM (cow): © Pixtal/age fotostock (RF); (tissue): © Ralph Eagle Jr./Photo Researchers

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Investigating Life: Scientific Detectives Follow HIV’s Trail Ancestor of HIV may be virus called simian immunodeficiency virus (SIV) 5 questions 1.Are genomes of HIV and SIV similar? 2.Do viral genes share sequences? 3.Is SIV common enough to spread? 4.Do HIV and SIV occur in same places? 5.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% Figure 15.9 HIV-1 from SIV. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SIVcpzANT U455 LAI ELI YBF30 SIVcpzGAB2 SIVcpzGAB1 SIVcpzUS ANT70 MVP5180 HIV-1 (group O) Central chimpanzee HIV-1 (group N) HIV-1 (group M) Eastern chimpanzee

15.7 Mastering Concepts How could researchers use a similar strategy to study the origin of a new influenza virus?