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PowerPoint® Lecture Presentations prepared by Bradley W. Christian, McLennan Community College CHAPTER 13 Viruses, Viroids, and Prions © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. General Characteristics of Viruses Learning Objective 13-1 Differentiate a virus from a bacterium. © 2016 Pearson Education, Inc. General Characteristics of Viruses • Obligatory intracellular parasites • Require living host cells to multiply • • • • Contain DNA or RNA Contain a protein coat No ribosomes No ATP-generating mechanism © 2016 Pearson Education, Inc. Table 13.1 Viruses and Bacteria Compared © 2016 Pearson Education, Inc. Host Range • The spectrum of host cells a virus can infect • Most viruses infect only specific types of cells in one host • Determined by specific host attachment sites and cellular factors • Bacteriophages—viruses that infect bacteria • Range from 20 nm to 1000 nm in length © 2016 Pearson Education, Inc. Figure 13.1 Virus sizes. Bacteriophage M13 800 × 10 nm 970 nm Bacteriophages f2, MS2 24 nm Ebola virus Poliovirus 30 nm Rhinovirus 30 nm Adenovirus 90 nm Rabies virus 170 × 70 nm Prion 200 × 20 nm Bacteriophage T4 225 nm Tobacco mosaic virus 250 × 18 nm Viroid 300 × 10 nm Vaccinia virus © 2016 Pearson Education, Inc. 300 × 200 × 100 nm 300 nm Chlamydia bacterium elementary body E. coli bacterium 3000 × 1000 nm Human red blood cell 10,000 nm in diameter Plasma membrane of red blood cell 10 nm thick Check Your Understanding How could the small size of viruses have helped researchers detect viruses before the invention of the electron microscope? 13-1 © 2016 Pearson Education, Inc. Viral Structure Learning Objective 13-2 Describe the chemical and physical structure of both an enveloped and a nonenveloped virus. © 2016 Pearson Education, Inc. Viral Structure • Virion—complete, fully developed viral particle • Nucleic acid—DNA or RNA can be single- or doublestranded; linear or circular • Capsid—protein coat made of capsomeres (subunits) • Envelope—lipid, protein, and carbohydrate coating on some viruses • Spikes—projections from outer surface © 2016 Pearson Education, Inc. Figure 13.2 Morphology of a nonenveloped polyhedral virus. Nucleic acid Capsomere © 2016 Pearson Education, Inc. Capsid Figure 13.3 Morphology of an enveloped helical virus. Nucleic acid Capsomere Spikes © 2016 Pearson Education, Inc. Envelope General Morphology • • • • Helical viruses—hollow, cylindrical capsid Polyhedral viruses—many-sided Enveloped viruses Complex viruses—complicated structures © 2016 Pearson Education, Inc. Figure 13.4 Morphology of a helical virus. Nucleic acid Capsomere Capsid © 2016 Pearson Education, Inc. Figure 13.5 Morphology of complex viruses. 65 nm Capsid (head) DNA Sheath Tail fiber Pin Baseplate A T-even bacteriophage Orthopoxvirus © 2016 Pearson Education, Inc. Check Your Understanding Diagram a nonenveloped polyhedral virus that has spikes. 13-2 © 2016 Pearson Education, Inc. Taxonomy of Viruses Learning Objectives 13-3 Define viral species. 13-4 Give an example of a family, genus, and common name for a virus. © 2016 Pearson Education, Inc. Taxonomy of Viruses • • • • Genus names end in -virus Family names end in -viridae Order names end in -ales Viral species: a group of viruses sharing the same genetic information and ecological niche (host) • Descriptive common names are used for species • Subspecies are designated by a number © 2016 Pearson Education, Inc. Check Your Understanding How does a virus species differ from a bacterial species? 13-3 Attach the proper endings to Papilloma- to show the family and genus that includes HPV, the cause of cervical cancer. 13-4 © 2016 Pearson Education, Inc. Isolation, Cultivation, and Identification of Viruses Learning Objectives 13-5 Describe how bacteriophages are cultured. 13-6 Describe how animal viruses are cultured. 13-7 List three techniques used to identify viruses. © 2016 Pearson Education, Inc. Growing Bacteriophages in the Laboratory • Viruses must be grown in living cells • Bacteriophages are grown in bacteria • Bacteriophages form plaques, which are clearings on a lawn of bacteria on the surface of agar • Each plaque corresponds to a single virus; can be expressed as plaque-forming units (PFU) © 2016 Pearson Education, Inc. Figure 13.6 Viral plaques formed by bacteriophages. Plaques © 2016 Pearson Education, Inc. Growing Animal Viruses in the Laboratory • In living animals • In embryonated eggs • Virus injected into the egg • Viral growth is signaled by changes or death of the embryo © 2016 Pearson Education, Inc. Figure 13.7 Inoculation of an embryonated egg. Shell Amniotic cavity Chorioallantoic membrane Air sac Chorioallantoic membrane inoculation Amniotic inoculation Yolk sac Allantoic inoculation Shell membrane Albumin © 2016 Pearson Education, Inc. Allantoic cavity Yolk sac inoculation Growing Animal Viruses in the Laboratory • In cell cultures • Tissues are treated with enzymes to separate cells • Virally infected cells are detected via their deterioration, known as the cytopathic effect (CPE) • Continuous cell lines are used © 2016 Pearson Education, Inc. Figure 13.8 Cell cultures. Normal cells A tissue is treated with enzymes to separate the cells. © 2016 Pearson Education, Inc. Cells are suspended in culture medium. Transformed cells Normal cells or primary cells grow in a monolayer across the glass or plastic container. Transformed cells or continuous cell cultures do not grow in a monolayer. Figure 13.9 The cytopathic effect of viruses. © 2016 Pearson Education, Inc. Viral Identification • Cytopathic effects • Serological tests • Western blotting—reaction of the virus with antibodies • Nucleic acids • RFLPs • PCR © 2016 Pearson Education, Inc. Check Your Understanding What is the plaque method? 13-5 Why are continuous cell lines of more practical use than primary cell lines for culturing viruses? 13-6 What tests could you use to identify influenza virus in a patient? 13-7 © 2016 Pearson Education, Inc. Viral Multiplication Learning Objectives 13-8 Describe the lytic cycle of T-even bacteriophages. 13-9 Describe the lysogenic cycle of bacteriophage lambda. 13-10 Compare and contrast the multiplication cycle of DNA- and RNA-containing animal viruses. © 2016 Pearson Education, Inc. Viral Multiplication • For a virus to multiply: • It must invade a host cell • It must take over the host's metabolic machinery • One-step growth curve © 2016 Pearson Education, Inc. Figure 13.10 A viral one-step growth curve. © 2016 Pearson Education, Inc. Multiplication of Bacteriophages • Lytic cycle • Phage causes lysis and death of the host cell • Lysogenic cycle • Phage DNA is incorporated in the host DNA • Phage conversion • Specialized transduction © 2016 Pearson Education, Inc. Viral Replication: Virulent Bacteriophages PLAY © 2016 Pearson Education, Inc. Animation: Viral Replication: Virulent Bacteriophages Viral Replication: Temperate Bacteriophages PLAY © 2016 Pearson Education, Inc. Animation: Viral Replication: Temperate Bacteriophages T-Even Bacteriophages: The Lytic Cycle • Attachment: phage attaches by the tail fibers to the host cell • Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell • Biosynthesis: production of phage DNA and proteins • Maturation: assembly of phage particles • Release: phage lysozyme breaks the cell wall © 2016 Pearson Education, Inc. Figure 13.11 The lytic cycle of a T-even bacteriophage. Bacterial Bacterial cell wall chromosome Capsid DNA Capsid (head) Sheath Attachment: Phage attaches to host cell. Tail fiber Baseplate Tail Pin Cell wall Plasma membrane Penetration: Phage penetrates host cell and injects its DNA. Sheath contracted Biosynthesis: Phage DNA directs synthesis of viral components by the host cell. Tail core Tail DNA Maturation: Viral components are assembled into virions. Capsid Release: Host cell lyses, and new virions are released. Tail fibers © 2016 Pearson Education, Inc. Bacteriophage Lambda (λ): The Lysogenic Cycle • Lysogeny: phage remains latent • Phage DNA incorporates into host cell DNA • Inserted phage DNA is known as a prophage • When the host cell replicates its chromosome, it also replicates prophage DNA • Results in phage conversion—the host cell exhibits new properties © 2016 Pearson Education, Inc. Figure 13.12 The lysogenic cycle of bacteriophage λ in E. coli. Phage DNA (double-stranded) Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle. Phage attaches to host cell and injects DNA. Bacterial chromosome Many cell divisions Lytic cycle Cell lyses, releasing phage virions. Lysogenic cycle Phage DNA circularizes and enters lytic cycle or lysogenic cycle. OR New phage DNA and proteins are synthesized and assembled into virions. © 2016 Pearson Education, Inc. Lysogenic bacterium reproduces normally. Prophage Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage. Bacteriophage Lambda (λ): The Lysogenic Cycle • Specialized transduction • Specific bacterial genes transferred to another bacterium via a phage • Changes genetic properties of the bacteria © 2016 Pearson Education, Inc. Figure 13.13 Specialized transduction. Prophage gal gene Bacterial DNA Prophage exists in galactose-using host (containing the gal gene). Galactosepositive donor cell gal gene Phage genome excises, carrying with it the adjacent gal gene from the host. gal gene Phage matures and cell lyses, releasing phage carrying gal gene. Phage infects a cell that cannot utilize galactose (lacking gal gene). Galactosenegative recipient cell Along with the prophage, the bacterial gal gene becomes integrated into the new host's DNA. Lysogenic cell can now metabolize galactose. Galactose-positive recombinant cell © 2016 Pearson Education, Inc. Transduction: Generalized Transduction PLAY © 2016 Pearson Education, Inc. Animation: Transduction: Generalized Transduction Transduction: Specialized Transduction PLAY © 2016 Pearson Education, Inc. Animation: Transduction: Specialized Transduction Check Your Understanding How do bacteriophages get nucleotides and amino acids if they don't have any metabolic enzymes? 13-8 Vibrio cholerae produces toxin and is capable of causing cholera only when it is lysogenic. What does this mean? 13-9 © 2016 Pearson Education, Inc. Table 13.3 Bacteriophage and Animal Viral Multiplication Compared © 2016 Pearson Education, Inc. Multiplication of Animal Viruses • • • • Attachment: viruses attach to the cell membrane Entry by receptor-mediated endocytosis or fusion Uncoating by viral or host enzymes Biosynthesis: production of nucleic acid and proteins • Maturation: nucleic acid and capsid proteins assemble • Release by budding (enveloped viruses) or rupture © 2016 Pearson Education, Inc. Figure 13.14 The entry of viruses into host cells. Host plasma membrane proteins at site of receptor-mediated endocytosis Entry of pig retrovirus by receptor-mediated endocytosis. © 2016 Pearson Education, Inc. Fusion of viral envelope and plasma membrane Entry of herpesvirus by fusion. Figure 13.20 Budding of an enveloped virus. Viral capsid Host cell plasma membrane Viral protein Bud Bud Envelope Release by budding Lentivirus © 2016 Pearson Education, Inc. Viral Replication: Overview PLAY © 2016 Pearson Education, Inc. Animation: Viral Replication: Overview Viral Replication: Animal Viruses PLAY © 2016 Pearson Education, Inc. Animation: Viral Replication: Animal Viruses The Biosynthesis of DNA Viruses • DNA viruses replicate their DNA in the nucleus of the host using viral enzymes • Synthesize capsid in the cytoplasm using host cell enzymes © 2016 Pearson Education, Inc. Figure 13.15 Replication of a DNA-Containing Animal Virus. Papovavirus RELEASE Virions are released. ATTACHMENT Virion attaches to host cell. A papovavirus is a typical DNA-containing virus that attacks animal cells. DNA Host cell Capsid MATURATION Virions mature. Nucleus ENTRY and UNCOATING Virion enters cell, and its DNA is uncoated. Cytoplasm Capsid proteins Viral DNA BIOSYNTHESIS Viral DNA is replicated, and some viral proteins are made. Capsid proteins mRNA Late translation; capsid proteins are synthesized. KEY CONCEPTS Viral replication in animals generally follows these steps: attachment, entry, uncoating, biosynthesis of nucleic acids and proteins, maturation, and release. Knowledge of viral replication phases is important for drug development strategies, and for understanding disease pathology. © 2016 Pearson Education, Inc. A portion of viral DNA is transcribed, producing mRNA that encodes "early" viral proteins. The Biosynthesis of DNA Viruses • Adenoviridae • Double-stranded DNA, nonenveloped • Respiratory infections in humans • Tumors in animals © 2016 Pearson Education, Inc. Figure 13.16a DNA-containing animal viruses. Capsomere Mastadenovirus © 2016 Pearson Education, Inc. The Biosynthesis of DNA Viruses • Poxviridae • Double-stranded DNA, enveloped • Cause skin lesions • Vaccinia and smallpox viruses (Orthopoxvirus) © 2016 Pearson Education, Inc. The Biosynthesis of DNA Viruses • Herpesviridae • Double-stranded DNA, enveloped • • • • • • HHV-1 and HHV-2—Simplexvirus; cause cold sores HHV-3—Varicellovirus; causes chickenpox HHV-4—Lymphocryptovirus; causes mononucleosis HHV-5—Cytomegalovirus HHV-6 and HHV-7—Roseolovirus HHV-8—Rhadinovirus; causes Kaposi's sarcoma © 2016 Pearson Education, Inc. Figure 13.16b DNA-containing animal viruses. Capsomeres Simplexvirus © 2016 Pearson Education, Inc. The Biosynthesis of DNA Viruses • Papovaviridae • Double-stranded DNA, nonenveloped • Papillomavirus • Causes warts • Can transform cells and cause cancer © 2016 Pearson Education, Inc. The Biosynthesis of DNA Viruses • Hepadnaviridae • Double-stranded DNA, enveloped • Hepatitis B virus • Use reverse transcriptase to make DNA from RNA © 2016 Pearson Education, Inc. The Biosynthesis of RNA Viruses • Virus multiplies in the host cell's cytoplasm using RNA-dependent RNA polymerase • ssRNA; + (sense) strand • Viral RNA serves as mRNA for protein synthesis • ssRNA; – (antisense) strand • Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis • dsRNA—double-stranded RNA © 2016 Pearson Education, Inc. Figure 13.17a Pathways of multiplication used by various RNA-containing viruses. Attachment Capsid RNA Nucleus Host cell Cytoplasm Entry and uncoating Maturation and release Translation and synthesis of viral proteins RNA replication by viral RNA– dependent RNA polymerase – strand is transcribed from + viral genome. KEY Capsid protein + or sense strand of viral genome Uncoating releases viral RNA and proteins. Viral genome (RNA) – or antisense strand of viral genome ss = single-stranded ds = double-stranded © 2016 Pearson Education, Inc. + strand mRNA is transcribed from the – strand. ssRNA; + or sense strand; Picornaviridae Viral protein Figure 13.17b Pathways of multiplication used by various RNA-containing viruses. Attachment Capsid RNA Translation and synthesis of viral proteins Capsid protein KEY RNA replication by viral RNA– dependent RNA polymerase The + strand (mRNA) must first be transcribed from the – viral genome before proteins can be synthesized. + or sense strand of viral genome – or antisense strand of viral genome ds = double-stranded © 2016 Pearson Education, Inc. Cytoplasm Host cell Entry and uncoating Maturation and release ss = single-stranded Nucleus – strands are incorporated into capsid Additional – strands are transcribed from mRNA. Uncoating releases viral RNA and proteins. Viral genome (RNA) ssRNA; – or antisense strand; Rhabdoviridae Viral protein Figure 13.17c Pathways of multiplication used by various RNA-containing viruses. Attachment Capsid RNA Cytoplasm Host cell Entry and uncoating Maturation and release Translation and synthesis of viral proteins KEY Nucleus RNA replication by viral RNA– dependent RNA polymerase RNA polymerase initiates production of – strands. The mRNA and – strands form the dsRNA that is incorporated as new viral genome. + or sense strand of viral genome mRNA is produced inside the capsid and released into the cytoplasm of the host. Uncoating releases viral RNA and proteins. Viral genome (RNA) Viral protein – or antisense strand of viral genome ss = single-stranded ds = double-stranded © 2016 Pearson Education, Inc. Capsid proteins and RNAdependent RNA polymerase dsRNA; + or sense strand with – or antisense strand; Reoviridae The Biosynthesis of RNA Viruses • Picornaviridae • Single-stranded RNA, + strand, nonenveloped • Enterovirus • Poliovirus and coxsackievirus • Rhinovirus • Common cold • Hepatitis A virus © 2016 Pearson Education, Inc. The Biosynthesis of RNA Viruses • Togaviridae • Single-stranded RNA, + strand, enveloped • Alphavirus • Transmitted by arthropods; includes chikungunya • Rubivirus • Rubella © 2016 Pearson Education, Inc. The Biosynthesis of RNA Viruses • Rhabdoviridae • Single-stranded RNA, − strand, one RNA strand • Lyssavirus • Rabies • Numerous animal diseases © 2016 Pearson Education, Inc. The Biosynthesis of RNA Viruses • Reoviridae • Double-stranded RNA, nonenveloped • Reovirus (respiratory enteric orphan) • Rotavirus (mild respiratory infections and gastroenteritis) © 2016 Pearson Education, Inc. Biosynthesis of RNA Viruses That Use DNA • Single-stranded RNA, produce DNA • Use reverse transcriptase to produce DNA from the viral genome • Viral DNA integrates into the host chromosome as a provirus • Retroviridae • Lentivirus (HIV) • Oncoviruses © 2016 Pearson Education, Inc. Figure 13.19 Multiplication and inheritance processes of the Retroviridae. Reverse transcriptase Capsid Envelope Virus Two identical + strands of RNA Host cell Retrovirus enters by fusion between attachment spikes and the host cell receptors. Mature retrovirus leaves the host cell, acquiring an envelope and attachment spikes as it buds out. DNA of one of the host cell's chromosomes Viral enzymes Uncoating releases the two viral RNA strands and the viral enzymes reverse transcriptase, integrase, and protease. Viral DNA Viral RNA Reverse transcriptase copies viral RNA to produce doublestranded DNA. Viral proteins are processed by viral protease; some of the viral proteins are moved to the host plasma membrane. Provirus Viral proteins Identical strands of RNA RNA Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that encodes the retrovirus capsid, enzymes, and envelope proteins. © 2016 Pearson Education, Inc. The new viral DNA is transported into the host cell's nucleus, where it's integrated into a host cell chromosome as a provirus by viral integrase. The provirus may be replicated when the host cell replicates. Table 13.2 Families of Viruses That Affect Humans (1 of 4) © 2016 Pearson Education, Inc. Table 13.2 Families of Viruses That Affect Humans (2 of 4) © 2016 Pearson Education, Inc. Table 13.2 Families of Viruses That Affect Humans (3 of 4) © 2016 Pearson Education, Inc. Table 13.2 Families of Viruses That Affect Humans (4 of 4) © 2016 Pearson Education, Inc. Check Your Understanding Describe the principal events of attachment, entry, uncoating, biosynthesis, maturation, and release of an enveloped DNA-containing virus. 13-10 © 2016 Pearson Education, Inc. Viruses and Cancer Learning Objectives 13-11 Define oncogene and transformed cell. 13-12 Discuss the relationship between DNA- and RNA-containing viruses and cancer. © 2016 Pearson Education, Inc. Viruses and Cancer • Several types of cancer are caused by viruses • May develop long after a viral infection • Cancers caused by viruses are not contagious • Sarcoma: cancer of connective tissue • Adenocarcinomas: cancers of glandular epithelial tissue © 2016 Pearson Education, Inc. The Transformation of Normal Cells into Tumor Cells • Oncogenes transform normal cells into cancerous cells • Oncogenic viruses become integrated into the host cell's DNA and induce tumors • A transformed cell harbors a tumor-specific transplant antigen (TSTA) on the surface and a T antigen in the nucleus © 2016 Pearson Education, Inc. DNA Oncogenic Viruses • Adenoviridae • Herpesviridae • Epstein-Barr virus • Poxviridae • Papovaviridae • Human papillomavirus • Hepadnaviridae • Hepatitis B virus © 2016 Pearson Education, Inc. RNA Oncogenic Viruses • Retroviridae • Viral RNA is transcribed to DNA (using reverse transcriptase), which can integrate into host DNA • HTLV-1 and HTLV-2 cause adult T cell leukemia and lymphoma © 2016 Pearson Education, Inc. Check Your Understanding What is a provirus? 13-11 How can an RNA virus cause cancer if it doesn't have DNA to insert into a cell's genome? 13-12 © 2016 Pearson Education, Inc. Latent Viral Infections and Persistent Viral Infections Learning Objectives 13-13 Provide an example of a latent viral infection. 13-14 Differentiate persistent viral infections from latent viral infections. © 2016 Pearson Education, Inc. Latent Viral Infections and Persistent Viral Infections • Latent virus remains in asymptomatic host cell for long periods • May reactivate due to changes in immunity • Cold sores, shingles • A persistent viral infection occurs gradually over a long period; is generally fatal • Subacute sclerosing panencephalitis (measles virus) © 2016 Pearson Education, Inc. Figure 13.21 Latent and persistent viral infections. Acute infection Latent infection Persistent infection © 2016 Pearson Education, Inc. Table 13.5 Examples of Latent and Persistent Viral Infections in Humans © 2016 Pearson Education, Inc. Check Your Understanding Is shingles a persistent or latent infection? 13-13, 13-14 © 2016 Pearson Education, Inc. Prions Learning Objective 13-15 Discuss how a protein can be infectious. © 2016 Pearson Education, Inc. Prions • Proteinaceous infectious particles • Inherited and transmissible by ingestion, transplant, and surgical instruments • Spongiform encephalopathies • • • • • "Mad cow disease" Creutzfeldt-Jakob disease (CJD) Gerstmann-Sträussler-Scheinker syndrome Fatal familial insomnia Sheep scrapie © 2016 Pearson Education, Inc. Prions • PrPC: normal cellular prion protein, on the cell surface • PrPSc: scrapie protein; accumulates in brain cells, forming plaques © 2016 Pearson Education, Inc. Prions: Overview PLAY © 2016 Pearson Education, Inc. Animation: Prions: Overview Prions: Characteristics PLAY © 2016 Pearson Education, Inc. Animation: Prions: Characteristics Prions: Diseases PLAY © 2016 Pearson Education, Inc. Animation: Prions: Diseases Figure 13.22 How a protein can be infectious. PrPSc PrPc PrPc produced by cells is secreted to the cell surface. PrPSc may be acquired or produced by an altered PrPc gene. PrPSc reacts with PrPc on the cell surface. PrPSc converts the PrPc to PrPSc. Lysosome Endosome PrPSc The new more PrPc. converts © 2016 Pearson Education, Inc. PrPSc The new is taken in, possibly by receptormediated endocytosis. PrPSc accumulates in endosomes. PrPSc continues to accumulate as the endosome contents are transferred to lysosomes. The result is cell death. Plant Viruses and Viroids Learning Objectives 13-16 Differentiate virus, viroid, and prion. 13-17 Describe the lytic cycle for a plant virus. © 2016 Pearson Education, Inc. Plant Viruses and Viroids • Plant viruses: enter through wounds or via insects • Plant cells are generally protected from disease by an impermeable cell wall • Viroids: short pieces of naked RNA • Cause potato spindle tuber disease © 2016 Pearson Education, Inc. Figure 13.23 Linear and circular potato spindle tuber viroid (PSTV). PSTV © 2016 Pearson Education, Inc. Table 13.6 Classification of Some Major Plant Viruses © 2016 Pearson Education, Inc. Check Your Understanding Contrast viroids and prions, and for each name a disease it causes. 13-15, 13-16 How do plant viruses enter host cells? 13-17 © 2016 Pearson Education, Inc.