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11/15/2016 PowerPoint® Lecture Presentations prepared by Bradley W. Christian, McLennan Community College CHAPTER 13 Viruses, Viroids, and Prions © 2016 Pearson Education, Ltd. Figure 7.6 Evaluation of disinfectants by the disk-diffusion method. Zone of inhibition Chlorine Hexachlorophene O-phenylphenol Quat Staphylococcus aureus (gram-positive) Chlorine O-phenylphenol Hexachlorophene Quat Escherichia coli (gram-negative) Chlorine Hexachlorophene O-phenylphenol Quat Pseudomonas aeruginosa (gram-negative) © 2016 Pearson Education, Ltd. 1 11/15/2016 General Characteristics of Viruses Learning Objective 13-1 Differentiate a virus from a bacterium. © 2016 Pearson Education, Ltd. 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, Ltd. 2 11/15/2016 Table 13.1 Viruses and Bacteria Compared © 2016 Pearson Education, Ltd. 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, Ltd. 3 11/15/2016 Figure 13.1 Virus sizes. Bacteriophage M13 800 × 10 nm 970 nm Bacteriophages f2, MS2 24 nm Poliovirus 30 nm Rhinovirus 30 nm Ebola virus 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 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 300 × 200 × 100 nm © 2016 Pearson Education, Ltd. 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, Ltd. 4 11/15/2016 Figure 13.2 Morphology of a nonenveloped polyhedral virus. Nucleic acid Capsomere Capsid © 2016 Pearson Education, Ltd. Figure 13.3 Morphology of an enveloped helical virus. Nucleic acid Capsomere Spikes Envelope Envelope is made of…? © 2016 Pearson Education, Ltd. 5 11/15/2016 General Morphology • • • • Helical viruses—hollow, cylindrical capsid Polyhedral viruses—many-sided Enveloped viruses Complex viruses—complicated structures © 2016 Pearson Education, Ltd. Figure 13.4 Morphology of a helical virus. Nucleic acid Capsomere Capsid © 2016 Pearson Education, Ltd. 6 11/15/2016 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, Ltd. Solunetti: © 2016 Pearson Education, Ltd. 7 11/15/2016 Isolation, Cultivation, and Identification of Viruses Learning Objectives 13-5 Describe how bacteriophages are cultured. 13-6 Describe how animal viruses are cultured. © 2016 Pearson Education, Ltd. 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, Ltd. 8 11/15/2016 Figure 13.6 Viral plaques formed by bacteriophages. Plaques © 2016 Pearson Education, Ltd. Figure 13.8 Cell cultures. Normal cells A tissue is treated with enzymes to separate the cells. 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. © 2016 Pearson Education, Ltd. 9 11/15/2016 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, Ltd. 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, Ltd. 10 11/15/2016 Figure 13.10 A viral one-step growth curve. © 2016 Pearson Education, Ltd. 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, Ltd. 11 11/15/2016 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, Ltd. 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, Ltd. 12 11/15/2016 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, Ltd. 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 Lysogenic cycle Lytic cycle Cell lyses, releasing phage virions. Phage DNA circularizes and enters lytic cycle or lysogenic cycle. OR New phage DNA and proteins are synthesized and assembled into virions. Lysogenic bacterium reproduces normally. Prophage Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage. © 2016 Pearson Education, Ltd. 13 11/15/2016 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, Ltd. 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, Ltd. 14 11/15/2016 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, Ltd. 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. Fusion of viral envelope and plasma membrane Entry of herpesvirus by fusion. © 2016 Pearson Education, Ltd. 15 11/15/2016 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, Ltd. 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, Ltd. 16 11/15/2016 Figure 13.15 Replication of a DNA-Containing Animal Virus. Papovavirus RELEASE Virions are released. ATTACHMENT Virion attaches to host cell. DNA A papovavirus is a typical DNA-containing virus that attacks animal cells. 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. A portion of viral DNA is transcribed, producing mRNA that encodes "early" viral proteins. 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, Ltd. 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, Ltd. 17 11/15/2016 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 Uncoating releases viral RNA and proteins. – strand is transcribed from + viral genome. KEY Viral protein Viral genome (RNA) Capsid protein + or sense strand of viral genome – or antisense strand of viral genome + strand ss = single-stranded ssRNA; + or sense strand; Picornaviridae mRNA is transcribed from the – strand. ds = double-stranded © 2016 Pearson Education, Ltd. 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 Cytoplasm Entry and uncoating Maturation and release ss = single-stranded Nucleus Host cell – strands are incorporated into capsid Additional – strands are transcribed from mRNA. Uncoating releases viral RNA and proteins. Viral genome (RNA) Viral protein ssRNA; – or antisense strand; Rhabdoviridae © 2016 Pearson Education, Ltd. 18 11/15/2016 Table 13.2 Families of Viruses That Affect Humans (1 of 4) © 2016 Pearson Education, Ltd. Table 13.2 Families of Viruses That Affect Humans (2 of 4) © 2016 Pearson Education, Ltd. 19 11/15/2016 Table 13.2 Families of Viruses That Affect Humans (3 of 4) © 2016 Pearson Education, Ltd. Table 13.2 Families of Viruses That Affect Humans (4 of 4) © 2016 Pearson Education, Ltd. 20 11/15/2016 Check Your Understanding ü Describe the principal events of attachment, entry, uncoating, biosynthesis, maturation, and release of an enveloped DNA-containing virus. © 2016 Pearson Education, Ltd. 21