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Characterizing and Classifying Viruses, Viroids, and Prions
CHAPTER 13 Characterizing and Classifying Viruses, Viroids, and Prions
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Characteristics of Viruses
Minuscule, acellular, infectious agent having either DNA or RNA Cause infections of humans, animals, plants, and bacteria Cause most of the diseases that plague the industrialized world Cannot carry out any metabolic pathway Neither grow nor respond to the environment Cannot reproduce independently Recruit the cell's metabolic pathways to increase their numbers No cytoplasmic membrane, cytosol, organelles Have extracellular and intracellular state
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Characteristics of Viruses
Extracellular state Called virion Protein coat (capsid) surrounding nucleic acid Nucleic acid and capsid also called nucleocapsid Some have phospholipid envelope Outermost layer provides protection and recognition sites for host cells Intracellular state Capsid removed Virus exists as nucleic acid
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Capsid (sectioned to show interior) Nucleic acid (viral genome)
Figure Virions, complete virus particles, include a nucleic acid, a capsid, and in some cases an envelope. Capsid (sectioned to show interior) Nucleic acid (viral genome)
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Characteristics of Viruses
Genetic Material of Viruses Show more variety in nature of their genomes than do cells Primary way scientists categorize and classify viruses May be DNA or RNA, but never both Can be dsDNA, ssDNA, dsRNA, ssRNA May be linear and segmented or single and circular Much smaller than genomes of cells
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Figure 13.2 The relative sizes of genomes.
Partial genome of E. coli Viral genome
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Characteristics of Viruses
Hosts of Viruses Most viruses infect only particular host's cells Due to affinity of viral surface proteins for complementary proteins on host cell surface May be so specific they only infect particular kind of cell in a particular host Generalists – infect many kinds of cells in many different hosts
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Figure 13.3 Some examples of plant, bacterial, and human hosts of viral infections.
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Figure 13.4 Sizes of selected virions.
E. coli (bacterium) (1000 nm x 3000 nm) Red blood cell (10,000 nm in diameter) Bacterial ribosomes (25 nm) Smallpox virus (200 nm x 300 nm) Poliovirus (30 nm) Bacteriophage T4 (50 nm x 225 nm) Bacteriophage MS2 (24 nm) Tobacco mosaic virus (15 nm x 300 nm)
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Characteristics of Viruses
Capsid Morphology Capsids Provide protection for viral nucleic acid Means of attachment to host's cells Composed of proteinaceous subunits called capsomeres Capsomere may be made of single or multiple types of proteins
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Figure 13.5 The shapes of virions.
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Figure 13.6 The complex shape of bacteriophage T4.
Head Tail fibers Tail Base plate
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Characteristics of Viruses
The Viral Envelope Acquired from host cell during viral replication or release Envelope is portion of membrane system of host Composed of phospholipid bilayer and proteins Some proteins are virally coded glycoproteins (spikes) Envelope proteins and glycoproteins often play role in host recognition
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Figure 13.7 Enveloped virion.
Glycoproteins Helical capsid Matrix protein Envelope Enveloped virus with helical capsid
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Viral Replication Dependent on hosts' organelles and enzymes to produce new virions Lytic replication Viral replication usually results in death and lysis of host cell Five stages of lytic replication cycle Attachment Entry Synthesis Assembly Release
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Viral Replication: Overview
PLAY Viral Replication: Overview
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Figure 13.8 The lytic replication cycle in bacteriophages.
Attachment Bacteriophage genome Entry Tail sheath Outer membrane Peptidoglycan Cytoplasmic membrane Bacterial chromosome 2 Entry 1 Attachment Phage DNA Lytic replication cycle of bacteriophage 3 Bacterial chromosome degraded 6 Release 4 Synthesis Phage proteins 5 Assembly Assembly Base Tail Sheath DNA Capsid Mature head Tail fibers Mature virion
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Figure 13.9 Pattern of virion abundance in lytic cycle.
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Viral Replication: Virulent Bacteriophages
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Viral Replication Lysogeny Modified replication cycle
Infected host cells grow and reproduce normally for generations before they lyse Temperate phages Prophages – inactive phages Lysogenic conversion Results when phages carry genes that alter phenotype of a bacterium
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Figure 13.11 The lysogenic replication cycle in bacteriophages.
Attachment 3 Prophage in chromosome 2 Entry Lambda phage Lytic cycle Lysogeny 6 Synthesis 8 Release 4 Replication of chromosome and virus; cell division 7 Assembly 5 Induction Further replications and cell divisions
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Viral Replication: Temperate Bacteriophages
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Viral Replication Replication of Animal Viruses
Same basic replication pathway as bacteriophages Differences result from Presence of envelope around some viruses Eukaryotic nature of animal cells Lack of cell wall in animal cells
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Viral Replication Replication of Animal Viruses
Attachment of animal viruses Chemical attraction between viral protein and cell receptor Animal viruses do not have tails or tail fibers Have glycoprotein spikes or other attachment molecules that mediate attachment
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Figure 13.12 Three mechanisms of entry of animal viruses.
Phage genome inside capsid Cytoplasmic membrane of host engulfs virus (endocytosis) 1 2 Capsid 3 2 1 3 4 Receptors on cytoplasmic membrane Viral genome Direct penetration 6 5 Viral genome Viral glycoproteins remain in cytoplasmic membrane 1 Viral glycoproteins Uncoating capsid 2 Envelope 3 Endocytosis 4 Receptors on cytoplasmic membrane of host Viral genome Uncoating capsid Membrane fusion
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Viral Replication Replication of Animal Viruses
Synthesis of DNA viruses of animals Each type of animal virus requires different strategy depending on its nucleic acid DNA viruses often enter the nucleus RNA viruses often replicate in the cytoplasm
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Viral Replication Replication of Animal Viruses Some exceptions
Poxvirus replication occurs in the cytoplasm Hepatitis B viruses replicate DNA from an RNA intermediary
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Viral Replication Replication of Animal Viruses
Parvoviruses have ssDNA genomes Host enzymes produce DNA strand complementary to viral genome to form dsDNA molecule dsDNA used for viral replication and transcription
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Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses.
Receptors on cytoplasmic membrane of host –ssRNA virus dsRNA virus +ssRNA virus +ssRNA –ssRNA dsRNA Transcription by viral RNA polymerase Transcription by RNA-dependent RNA transcriptase Unwinding –ssRNA +ssRNA acts as mRNA Translation of viral proteins, genome acts as mRNA Complementary +ssRNA to act as template and as mRNA Transcription by viral RNA polymerase to make complementary RNA strands Complementary –ssRNA to act as template Further transcription Further transcription Translation of viral proteins Copies of –ssRNA Translation of viral proteins Copies of +ssRNA Assembly Assembly Assembly Positive-sense ssRNA virus Negative-sense ssRNA virus Double-stranded RNA virus
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Viral Replication Replication of Animal Viruses
Synthesis of RNA viruses of animals Retroviruses Do not use their genomes as mRNA Use DNA intermediary transcribed by viral reverse transcriptase as template to produce viral genomes
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Viral Replication Replication of Animal Viruses
Assembly and release of animal viruses Most DNA viruses assemble in nucleus Most RNA viruses develop solely in cytoplasm Number of viruses produced depends on type of virus and size and initial health of host cell Enveloped viruses cause persistent infections Naked viruses are released by exocytosis or lysis
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Figure 13.14: The process of budding in enveloped viruses.
virion 5 Budding of enveloped virus 3 4 2 1 Viral glycoproteins Cytoplasmic membrane of host Viral capsid
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Figure 13.15 Pattern of virion abundance in persistent infections.
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Viral Replication: Animal Viruses
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Viral Replication Replication of Animal Viruses
Latency of animal viruses When animal viruses remain dormant in host cells Viruses are called latent viruses or proviruses May be prolonged for years with no viral activity Some latent viruses do not become incorporated into host chromosome Incorporation of provirus into host DNA is permanent
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The Role of Viruses in Cancer
Cell division is under strict genetic control Genes dictate that some cells can no longer divide at all Cells that can divide are prevented from unlimited division Genes for cell division "turned off" or genes inhibiting division "turned on" Neoplasia Uncontrolled cell division in multicellular animal Mass of neoplastic cells is tumor Benign vs. malignant tumors Malignant tumors also called cancers Metastasis occurs when tumors spread
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Figure 13.16 The oncogene theory of the induction of cancer in humans.
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The Role of Viruses in Cancer
Viruses cause 20–25% of human cancers Some carry copies of oncogenes as part of their genomes Some promote oncogenes already present in host Some interfere with tumor repression Specific viruses are known to cause ~15% of human cancers Burkitt's lymphoma Hodgkin's disease Kaposi's sarcoma Cervical cancer
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Culturing Viruses in the Laboratory
Viruses cannot grow in standard microbiological media Cultured inside host cells Three types of media for culturing viruses Media consisting of mature organisms Embryonated eggs Cell cultures
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Culturing Viruses in the Laboratory
Culturing Viruses in Mature Organisms Culturing viruses in bacteria Phages grown in bacteria in liquid cultures or on agar plates Lysis of bacteria produces plaques Allows estimation of phage numbers by plaque assay
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Bacterial lawn Viral plaques
Figure Viral plaques in a lawn of bacterial growth on the surface of an agar plate. Bacterial lawn Viral plaques
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Culturing Viruses in the Laboratory
Culturing Viruses in Mature Organisms Culturing viruses in plants and animals Numerous plants and animals have been used to culture viruses Laboratory animals can be difficult and expensive to maintain Ethical concerns
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Culturing Viruses in the Laboratory
Culturing Viruses in Embryonated Chicken Eggs Inexpensive Among the largest of cells Free of contaminating microbes Contain a nourishing yolk Fertilized chicken eggs are often used Embryonic tissues provide ideal site for growing viruses Some vaccines prepared in chicken cultures
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Chorioallantois/mesodermal layers
Figure Inoculation sites for the culture of viruses in embryonated chicken eggs. Air sac Injection into chorioallantoic membrane Injection into Chorioallantois/mesodermal layers Injection into embryo Injection into amnion Injection into yolk sac
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Figure 13.19 An example of cell culture.
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Are Viruses Alive? Some consider them complex pathogenic chemicals
Others consider them to be the least complex living entities Use sophisticated methods to invade cells Have the ability to take control of their host cell Are able to replicate themselves
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Other Parasitic Particles: Viroids and Prions
Characteristics of Viroids Extremely small, circular pieces of RNA that are infectious and pathogenic in plants Similar to RNA viruses, but lack capsid May appear linear due to hydrogen bonding
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(Potato spindle tuber viroid) Genome of bacteriophage T7
Figure The RNA strand of the small potato spindle tuber viroid (PSTV). PSTV (Potato spindle tuber viroid) Genome of bacteriophage T7
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Figure 13.21 One effect of viroids on plants.
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Other Parasitic Particles: Viroids and Prions
Characteristics of Prions Proteinaceous infectious agents Cellular PrP Made by all mammals Normal, functional structure has -helices Prion PrP Disease-causing form has -sheets Prion PrP causes cellular PrP to refold into prion PrP
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Prions: Overview
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-helices -pleated sheet
Figure The two stable, three-dimensional forms of prion protein (PrP). -helices -pleated sheet Cellular PrP Prion PrP
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Prions: Characteristics
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Other Parasitic Particles: Viroids and Prions
Characteristics of Prions Prion diseases Spongiform encephalopathies Large vacuoles form in brain Characteristic spongy appearance BSE, vCJD, kuru Transmitted by ingestion, transplantation, or contact of mucous membranes with infected tissues Prions destroyed by incineration or autoclaving in concentrated sodium hydroxide
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Figure A brain showing the large vacuoles and spongy appearance typical in prion-induced diseases. Vacuole
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Prions: Diseases PLAY Prions: Diseases
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