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INTRODUCTION TO VIRUSES
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Characteristics of Viruses
Minuscule, acellular infectious agent having either DNA or RNA Causes many infections of humans, animals, plants, and bacteria Causes most of the diseases that plague the industrialized world © 2012 Pearson Education Inc. 2
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Characteristics of Viruses
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 (with one exception) © 2012 Pearson Education Inc. 3
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Characteristics of Viruses
Virus Structure 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 © 2012 Pearson Education Inc. 4
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Figure 13.1 Virions-overview
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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 dsDNA, ssDNA, dsRNA, ssRNA Single or segmented and linear and circular Much smaller than genomes of cells © 2012 Pearson Education Inc. 6
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Characteristics of Viruses
Hosts of Viruses Most viruses infect only particular host’s cells Affinity of viral surface proteins for proteins on host cell May be so specific they infect only particular kind of cell in a particular host Generalists – infect many kinds of cells in many different hosts © 2012 Pearson Education Inc. 7
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Figure 13.3 Hosts of viral infections-overview
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Figure 13.4 Sizes of selected virions
E. coli (bacterium) (1000 nm 3000 nm) Red blood cell (10,000 nm in diameter) Bacterial ribosomes (25 nm) Smallpox virus (200 nm 300 nm) Poliovirus (30 nm) Bacteriophage T4 (50 nm 225 nm) Bacteriophage MS2 (24 nm) Tobacco mosaic virus (15 nm 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 made of single or multiple types of proteins © 2012 Pearson Education Inc. 10
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Characteristics of Viruses
Viral Shapes Three basic shapes Helical Polyhedral Complex © 2012 Pearson Education Inc. 11
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Figure 13.5 The shapes of virions-overview
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Figure 13.6 Bacteriophage T4-overview
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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’s proteins and glycoproteins often play role in host recognition © 2012 Pearson Education Inc. 14
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Figure 13.7 Enveloped virion-overview
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Table 13.2 Families of Human Viruses
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Dependent on hosts’ organelles and enzymes to produce new virions
Viral Replication Dependent on hosts’ organelles and enzymes to produce new virions Lytic replication Replication cycle usually results in death and lysis of host cell Stages of replication cycle Attachment Entry Synthesis Assembly Release © 2012 Pearson Education Inc. 17
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Replication of Viruses
Viral Replication Replication of Viruses Attachment of animal viruses Chemical attraction Have glycoprotein spikes or other attachment molecules that mediate attachment © 2012 Pearson Education Inc. 18
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Figure 13.12 Three mechanisms of entry of animal viruses-overview
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Replication of Animal Viruses
Viral Replication Replication of Animal Viruses Synthesis of animal viruses Requires different strategy depending on its nucleic acid DNA viruses often enter the nucleus RNA viruses often replicate in the cytoplasm Must consider How mRNA is synthesized What serves as template for nucleic acid replication © 2012 Pearson Education Inc. 20
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Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses-overview
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Replication of Animal Viruses
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 © 2012 Pearson Education Inc. 22
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Figure 13.14 The process of budding in enveloped viruses
Enveloped virion Budding of enveloped virus Viral glycoproteins Cytoplasmic membrane of host Viral capsid
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Figure 13.15 Pattern of virion abundance in persistent infections
Number of infective virions in medium Synthesis and assembly Release of virions Entry Time Attachment
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Replication of Animal Viruses
Viral Replication Replication of Animal Viruses Latency of animal viruses When animal viruses remain dormant in host cells 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 © 2012 Pearson Education Inc. 25
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The Role of Viruses in Cancer
Animal’s genes dictate that some cells can no longer divide or 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 Metastasis Cancers © 2012 Pearson Education Inc. 26
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Figure 13.16 The oncogene theory of the induction of cancer in humans
Normal state: DNA Protooncogene Gene for repressor Represses mRNA Repressor Result: No cancer First “hit”: Virus inserts promoter DNA Oncogene Gene for repressor Represses mRNA Repressor Result: Still no cancer Second “hit”: Virus inserts into represssor gene DNA Oncogene No repressor protein because gene is segmented mRNA Protein Causes cell division Result: Cancer
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The Role of Viruses in Cancer
Environmental factors that contribute to the activation of oncogenes Ultraviolet light Radiation Carcinogens Viruses © 2012 Pearson Education Inc. 28
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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 © 2012 Pearson Education Inc. 29
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