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Characterizing and Classifying Viruses and Prions
Chapter 13 Characterizing and Classifying Viruses and Prions
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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, but some have a membrane-like envelope. Have extracellular and intracellular state © 2012 Pearson Education Inc. 3
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Characteristics of Viruses
Extracellular State Called virion Protein coat (capsid) surrounding nucleic acid Some have phospholipid envelope Outermost layer provides protection and recognition sites for host cells Intracellular State Capsid removed Virus exists as nucleic acid © 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 Much smaller than genomes of cells © 2012 Pearson Education Inc. 6
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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 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 Bacteriophages are viruses that infect bacteria. © 2012 Pearson Education Inc. 8
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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 © 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 Envelope’s proteins 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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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 lytic replication cycle Attachment Entry Synthesis Assembly Release © 2012 Pearson Education Inc. 16
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Figure 13.8 The lytic replication cycle in bacteriophages-overview
Attachment Bacteriophage genome Entry Tail sheath Outer membrane Peptidoglycan Cytoplasmic membrane Bacterial chromosome Entry Attachment Phage DNA Lytic replication cycle of bacteriophage Bacterial chromosome degraded Release Synthesis Phage proteins Assembly Assembly Base Tail Sheath DNA Capsid Mature head Tail fibers Mature virion
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Viral Replication Lysogeny Modified replication cycle
Infected host cells grow and reproduce normally for generations before they lyse Virus gets inserted into host cell DNA. © 2012 Pearson Education Inc. 18
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Figure 13.11 The lysogenic replication cycle in bacteriophages: phage lambda and E. coli
Attachment Entry Prophage in chromosome Lambda phage Lytic cycle Lysogeny Synthesis Release Replication of chromosome and virus; cell division Assembly Induction Further replications and cell divisions
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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 © 2012 Pearson Education Inc. 20
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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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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 Herpesvirus Incorporation of provirus into host DNA is permanent © 2012 Pearson Education Inc. 22
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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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Some consider them complex pathogenic chemicals
Are Viruses Alive? Some consider them complex pathogenic chemicals Others consider them 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 © 2012 Pearson Education Inc. 24
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Other Parasitic Particles: Viroids and Prions
Characteristics of Prions Proteinaceous infectious agents. Prion diseases Fatal neurological degeneration and loss of brain matter Large vacuoles form in brain Characteristic spongy appearance Spongiform encephalopathies Prions are extremely difficult to kill. Must be incinerated. © 2012 Pearson Education Inc. 25
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Figure 13.23 The brain of a sheep with the prion disease called scrapie
Vacuole
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