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Chapter 13 Characterizing Viruses, Viroids, and Prions
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Not all pathogens are cellular!
Many infections of humans, animals, plants, and even bacteria are caused by _________ ____________________ Acellular infectious particles include _____________
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General characteristics of viruses, viroids, and prions
Simple compared to cells Lack cell membranes Composed of 1 or a few organic molecules Lack most of the characteristics of life described in chapters 3, 5, 6, and 7
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Shared Characteristics of Viruses
Minuscule (usually measured in nanometers), acellular infectious agent having either ____ or ______ _______________________ Cause many infections in humans, animals, plants, and bacteria Cause most of the diseases that plague the industrialized world Examples: common cold, influenza, herpes, SARS, Polio, HIV 4
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Shared Characteristics of Viruses
__________carry out any metabolic pathway outside of a cell Neither _________________to the environment Cannot _____________independently Recruit the cell’s (animal, plant, or bacterial cells) metabolic pathways to reproduce No cytoplasmic membrane, cytosol (liquid portion of cytoplasm), organelles Have extracellular and intracellular state 5
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Shared Characteristics of Viruses
Extracellular State Protein coat (________) surrounding nucleic acid Some have a phospholipid ____________which surrounds the capsid Outermost layer provides ____________and _____________sites for host cells Intracellular State Capsid removed Virus exists as nucleic acid 6
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Figure 13.1 Virions-overview
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Differentiating Viruses
We can differentiate viruses from one another based on their _________________ Show more variety in genomes than cells The genetic material a virus contains is the primary way scientists categorize and classify viruses 8
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Differentiating Viruses
We can differentiate viruses from one another based on their Genetic Material Viral genome may be DNA or RNA, but never both _________________________________ ds= double stranded, ss= single stranded Linear and segmented or single and circular Influenza virus genome has 8 linear segments of ssRNA Much smaller than genomes of cells Cells __________have ___________________DNA ssDNA and dsRNA are almost nonexistent in cells
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Differentiating Viruses
Hosts of Viruses Most viruses infect only particular host’s cells _________________ Dog viruses don’t infect humans May be so specific they infect only particular kind of cell in a particular host HIV attacks helper T lymphocytes in humans but does not infect muscle or bone cells _______________– infect many kinds of cells in many different hosts Rabies 10
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Differentiating Viruses
Host specificity Due to viral surface ____________which have a precise __________ (attraction) for complementary proteins on the host cell membranes
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Figure 13.3 Hosts of viral infections-overview
Tobacco mosaic virus infected leaf on left Bacteria (blue/gray) under attack from a bacteriophage (pink) Human WBC cytoplasmic membrane with HIV particles (blue) attached
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Differentiating Viruses
Hosts of Viruses ____________________can be infected by a virus Archaeal, bacterial, plant, protozoan, fungal, animal Most research focuses on animal and bacterial viruses A virus that infects a bacterium is called a ________________or phage Bacteriophages out number all bacteria, archaea, and eukaryotes put together!
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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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Differentiating Viruses
Capsid Morphology Capsids Provide ________________for viral nucleic acid Means of _________________to host’s cells Composed of proteinaceous subunits called _______________ Capsomere made of single or multiple types of proteins 15
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Differentiating Viruses
Viral Shapes Three basic shapes ___________(capsomeres bond in a spiral fashion) ___________(close to spherical, geodesic dome) ___________(many different shapes, bullet shaped rabies virus) 16
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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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Differentiating Viruses
The ________________ Acquired from host cell during viral replication or release Envelope is portion of _____________system of _______ Composed of ____________bilayer and _________ Some proteins are virally coded glycoproteins (spikes) Envelope’s proteins and glycoproteins often play role in host recognition 19
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Figure 13.7 Enveloped virion-overview
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Viral Replication Dependent on hosts’ organelles and enzymes to produce new viral particles ___________________ Replication cycle usually results in ______and ______of host cell Basic stages of lytic replication cycle Recognition and Attachment Entry Chromosome degraded Synthesis Assembly Release 21
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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 __________________ Modified replication cycle
Infected host cells _______________________for generations before they lyse Inactive bacteriophage is called a _____________ ______________occurs and the prophage is excised from the host chromosome Induction can occur through DNA damaging chemicals, UV light, X rays After induction the _______________will occur 23
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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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Transduction
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Viral Replication Replication of Animal Viruses
Same basic replication pathway as bacteriophages Recognition and Attachment Entry Chromosome degraded Synthesis Assembly Release 26
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Viral Replication Replication of Animal Viruses
Attachment of animal viruses Chemical attraction between glycoproteins or proteins on the virus exactly fitting _______________________on animal cells Animal viruses do not have tails or tail fibers like bacteriophages do Have glycoprotein spikes or other attachment molecules that mediate attachment 27
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Figure 13.12 Three mechanisms of entry of animal viruses-overview
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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 29
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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 ____________________ Released from cell by _______________ Naked viruses are released by __________or _____ 30
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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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Viral Replication Replication of Animal Viruses
Latency of animal viruses (chicken pox and herpes virus) Called __________viruses or proviruses When animal viruses remain dormant in host cells May be prolonged for years with no viral activity Different than ___________ Some latent viruses do not become incorporated into host chromosome _______________________________________________________ Incorporation of provirus into host DNA is _________ HIV 32
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The Role of Viruses in Cancer
Viruses cause ___________ of human cancers Some carry copies of oncogenes as part of their genomes Oncogenes are involved in cell division and are usually repressed (not activated) and no cancer results Some promote oncogenes already present in host Some interfere with tumor repression Specific viruses are known to cause human cancers Kaposi’s sarcoma (HIV) Cervical cancer (HPV) 33
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Culturing Viruses in the Laboratory
Viruses must be _____________________________ ________________________________________ Culturing Viruses in Mature Organisms In bacteria In plants and animals Culturing Viruses in Embryonated Chicken Eggs Inexpensive, among the largest of cells, free of contaminating microbes, and contain a nourishing yolk Culturing Viruses in Cell (Tissue) Culture 34
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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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Are Viruses Alive? Living characteristics: Nonliving characteristics:
Infectious agents with both living and non-living characteristics Living characteristics: Nonliving characteristics: 36
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Not all viruses are bad! A previously unknown virus has been found which attackes tiny marine alga that multiplies and forms algal blooms Algal blooms can often be seen from space! Deplete water of oxygen Harming fish and other marine life A bacteriophage has been discovered which transfers genes for photosynthetic machinery into cyanobacteria increasing the rate of photosynthesis Up to 10 million of these viruses have been found in a single milliliter of seawater! Much of the oxygen we breath may be attributed to the action of this virus on cyanobacteria
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Other Parasitic Particles: Viroids and Prions
Characteristics of ____________ Extremely small, _______________________ __________________________ Similar to RNA viruses, but ________capsid No known animal diseases are known to be caused by viroids 39
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Figure 13.21 One effect of viroids on plants
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Characteristics of Prions
Proteinaceous infectious agents Cause spongiform encephalopathies: Mad cow Scrapie Kuru Creutzfeld-Jakob syndrome _____________to proteases, UV light, heat, disinfectants
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Prions Characteristics of Prions Prion diseases
Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter Large vacuoles form in brain Characteristic spongy appearance Spongiform encephalopathies Prions only destroyed by incineration or autoclaving in 1 N NaOH 42
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Prions Characteristics of Prions Proteinaceous infectious agents
Cellular PrP protein Made by all mammals Normal structure with -helices called cellular PrP Prion PrP Disease-causing form with -pleated sheets called prion PrP Prion PrP changes shape of cellular PrP so it becomes prion PrP 43
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Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP)-overview
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