1 Chapter 13 Viruses, Viroids & Prions

2 Viruses - General Characteristics obligate intracellular parasites (completely inert when not in a host cell) very small (filterable) contain a single type of nucleic acid (RNA or DNA) contain a protein coat which surrounds the nucleic acid (sometimes the coat is surrounded by a lipid envelope) multiply inside living cells by using the cell’s own molecular machinery cause the synthesis of special structures to facilitate the transfer of the virus to other cells

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4 Host Range most viruses can only infect a limited range of cell types (& usually only one host species) bacteriophages (or phages) are viruses which infect bacteria host range is determined by the specific binding of the virus to particular structures, called receptors, on the surface of the host cell virus binds to the receptor via hydrogen bonding Viral size – range 20nm – 975nm (refer to fig 13.1)

5 Viral Structure Virion - complete, fully developed, infectious viral particle composed of nucleic acid which is surrounded by a protein coat which protects it from the environment and is the vehicle of transmission from one host cell to the next Variations seen in:  type of nucleic acid  type of protein coat or envelope  morphology

6 Nucleic Acids Variations single-stranded DNA viruses double-stranded DNA viruses single-stranded RNA viruses (+ strand or – strand) + strand viruses are able to directly synthesize proteins from their genome - strand viruses must transcribe a + strand to serve as mRNA before synthesizing proteins double-stranded RNA viruses depending on the virus, the nucleic acid can be linear or circular (some, like influenza virus, have multiple nucleic acid segments)

7 Capsid and Envelope capsid: the protein coat surrounding the nucleic acid of the virus capsomeres: protein subunits of the capsid envelope: the capsid of some viruses is surrounded by a lipid, protein, and carbohydrate envelope in some animal viruses, this envelope is derived from the host cell membrane as viral particles are extruded from the cell – think about this, how would this strategy benefit the virus? spikes composes of protein-carbohydrate complexes on the surface of the envelope can be used in attachment to host cells, also used in identification of viruses

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9 General Morphology Various shapes seen in viruses include… Polyhedral viruses Helical viruses Enveloped viruses Complex viruses

10 many-sided icosahedron (20 triangular faces) is the most common form of ‘many-sided’

11 long rods (rigid or flexible) consisting of a hollow, cylindrical capsid which has a helical structure. Nucleic acid is within the hollow capsid

12 surrounded by an envelope, are roughly spherical can have helical or polyhedral capsid

13 Complex viruses have complicated structures, often both helical and polyhedral portions

14 Viral Taxonomy Viruses are grouped based on:  nucleic acid type  strategy for replication  morphology a viral species is a group of viruses sharing the same genetic information and ecological niche no species epithets (names) have been given instead common names are used such as human immunodeficiency virus (HIV) see Table 13.2 for virus families

15 Cultivation of Viruses All viruses can only multiply inside host cells Different techniques are required to grow & maintain viruses. Techniques used depend on the normal host of the virus

16 Cultivation of Bacteriophages Bacteriophages – viruses that prey upon bacteria grown in either liquid cultures of bacteria or in bacterial cultures on solid medium in a bacterial lawn on a plate, viruses can be detected by the formation of plaques, zones of clearing produced by the killing of bacteria (figure will appear momentarily) like colony counting in bacteria, each plaque represents one viral infection, or plaque-forming unit (pfu)

17 Cultivation of Animal Viruses may be cultivated in living animals, embryonated (fertilized) eggs, or in cell culture Observers see cytopathic effect a cell deterioration as a result of virus activity This effect can be observed and counted similar to counting viral plaques in bacteriophage

18 Healthy mouse cells in culture Same cells 24 hours after viral infection

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20 Transduction the process of….. transferring DNA from one bacterial cell to another by a bacteriophage

21 Multiplication of Bacteriophages Lytic Cycle - all infected cells are lysed and killed (fig , p. 387) Lysogenic Cycle - most infected cells are lysed and killed (fig , p. 389) Some cells have a phage which inserts in the chromosome and becomes dormant (a prophage), this is called lysogeny Cell replicates normally; at some later time, phage can enter lytic cycle This phenomenon makes transduction possible  specialized transduction transfers specific genes;  generalized transfers any gene

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26 Multiplication of Animal Viruses similar steps as in bacteriophages, but some differences (see Table 13.3, p. 391) also see differences in multiplication of nucleic acid between DNA and RNA viruses (Table 13.4, p. 393) General steps: (in order)  Attachment to cell  Penetration of cell membrane  Uncoating  Biosynthesis  Maturation & Release

27 Viruses and Cancer viruses first shown to cause some cancers in 1908 (chicken leukemia) hard to establish link due to:  cancer may develop long after the viral infection  cancer are not contagious like viral diseases Transformation (in the context of cancer): the process of altering a normal cell with a virus to make it cancerous – generally by inserting DNA (done by an oncovirus ) an oncogene is the area of the cells DNA which is altered transformed cells no longer demonstrate contact inhibition (p 396)

28 Latent and Slow Viral Infections Latent infection: virus remains dormant in the host and does not produce the disease for some length of time example: herpes simplex virus (cold sores) Slow Viral Infection: disease progresses slowly

29 Prions a prion is an infectious protein particle Prion  proteinaceous infectious particle the protein is an altered normal cellular protein, the altered form is capable of catalyzing the alteration of other molecules of the same protein example: scrapie, mad cow disease, Creutzfeldt-Jakob disease Thus far 9 animal diseases fall into this category – all are neurological spongiform encephalopathys (large voids develop in brain) “Spongy brains”

30 Healthy Cell mRNA translated into normal proteins Prion (altered/infectious protein) enters normal cell Prion transforms normal protein into prion prion

31 Prion Diseases – Non-Human Animals Scrapie: sheep TME (transmissible mink encephalopathy): mink CWD (chronic wasting disease): muledeer, elk BSE (bovine spongiform encephalopathy): cows (“Mad Cow”)

32 Prion Diseases - Human CJD: Creutzfeld-Jacob Disease. Cerebral involvement so dementia is more common and patient seldom survives a year (originally thought to be sporadic, but now known to be familial as well). GSS: Gerstmann-Straussler-Scheinker syndrome. characterised by cerebellar ataxia and concomitant motor problems, dementia less common and disease course lasts several years to death. FFI: Fatal familial Insomnia (characterized by severe selective atrophy of the thalamus ) Alpers Syndrome - Prion disease of infants Kuru highlands of New Guinea. Funeral ritual of respect - ingesting brain tissue of dead relatives

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