Chapter 13 187 Characterizing and Classifying Viruses, Viroids, and Prions In 1892, Dmitri Ivanowski discovered tobacco mosaic virus (TMV). Frederick W. Twort in 1915, and Felix d’Herelle in 1917, indepdently discovered bacteriophage. The agent caused bacterial lysis, known as Twort-d’Herelle phenomenon.

187 Characteristics of Viruses Submicroscopic Filterable Contain either DNA or RNA, never both Contain no enzyme systems Contain no cellular organelles Obligate intracellular parasites

187 Genome: Capsid: Nucleocapsid Capsomers (capsomeres): Protomers: Peplomers (spikes): Virion:

Virions, complete virus particles Figure 13.1

188 Shapes of Viruses Helical: Polyhedral: Complex: Filamentous: Tobacco mosaic virus (TMV) Polyhedral: Herpes viruses Adenoviruses (icosahedron viruses) Complex: All enveloped viruses (cowpox, rabies, arboviruses) Filamentous: Ebola, Marburg, Granulosis virus (insects)

Enveloped viruses Figure 13.7

The complex shape of bacteriophage T4 Figure 13.6

Fig. 6.6.b

Basic types of viral morphology:

189 Classification of Viruses: Based on their structure, chemical composition and genetic makeup: 7 families of DNA viruses 15 families of RNA viruses

Contd.

189 Replication of Viruses: 1. Attachment (Adsorption) 2. Penetration Injection Endocytosis (phagocytosis or viropexis) Fusion 3. Eclipse phase 4. Replication 5. Assembly and maturation 6. Release Lysis Budding Exocytosis

Fig. 6.18.b

Three mechanisms of entry of animal viruses Figure 13.12

The process of budding in enveloped viruses Figure 13.14

190 Life Cycles: Lytic cycle: The host cell is destroyed by the virus. The virus is known as the virulent virus (phage).

The lytic replication cycle in bacteriophage Figure 13.8

The lysogenic replication cycle in bacteriophages Figure 13.11

190 Lysogenic cycle: Lysogeny is a process in which the viral DNA and the host DNA are joined together. The integrated viral DNA is known as the prophage or provirus. Both the cell and the virus can survive. The virus that carries out this type of life cycle is known as the temperate phage or avirulent phage.

Viral Replication Animation: Viral Replication: Temperate Bacteriophages

191 Replication of : dsDNA dsDNA assembled into dsDNA viruses mRNA Proteins Hepatitis B virus, herpesviruses, papillomaviruses and adenoviruses

192 ssDNA ssDNA dsDNA assembled into ssDNA viruses mRNA Proteins Parvoviruses (gastroenteritis), satellite viruses and insect viruses

192 Minus (-) Plus (+) strand strand RNA RNA Plus (+) strand assembled into RNA Proteins + strand RNA viruses Picornaviruses (poliovirus, coxsackievirus), coronaviruses (common cold virus) and hepatitis A virus

192 Minus (-) strand RNA Minus (-) assembled Strand RNA Plus (+) strand into – strand RNA RNA viruses Proteins Influenza virus, mumps virus and rabies virus

193 Plus (+) strand RNA Protein Minus (-) strand RNA assembled dsRNA into dsRNA viruses Plus (+) strand RNA dsRNA Minus (-) strand RNA Rotavirus, Colorado tick fever virus

193 reverse RNA DNA RNA assembled into mRNA Proteins RNA viruses transcriptase RNA DNA RNA assembled into mRNA Proteins RNA viruses Retroviruses: HIV, HTLV-1, HTLV-2

193-194 Roles of Viruses in Cancer DNA oncogenic viruses: Papilloma viruses: wart and cervical cancer Epstein-Barr virus: Burkitt’s lymphoma Herpes simplex type II: cervical cancer Hepatitis B virus: liver cancer Adenoviruses: respiratory, enteric and eye infections

Figure 13.16 The oncogene theory of the induction of cancer in humans.

194 RNA oncogenic viruses: HTLV-I: adult T-cell leukemia HTLV-II: hairy-cell leukemia

194 Culturing Viruses in the Laboratory: Cell culture or tissue culture Primary tissue culture Continuous tissue culture 2. Culturing viruses in bacteria 3. Culturing viruses in plants and animals Culturing viruses in embryonated chicken eggs

Fig. 6.20.c Plaque formation on the lawn of E. coli.

Fig. 6.21 Inoculation of chick embryos.

194 Monolayer: Hela cell line: Plaque: Plaque-forming unit (PFU): Cytopathic effect (CPE):

Cytopathic changes in animal cells.

195 Viroids: single-stranded or double-stranded RNA, no protein coat, 270-380 nucleotides, infect plants causing: potato spindle tuber disease citrus exocortis chrysanthemum stunt cucumber pale fruit disease cadang cadang in coconuts

One effect of viroids on plants Figure 13.21

Characteristics of Prions Infectious protein particles that cause neurological diseases Contain no DNA or RNA Cannot be digested by nucleases Can be digested by proteolytic enzymes Resistant to radiation Resistant to high temperture up to 90oC

195 Diseases Caused by Prions: Kuru disease Creutzfeldt-Jakob disease Scrapie of sheep Chronic wasting disease (CWD) in deer and elk Mad cow disease

Formation of Prions Two types of Prions: 1. PrPC : is the normal cellular prion protein Made by all mammals Normal structure with -helices called cellular PrPC 2. PrPSc: (PrPSc stands for scrapie prion protein). It is the disease-causing form with -sheets. The PrPSc converts cellular prion protein (PrPC) into the disease-causing form PrPSc by inducing conformational change.

Other Parasitic Particles: Viroids and Prions Animation: Prions: Characteristics

Fig. 6.22 Mad cow disease (bovine spongiform encephalopathy) caused by prions.

196 Satellite viruses: These are defective viruses that require a helper virus in order to replicate. Examples: Adeno-associated virus (AAV) requires adenovirus. Delta agent requires hepatitis B virus.