Talaro

Chapter 6  Louis Pasteur postulated that was caused by a (1884)  Ivanovski and Beijerinck showed a disease in tobacco was caused by a virus (1890s)  1950s was a multifaceted discipline  : noncellular particles with a definite size, shape, and chemical composition

2  There is no universal agreement on how and when viruses originated  Viruses are considered the most abundant microbes on earth  Viruses played a role in evolution of , , and Eukarya  Viruses are obligate intracellular parasites

3 4  Size range –  most <0.2 μm; requires electron microscope

5 Viruses bear no resemblance to cells Lack protein-synthesizing machinery

Viruses contain only the parts needed to invade and control a host

6 7   All viruses have capsids - protein coats that enclose and protect their  The together with the nucleic acid are nucleocapsid  Some viruses have an external covering called envelope; those lacking an envelope are naked  Each capsid is constructed from identical subunits called capsomers made of protein

8 9  Two structural types:  Helical - continuous helix of capsomers forming a cylindrical nucleocapsid  Icosahedral - 20-sided with 12 corners  Vary in the number of capsomers  Each capsomer may be made of 1 or several proteins  Some are enveloped

10 11 Figure 6.7 Figure 6.8

12   Mostly viruses  Acquired when the virus leaves the host cell  Exposed proteins on the outside of the envelope, called spikes, essential for attachment of the virus to the host cell

13  Protects the nucleic acid when the virus is outside the host cell

 Helps the virus to bind to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell

14  Complex viruses: atypical viruses  Poxviruses lack a typical capsid and are covered by a dense layer of lipoproteins

 Some have a polyhedral nucleocapsid along with a helical tail and attachment fibers

15 Figure 6.9

16 17  Viral – either DNA or RNA but never both  Carries necessary to invade host cell and redirect cell’s activity to make new viruses  Number of genes varies for each type of virus – few to hundreds

18  DNA viruses  Usually double stranded (ds) but may be single stranded (ss)  Circular or linear

 RNA viruses  Usually single stranded, may be double stranded, may be segmented into separate RNA pieces  ssRNA ready for immediate are positive-sense RNA  ssRNA genomes that must be converted into proper form are negative-sense RNA

19  Pre-formed enzymes may be present  Polymerases – DNA or RNA  Replicases – copy RNA  Reverse transcriptase – synthesis of DNA from RNA (AIDS virus)

20  Main criteria presently used are structure, chemical composition, and genetic makeup  Currently recognized: 3 orders, 63 families, and 263 genera of viruses  Family name ends in -viridae, i.e.  Genus name ends in -virus, Simplexvirus  Herpes simplex virus I (HSV-I)

21 22 General phases in animal virus multiplication cycle: 1. Adsorption – binding of virus to specific molecule on host cell 2. Penetration – genome enters host cell 3. Uncoating – the viral nucleic acid is released from the capsid 4. Synthesis – viral components are produced 5. Assembly – new viral particles are constructed 6. Release – assembled viruses are released by budding (exocytosis) or cell lysis 23 24  Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane  Spectrum of cells a virus can infect – host range  Hepatitis B – human liver cells  Poliovirus – primate intestinal and nerve cells  Rabies – various cells of many mammals

25 Figure 6.12

26  Flexible cell membrane is penetrated by the whole virus or its nucleic acid by:  Endocytosis – entire virus is engulfed and enclosed in a vacuole or vesicle  Fusion – envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm

27 Figure 6.13

28  Varies depending on whether the virus is a DNA or RNA virus  DNA viruses generally are replicated and assembled in the nucleus  RNA viruses generally are replicated and assembled in the cytoplasm  Positive-sense RNA contain the message for translation  Negative-sense RNA must be converted into positive-sense message

29  Assembled viruses leave host cell in one of two ways:  Budding – exocytosis; nucleocapsid binds to membrane which pinches off and sheds the viruses gradually; cell is not immediately destroyed  Lysis – nonenveloped and complex viruses released when cell dies and ruptures  Number of viruses released is variable  3,000-4,000 released by poxvirus  >100,000 released by poliovirus

30 Figure 6.15

31 Cytopathic effects - virus-induced damage to cells 1. Changes in size and shape 2. Cytoplasmic inclusion bodies 3. Inclusion bodies 4. Cells fuse to form multinucleated cells 5. Cell lysis 6. Alter DNA 7. Transform cells into cancerous cells

32 Figure 6.16

33 34  Persistent infections - cell harbors the virus and is not immediately lysed  Can last weeks or host’s lifetime; several can periodically reactivate – chronic latent state  Measles virus – may remain hidden in brain cells for many years  Herpes simplex virus – cold sores and genital herpes  Herpes zoster virus – chickenpox and shingles

35  Some animal viruses enter host cell and permanently alter its genetic material resulting in cancer – transformation of the cell  Transformed cells have increased rate of growth, alterations in , and capacity to divide for indefinite time periods resulting in tumors  Mammalian viruses capable of initiating tumors are called  Papillomavirus – cervical cancer  Epstein-Barr virus – Burkitt’s lymphoma

36  Bacteriophages – bacterial viruses (phages)  Most widely studied are those that infect – complex structure, DNA  Multiplication goes through similar stages as animal viruses  Only the nucleic acid enters the cytoplasm - uncoating is not necessary  Release is a result of cell lysis induced by viral enzymes and accumulation of viruses - lytic cycle

37 1. Adsorption – binding of virus to specific molecule on host cell 2. Penetration – genome enters host cell 3. Replication – viral components produced 4. Assembly – viral components assembled 5. Maturation – completion of viral formation 6. Release – viruses leave cell to infect other cells 38 Figure 6.17

39 Figure 6.18

Figure 6.19

40  Not all phages complete the lytic cycle  Some DNA phages, called temperate phages, undergo adsorption and penetration but don’t replicate  The viral genome inserts into bacterial genome and becomes an inactive prophage – the cell is not lysed  Prophage is retained and copied during normal resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny  Induction can occur resulting in activation of lysogenic prophage followed by and cell lysis

41 Figure 6.17

42  Lysogeny results in the spread of the virus without killing the host cell  Phage genes in the bacterial can cause the production of toxins or enzymes that cause pathology – lysogenic conversion  diphtheriae   Clostridium botulinum

43 44  Obligate intracellular parasites that require appropriate cells to replicate  Methods used:  Cell (tissue) cultures – cultured cells grow in sheets that support viral replication and permit observation for  Bird embryos – incubating egg is an ideal system; virus is injected through the shell  Live animal inoculation – occasionally used when necessary

45 Figure 6.20

46 Figure 6.21

47  Viruses are the most common cause of acute infections  Several billion viral infections per year  Some viruses have high mortality rates  Possible connection of viruses to chronic afflictions of unknown cause  Viruses are major participants in the earth’s ecosystem

48 - misfolded proteins, contain no nucleic acid  Cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases  Common in :  Scrapie in sheep and goats  Bovine spongiform encephalopathies (BSE), a.k.a. mad cow disease  Wasting disease in elk  Humans – Creutzfeldt-Jakob Syndrome (CJS)

 Extremely resistant to usual sterilization techniques

49 Figure 6.22

50  viruses – dependent on other viruses for replication  Adeno-associated virus – replicates only in cells infected with adenovirus  Delta agent – naked strand of RNA expressed only in the presence of hepatitis B virus  – short pieces of RNA, no protein coat; only been identified in 51  More difficult than other agents  Consider overall clinical picture  Take appropriate sample  Infect cell culture – look for characteristic cytopathic effects  Screen for parts of the virus  Screen for immune response to virus (antibodies)  Antiviral drugs can cause serious side effects

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