Viruses, Viroids, and Prions Chapter 13 Viruses, Viroids, and Prions

General Characteristics of Viruses Differentiate a virus from a bacterium. Obligatory intracellular parasites Contain DNA or RNA Contain a protein coat Some are enclosed by an envelope Some viruses have spikes Most viruses infect only specific types of cells in one host Host range is determined by specific host attachment sites and cellular factors

Virus Sizes How could the small size of viruses have helped researchers detect viruses before the invention of the electron microscope? p. 369 Figure 13.1

Virion Structure Nucleic acid Capsid Envelope Spikes DNA or RNA Capsomeres Envelope Spikes p. 372 Figure 13.2a

Morphology of an Enveloped Virus Figure 13.3

Enveloped Viruses p. 387 Fig. 13.16 b Herpesvirus (DNA, enveloped) Figure 13.16b

Morphology of a Helical Virus Figure 13.4

Morphology of a Complex Virus Figure 13.5

Taxonomy of Viruses Family names end in -viridae. Herpesviridae Genus names end in -virus. Herpesvirus Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species. Subspecies are designated by a number. Human herpes virus HHV-1, HHV-2, HHV-3

Growing Viruses Viruses must be grown in living cells Bacteriophages form plaques on a lawn of bacteria p. 377 Figure 13.6

Growing Viruses Animal viruses may be grown in living animals or in embryonated eggs p. 377 Figure 13.7

Growing Viruses Animal and plant viruses may be grown in cell culture Continuous cell lines may be maintained indefinitely Figure 13.8

Virus Identification Cytopathic effects: Detectable changes in the host cell due to viral infection (p. 441) Blood serum (serology) tests Detect antibodies against viruses in a patient Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot Nucleic acids: Studies of the DNA/RNA

Virus Identification: Cytopathic Effect a) Uninfected cells growing in a monolayer b) Infected cells piling/rounding up Figure 13.9

The Lytic Cycle Attachment: Phage attaches by tail fibers to host cell Penetration: Phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell (analogy hypodermic syringe) Biosynthesis: Production of phage DNA and proteins (What is the eclipse period?) Maturation: Spontaneous assembly of phage particles Release: Phage lysozyme breaks cell wall Text discussion of T-even bacteriophage lytic cycle steps p. 380

Lytic Cycle of a T-Even Bacteriophage 1 2 p. 381 3 Figure 13.11

Lytic Cycle of a T-Even Bacteriophage 4 Continued… p. 381 Figure 13.11

The Lysogenic Cycle p. 382 Figure 13.12

Results of Multiplication of Bacteriophages Lytic cycle Phage causes lysis and death of host cell Lysogenic cycle: Prophage DNA incorporated in host DNA may result in… Lysogenic (bacterial) cells become immune to reinfection of the same virus (but not other types of phage) Phage conversion: Bacterial cells that have new genetic properties (Examples: Clostridium botulinium, Corynebacterium diphtheriae, Streptococcus pyogenes scarlet fever toxin genes are brought into the cell & incorporated into the cell DNA by the virus) Specialized transduction: Bacterial DNA next to the phage DNA that is accidentally excised out of the bacterial DNA and packaged with the phage DNA (See slides 20 & 21)

Generalized Transduction 2 3 4 Chap. 8 Fig. 8.28 Generalized transduction, in which any bacterial DNA can be transferred from one cell to another. p. 239 5 6 Figure 8.28

Specialized Transduction Figure 13.13

Multiplication of Animal Viruses Attachment: Viruses attach to cell membrane Penetration by endocytosis or fusion (HIV) Uncoating by viral or host enzymes Biosynthesis: Production of nucleic acid and proteins Maturation: Nucleic acid and capsid proteins assemble Release by budding (enveloped viruses) or rupture Text discussion beginning p. 382

Attachment, Penetration, Uncoating By pinocytosis p. 384 Figure 13.14a

Attachment, Penetration, Uncoating By fusion p. 384 Figure 13.14b

Budding of an Enveloped Virus Figure 13.20

Multiplication of DNA Virus Figure 13.15

Sense Strand (+ Strand) RNA Virus p. 388 Figure 13.17a

Antisense Strand (– Strand) RNA Virus p. 388 Figure 13.17b

Double-Stranded RNA Virus p. 388 Figure 13.17c

Multiplication of a Retrovirus Figure 13.19

p. 385

p. 384

Prions Proteinaceous Infectious particle Inherited and transmissible by ingestion, transplant, and surgical instruments Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease PrPC: Normal cellular prion protein, on cell surface PrPSc: Scrapie protein; accumulates in brain cells, forming plaques

Filoviridae Single-stranded RNA, – strand, one RNA strand Filovirus Enveloped, helical viruses Ebola and Marburg viruses Figure 23.21

Retroviridae Single-stranded RNA, 2 RNA strands, produce DNA Use reverse transcriptase to produce DNA from viral genome Lentivirus (HIV) Oncogenic viruses Includes all RNA tumor viruses Figure 19.13