1. Viruses, Viroids, and Prions
Chapter 13, part A
Viruses, Viroids, and Prions

2. Viruses Are very small Viruses contain DNA or RNA And a protein coat
Capsid with Capsomeres
Some are enclosed by an envelope
Acellular
Obligate intracellular parasites
No ATP generating system
No Ribosomes or means of Protein Synthesis
Some viruses have spikes
Most viruses infect only specific types of cells in one host

3. Viruses: only 1 nucleic acid
DNA or RNA (But never both)
ssDNA
ds DNA
ss RNA
ds RNA

4. Viruses are small
Figure 13.1

5. Shapes of Viruses: Helical
Figure 13.4a, b

6. Shapes: Polyhedral
icosahedral
Figure 13.2a, b

7. Shapes: Complex Viruses
Figure 13.5a

8. Envelope
Membrane may be around capsid
It may contain spikes

9. Viral Taxonomy 1. Nucleic Acid 2. Morphology
3. Strategy for replication
4. Symptoms
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 e.g. HHVII

10. Growing Viruses Viruses must be grown in living cells.
The are never grown in culture media
Bacteriophages form plaques on a lawn of bacteria.
Plaques on E. coli Ø

11. Growing Viruses
Animal viruses may be grown in living animals or in embryonated eggs.
Vaccine virus in eggs
Figure 13.7

12. Viral cultures 1. Primary Cell Lines die out after a few generations
2. Diploid Cell Lines
derived from human embryos
maintained for up to 100 generations
3. Continuous Cell Lines
Transformed Cells (Cancerous Cells)
may be maintained indefinitely
HeLa Cells
Henrietta Lacks (Cervical Cancer)

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

14. Virus Identification Cytopathic effects CPE - Negri bodies rabies
Serological tests
Detect antibodies against viruses in a patient
Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot
Nucleic acids
RFLPs
PCR

15. Cytopathic effects CPE
Virus Identification
Cytopathic effects CPE
Figure 13.9

16. Multiplication of Bacteriophages (Lytic Cycle)
APBMR - Bacteriophage Lytic multiplication 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
Biosynthesis Production of phage DNA and proteins
Maturation Assembly of phage particles
Release Phage lysozyme breaks cell wall

17. 1 2 3 Bacterial cell wall Bacterial chromosome Capsid DNA Capsid
Sheath
Tail fiber 1
Attachment: Phage attaches to host cell.
Tail
Base plate
Pin
Cell wall
Plasma membrane 2
Penetration: Phage penetrates host cell and injects its DNA.
Sheath contracted
Tail core 3
Biosynthesis: DNA is copied and capsomeres are produced
Figure

18. 4 5 Tail DNA Maturation: Viral components are assembled into virions.
Capsid 5
Release: Host cell lyses and new virions are released.
Tail fibers
Figure

19. Burst size and Burst time during Eclipse Period
One-step Growth Curve
Burst size and Burst time during Eclipse Period
Eclipse period no whole virions
Figure 13.11

20. Lytic cycle versus Lysogenic
Lytic cycle Phage causes lysis and death of host cell
Lysogenic cycle Prophage DNA incorporated in host DNA

21. The Lysogenic Cycle
Figure 13.12

22. Specialized Transduction
gal gene
Prophage
Bacterial DNA 1
Prophage exists in galactose-using host (containing the gal gene).
Galactose-positive donor cell
gal gene 2
Phage genome excises, carrying with it the adjacent gal gene from the host. 3
Phage matures and cell lyses, releasing phage carrying gal gene.
gal gene 4
Phage infects a cell that cannot utilize galactose (lacking gal gene).
Galactose-negative recipient cell 5
Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA. 6
Lysogenic cell can now metabolize galactose.
Galactose-positive recombinant cell
Figure 13.13

23. Multiplication of Animal viruses
Same as bacteriophage except must uncoat
Attachment Viruses attaches to cell membrane
Penetration By endocytosis or fusion
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