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

2. Viruses Viruses contain DNA or RNA And 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

3. Viruses
Figure 13.1

4. Helical Viruses
Figure 13.4a, b

5. Polyhedral Viruses
Figure 13.2a, b

6. Complex Viruses
Figure 13.5a

7. Viral Taxonomy Family names end in -viridae Genus names end in -virus
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

8. Viral Taxonomy Herpesviridae Retroviridae Herpesvirus Lentivirus
Human herpes virus 1, HHV 2, HHV 3
Retroviridae
Lentivirus
Human Immunodeficiency Virus 1, HIV 2

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

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

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

12. Virus Identification Cytopathic effects 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

13. Virus Identification
Figure 13.9

14. Multiplication of Bacteriophages (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
Biosynthesis Production of phage DNA and proteins
Maturation Assembly of phage particles
Release Phage lysozyme breaks cell wall

15. 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 pnetrates host cell and injects its DNA.
Sheath contracted
Tail core 3
Merozoites released into bloodsteam from liver may infect new red blood cells
Figure

16. 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

17. One-step Growth Curve
Figure 13.11

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

19. The Lysogenic Cycle
Figure 13.12

20. 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

21. Multiplication of Animal viruses
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