1. The Genetics of Viruses
Chapter 19:
The Genetics of Viruses

2. The Discovery of Viruses
Tobacco mosaic disease:
stunts growth, gives leaves a mosaic coloration
Late 1800s, researchers hypothesized a particle smaller than bacteria caused the disease
In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

3. RESULTS Extracted sap from tobacco plant with tobacco mosaic disease
Fig. 19-2
RESULTS 1
Extracted sap
from tobacco
plant with
tobacco
mosaic
disease 2
Passed sap
through a
porcelain
filter known
to trap
bacteria 3
Rubbed filtered
sap on healthy
tobacco plants
Figure 19.2 What causes tobacco mosaic disease? 4
Healthy plants
became infected

4. Structure of Viruses Viruses are: Viruses consist of: not cells
means “poison” in Latin
very small infectious particles
Viruses consist of:
Nucleic acid (DNA or RNA)
Protein coat (capsid) surrounding DNA/RNA
Sometimes a membranous envelope

5. Viral structure

6. Viral reproduction: Lytic Cycle
Host range:
infect a limited range of host cells
Based on receptor molecules on cell surface
The lytic cycle:
attachment
injection
hydrolyzation
assembly
release
Results in host cell death
Virulent virus: reproduce only via lytic cycle
Simplified Viral Reproductive Cycle
Animation: Phage T4 Lytic Cycle

7. Animation: Phage Lambda Lysogenic and Lytic Cycles

8. Attachment Entry of phage DNA and degradation of host DNA Release
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA 5
Release
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage 4
Assembly 3
Synthesis of viral
genomes and
proteins
Head
Tail
Tail fibers

9. Viral reproduction: Lysogenic Cycle
Genome replicated w/o destroying host cell
Viral genetic material incorporated into host cell DNA (prophage)
Temperate virus (capable of using lytic and lysogenic cycles)
May give rise to lytic cycle

10. Mutations All viruses can mutate (reproduction)
Organisms often not immune to new strains
Epidemic: outbreak from new strain (flu)
Pandemic: global outbreak usually lethal (Spanish flu)

11. Animation: HIV Reproductive Cycle
RNA viruses
Retroviruses: transcribe DNA from RNA (RNA--->DNA)
Reverse transcriptase
Most plant viruses (TMV)
No proofing, more mutations
HIV--->AIDS
Animation: HIV Reproductive Cycle

12. Viroids and prions Viroids: Prions:
tiny, naked, circular RNA that infect plants
Prions:
“mad cow disease”
a transmissible protein

13. Table 19-1a
Table 1

14. Table 19-1b
Table 1

15. You should now be able to:
Explain how capsids and envelopes are formed
Distinguish between the lytic and lysogenic reproductive cycles
Explain why viruses are obligate intracellular parasites
Describe the reproductive cycle of an HIV retrovirus
Describe three processes that lead to the emergence of new diseases
Describe viroids and prions