Download Ch. 19 2012

Chapter 19:
The Genetics of Viruses
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)
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
4 Healthy plants
became infected
3 Rubbed filtered
sap on healthy
tobacco plants
Structure of Viruses
• Viruses are:
– 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
Viral structure
Viral reproduction: Lytic Cycle
• Host range:
– infect a limited range of host cells
– Based on receptor molecules on cell surface
• The lytic cycle:
1.
2.
3.
4.
5.
attachment
injection
hydrolyzation
assembly
release
Simplified Viral Reproductive Cycle
Animation: Phage T4 Lytic Cycle
• Results in host cell death
• Virulent virus: reproduce only via lytic cycle
Animation: Phage Lambda Lysogenic and Lytic Cycles
Fig. 19-5-5
1 Attachment
2 Entry of phage
5 Release
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
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
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)
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
Viroids and prions
• Viroids:
– tiny, naked, circular RNA that infect plants
• Prions:
– “mad cow disease”
– a transmissible protein
Table 19-1a
Table 19-1b
You should now be able to:
1. Explain how capsids and envelopes are formed
2. Distinguish between the lytic and lysogenic
reproductive cycles
3. Explain why viruses are obligate intracellular
parasites
4. Describe the reproductive cycle of an HIV retrovirus
5. Describe three processes that lead to the emergence
of new diseases
6. Describe viroids and prions