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1
VIRUSES
Non-cellular infectious agents
(Chapter 10, sections 17-21)
VIRUSES
• Two major characteristics
DNA
protein coat
sheath
base plate tail
fiber
65–nm diameter head, 225 nm total length
Fig. 21-13c, p.342
2
18 nm diameter, 250 nm length
protein subunits of coat
viral RNA
80 nm diameter
80 nm diameter
lipid envelope: proteins span the envelope, line its inner surface, spike out above it
viral RNA
reverse transcriptase
viral coat (proteins)
100-120 nm diameter
Fig. 21-13d, p.342
VIRUSES
• Examples
3
Fig. 21-15a, p.344
Bacteriophage
SARS virus
Ebola virus
HIV
VIRUSES
How they multiply
4
Lytic Pathway
Virus particles bind to wall of suitable host. Viral genetic material enters cell cytoplasm.
Viral DNA directs host machinery to produce viral proteins and viral DNA.
Viral protein molecules are assembled into coats; DNA is packaged inside.
Tail fibers and other parts are added to coats.
Lysis of host cell is induced; infectious particles escape.
Fig. 21-15b, p.344
Stepped Art
Viral DNA usually becomes integrated into the bacterial chromosome.
Lysogenic Pathway
Prior to prokaryotic fission, the chromosome and integrated viral DNA are replicated.
After binary fission, each daughter cell will have recombinant DNA.
Viral DNA is excised from chromosome and cell enters lytic pathway.
Stepped Art
Fig. 21-15c, p.344
a Virus particle injects genetic material into a suitable host cell after binding to its wall.
e Lysis of host cell is induced; infectious particles escape.
d The coats get tail fibers, other parts.
c Viral proteins are assembled into coats around viral DNA.
b Viral DNA directs host cell to make viral proteins and replicate viral DNA.
LyticPathway
a-1 Viral DNA is integrated into the host’s chromosome.
a-4 Viral DNA is excised from the chromosome.
a-2 Before prokaryotic fission, the bacterial chromosome with the integrated viral DNA is replicated.
a-3 After cell division, each daughter cell will have recombinant DNA.
LysogenicPathway
Fig. 21-15, p.344
5
Fig. 21-16, p.345
animation
Infectious Diseases and Evolution
6
Evolution and Disease
• Host and pathogen are coevolving• If a pathogen kills too quickly, it might
disappear along with the individual host • Most dangerous if pathogen
– Is overwhelming in numbers– Is in a novel host– Is a mutant strain
Mycobacterium tuberculosis SARS virus
Ebola virus
7
New Threats
• Emerging Pathogens– Ebola virus – Monkeypox virus– SARS virus
• Drug-resistant strains• Food poisoning
– E. coli– Salmonella
8
Fig. 21-18bp.349
Salmonella attached to a knife blade