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VIRUSES
Non-cellular infectious agents
(Chapter 10, sections 17-21)
VIRUSES
• Two major characteristics
65–nm diameter head, 225 nm total length
DNA
protein coat
sheath
base plate
tail fiber
Fig. 21-13c, p.342
1
viral RNA
protein subunits of coat
80 nm diameter
80 nm diameter
18 nm diameter, 250 nm length
viral coat (proteins)
reverse transcriptase
100-120 nm diameter
viral RNA
lipid envelope: proteins span the envelope, line its inner surface, spike out above it
Fig. 21-13d, p.342
VIRUSES
• Examples
2
Bacteriophage
Fig. 21-15a, p.344
HIV
SARS virus
Ebola virus
VIRUSES
How they multiply
3
Lytic Pathway
Lysis of host cell is induced; infectious particles escape.
Tail fibers and other parts are added to coats.
Virus particles bind to wall of suitable host. Viral genetic material enters cell cytoplasm.
Viral protein molecules are assembled into coats; DNA is packaged inside.
Viral DNA directs host machinery to produce viral proteins and viral DNA.
Stepped Art
Fig. 21-15b, p.344
Lysogenic Pathway
Viral DNA usually becomes integrated into the bacterial chromosome.
Prior to prokaryotic fission, the chromosome and integrated viral DNA are replicated.
Viral DNA is excised from chromosome and cell enters lytic pathway.
After binary fission, each daughter cell will have recombinant DNA. Stepped Art
Fig. 21-15c, p.344
e Lysis of host cell is induced; infectious particles escape.
d The coats get tail fibers, other parts.
Lytic Pathway
a Virus particle injects genetic material into a suitable host cell after binding to its wall.
c Viral proteins are assembled into coats around viral DNA.
b Viral DNA directs host cell to make viral proteins and replicate viral DNA.
a-1 Viral DNA is integrated into the host’s chromosome.
a-4 Viral DNA is excised from the chromosome.
Lysogenic Pathway
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.
Fig. 21-15, p.344
4
animation
Fig. 21-16, p.345
Infectious Diseases and Evolution
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