1. VIRUSES
Non-cellular infectious agents
(Chapter 10, sections 17-21)
VIRUSES
• Two major characteristics
65–nm diameter head,
DNA
225 nm total length
protein coat
sheath
base tail
plate fiber
Fig. 21-13c, p.342
1

2. 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

3. Bacteriophage
Fig. 21-15a, p.344
HIV
SARS virus
Ebola virus
VIRUSES
How they multiply
3

4. Lysis of host cell is induced;
Lytic Pathway 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 Stepped Art
machinery to produce viral
proteins and viral DNA. Fig. 21-15b, p.344
Lysogenic Pathway
Viral DNA is excised from
chromosome and cell
Viral DNA usually becomes enters lytic pathway.
integrated into the bacterial
chromosome.
After binary fission, each
Prior to prokaryotic fission, the
daughter cell will have
chromosome and integrated
recombinant DNA. Stepped Art
viral DNA are replicated.
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
c Viral proteins are assembled
material into a suitable host cell
into coats around viral DNA.
after binding to its wall.
b Viral DNA directs host
cell to make viral proteins
and replicate viral DNA.
a-1 Viral DNA is integrated a-4 Viral DNA is excised
into the host’s chromosome. from the chromosome.
Lysogenic
Pathway
a-2 Before prokaryotic fission, the a-3 After cell division, each daughter
bacterial chromosome with the cell will have recombinant DNA.
integrated viral DNA is replicated.
Fig. 21-15, p.344
4

5. animation
Fig. 21-16, p.345
Infectious Diseases and Evolution
5

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
6

7. New Threats
• Emerging Pathogens
– Ebola virus
– Monkeypox virus
– SARS virus
• Drug-resistant strains
• Food poisoning
– E. coli
– Salmonella
7

8. Salmonella attached to a knife blade
Fig. 21-18b
p.349
8