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

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Fig. 21-16, p.345

animation

Infectious Diseases and Evolution

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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

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New Threats

• Emerging Pathogens– Ebola virus – Monkeypox virus– SARS virus

• Drug-resistant strains• Food poisoning

– E. coli– Salmonella

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Fig. 21-18bp.349

Salmonella attached to a knife blade