1. Overview: A Borrowed Life
Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli
Viruses lead “a kind of borrowed life” between life-forms and chemicals
The origins of molecular biology lie in early studies of viruses that infect bacteria

2. Fig. 19-1
Figure 19.1 Are the tiny viruses infecting this E. coli cell alive?
0.5 µm

3. Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat
Viruses were detected indirectly long before they were actually seen

4. RESULTS Extracted sap from tobacco plant with tobacco mosaic disease
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 3
Rubbed filtered
sap on healthy
tobacco plants
Figure 19.2 What causes tobacco mosaic disease? 4
Healthy plants
became infected

5. Structure of Viruses
Viruses are not cells
Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

6. Viral genomes may consist of either
Double- or single-stranded DNA, or
Double- or single-stranded RNA
Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

7. RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath
Fig. 19-3
RNA
DNA
Membranous
envelope
Head
RNA
Capsomere
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Tail
fiber
Glycoprotein
Glycoproteins
18  250 nm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
Figure 19.3 Viral structure
20 nm
50 nm
50 nm
50 nm
(a) Tobacco mosaic
virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4

8. 18  250 nm RNA Capsomere of capsid (a) Tobacco mosaic virus 20 nm
Fig. 19-3a
RNA
Capsomere
of capsid
18  250 nm
Figure 19.3 Viral structure
20 nm
(a) Tobacco mosaic
virus

9. DNA Capsomere Glycoprotein 70–90 nm (diameter) (b) Adenoviruses 50 nm
Fig. 19-3b
DNA
Capsomere
Glycoprotein
70–90 nm (diameter)
Figure 19.3 Viral structure
50 nm
(b) Adenoviruses

10. Membranous envelope RNA Capsid Glycoproteins 80–200 nm (diameter)
Fig. 19-3c
Membranous
envelope
RNA
Capsid
Glycoproteins
80–200 nm (diameter)
Figure 19.3 Viral structure
50 nm
(c) Influenza viruses

11. Head DNA Tail sheath Tail fiber 80  225 nm (d) Bacteriophage T4 50 nm
Fig. 19-3d
Head
DNA
Tail
sheath
Tail
fiber
80  225 nm
Figure 19.3 Viral structure
50 nm
(d) Bacteriophage T4

12. Concept 19.2: Viruses reproduce only in host cells
Viruses are intracellular parasites, which means they can reproduce only within a host cell
Each virus has a host range, a limited number of host cells that it can infect

13. General Features of Viral Reproductive Cycles
Once a viral genome has entered a cell, the cell begins to manufacture viral proteins
The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
Animation: Simplified Viral Reproductive Cycle

14. VIRUS Entry and uncoating DNA Capsid Transcription and manufacture
Fig. 19-4
VIRUS
Entry and
uncoating 1
DNA
Capsid
Transcription
and manufacture
of capsid proteins 3 2
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Figure 19.4 A simplified viral reproductive cycle
Self-assembly of
new virus particles
and their exit from
the cell 4

15. Reproductive Cycles of Phages
Phages are the best understood of all viruses

16. Fig 1
Attachment
Figure 19.5 The lytic cycle of phage T4, a virulent phage

17. Attachment Entry of phage DNA and degradation of host DNA 1 2
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage

18. Attachment Entry of phage DNA and degradation of host DNA
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage 3
Synthesis of viral
genomes and
proteins

19. Attachment Entry of phage DNA and degradation of host DNA
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage 4
Assembly 3
Synthesis of viral
genomes and
proteins
Head
Tail
Tail fibers

20. Attachment Entry of phage DNA and degradation of host DNA Release
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA 5
Release
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage 4
Assembly 3
Synthesis of viral
genomes and
proteins
Head
Tail
Tail fibers

21. Table 19-1a
Table 1

22. Table 19-1b
Table 1

23. Viral Envelopes
Many viruses that infect animals have a membranous envelope
Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit

24. Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

25. Capsid and viral genome enter the cell Capsid
Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Capsid
proteins ER
Copy of
genome (RNA)
Glyco-
proteins
Figure 19.7 The reproductive cycle of an enveloped RNA virus
New virus

26. RNA as Viral Genetic Material
The broadest variety of RNA genomes is found in viruses that infect animals
Retroviruses use reverse transcriptase to copy their RNA genome into DNA
HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

27. Fig. 19-8
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HIV
Membrane of
white blood cell
HIV
HOST CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Provirus
Chromosomal
DNA
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
RNA genome
for the
next viral
generation
mRNA
New virus
New HIV leaving a cell

28. Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse
Fig. 19-8a
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HOST CELL
HIV
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
mRNA
New virus

29. Membrane of white blood cell HIV HIV entering a cell
Fig. 19-8b
Membrane of
white blood cell
HIV
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
0.25 µm
HIV entering a cell
New HIV leaving a cell

30. Evolution of Viruses
Viruses do not fit our definition of living organisms
Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid

31. Viral Diseases in Animals
Viruses may damage or kill cells by causing the release of digestive enzymes from lysosomes
Some viruses cause infected cells to produce poisons (toxins) that lead to disease symptoms
Others have envelope proteins that are toxic

32. Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual virus
Vaccines can prevent certain viral illnesses
Viral infections cannot be treated by antibiotics
Antiviral drugs can help to treat, though not cure, viral infections

33. Emerging Viruses
Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists
Severe acute respiratory syndrome (SARS) recently appeared in China
Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

34. Flu epidemics are caused by new strains of influenza virus to which people have little immunity
Viral diseases in a small isolated population can emerge and become global
New viral diseases can emerge when viruses spread from animals to humans
Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

35. These strains can cause pandemics, global epidemics
The “bird flu” is a virus that recently appeared in humans and originated in wild birds

36. (a) The 1918 flu pandemic (b) Influenza A H5N1 virus
Fig. 19-9
(a) The 1918 flu pandemic
0.5 µm
Figure 19.9 Influenza in humans and other animals
For the Discovery Video Emerging Diseases, go to Animation and Video Files.
(b) Influenza A
H5N1 virus
(c) Vaccinating ducks

37. (a) The 1918 flu pandemic Fig. 19-9a
Figure 19.9 Influenza in humans and other animals
(a) The 1918 flu pandemic

38. (b) Influenza A H5N1 virus 0.5 µm Fig. 19-9b
Figure 19.9 Influenza in humans and other animals
(b) Influenza A H5N1
virus

39. (c) Vaccinating ducks Fig. 19-9c
Figure 19.9 Influenza in humans and other animals
(c) Vaccinating ducks

40. Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots

41. Fig a
Figure Viral infection of plants

42. Fig b
Figure Viral infection of plants

43. Fig c
Figure Viral infection of plants

44. Viroids and Prions: The Simplest Infectious Agents
Viroids are circular RNA molecules that infect plants and disrupt their growth
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Prions propagate by converting normal proteins into the prion version
Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions

45. Original Prion prion Aggregates of prions New prion Normal protein
Fig
Original
prion
Prion
Aggregates
of prions
New
prion
Normal
protein
Figure Model for how prions propagate