1. Chapter Viruses
In 1883, A. Mayer discovered that the sap extracted from tobacco plants infected with tobacco mosaic disease was able to cause the same disease when it was sprayed onto the healthy tobacco plants. He concluded that it was caused by a submicroscopic bacterium. A decade later, D. Ivanowsky, a Russian scientist, discovered that it was caused by tobacco mosaic virus (TMV).

2. 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

3. 234 Characteristics of Viruses: Submicroscopic and filterable
Contain either DNA or RNA, never both
Contain no enzyme system
Contain no cellular organelles
Obligate intracellular parasites
Capsid:
Capsomere (capsomer):
Nucleocapsid:
Polyhedron:
Icosahedron:
Bacteriophage:

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

5. 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

6. 235 Replication of Virus: Adsorption (Attachment):
Penetration: Injection, Fusion, Phagocytosis
Eclipse phase:
Replication:
Assembly and Maturation:
Release:
Lysis
Budding
Exocytosis

7. 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

8. 235-236 Lytic Cycle: Virulent phage: Lysogenic Cycle:
Avirulent (temperate) phage:
Prophage:
Lysogenic conversion is a change in the characteristic of the host cell once the cell is infected with the virus.

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

10. 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

11. 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

12. 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

13. 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

14. The phage injects its DNA.
Fig. 19-6
Daughter cell
with prophage
Phage
DNA
The phage injects its DNA.
Cell divisions
produce
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial
chromosome,
initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced or
Lysogenic cycle
is entered
Figure 19.6 The lytic and lysogenic cycles of phage λ, a temperate phage
Prophage
New phage DNA and proteins
are synthesized and
assembled into phages.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.

15. Table 19-1
Table 1

16. Table 19-1a
Table 1

17. Table 19-1b
Table 1

18. 238 DNA Viruses: Replication DNA DNA Assembled into mRNA Protein
Transcription DNA virus
mRNA Protein
Translation

19.
RNA Viruses:
Some RNA viruses such as poliovirus and hepatitis A virus are positive (+) or sense strand RNA viruses. They serve as mRNA which can be directly translated into proteins.
The negative (-) or no sense strand RNA viruses such as influenza virus, mumps virus and rabies virus, have to be converted into positive (+) before they can be translated into proteins. The positive strand is used to make negative (-) strand which is then assembled with proteins to form negative strand RNA viruses.

20. (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

21. 239 Minus (-) Plus (+) strand strand RNA RNA Assembled
Plus (+) into plus (+) strand RNA translated into Proteins strand RNA
viruses

22. 239 Retroviruses: RNA DNA RNA assembled into transcription RNA viruses
reverse
transcriptase transcription
RNA DNA RNA
assembled into
transcription RNA viruses
translation
mRNA Protein

23. 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

24. 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

25. 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

26. Viruses and Cancer:
DNA oncogenic viruses:
Hepatitis B virus
Epstein Barr virus causes Burkitt’s lymphoma
Human papilloma virus
RNA oncogenic viruses:
HTLV-I causes adult T-cell leukemia
HTLV-II causes hairy-cell leukemia

27. 240 Three main hypotheses on the origin of viruses:
They are fragments of host genetic material that broke off but remained in the host cytoplasm.
Their ancestors were free-living noncellular macromolecules that became parasites of the cellular organisms when the organic nutrients of the primordial seas disappeared.
They are organisms that reached the extreme of evolutionary specialization for parasitism by losing all the cellular organelles.

28. 241
Viroids are naked RNA viruses with no capsid. They cause cadang cadang disease in coconuts, and other diseases in chrysanthemum, potato and tomato.

29. Fig
Figure Viral infection of plants

30. 241
Prions are infectious proteins that contain no nucleic acid. They cause scrapie in sheep, mad cow disease in cattle, kuru disease and Creutzfeldt-Jacob disease in humans.

31. 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