1. VIRUSES Chapter 19

2. What is a virus?
A virus is a submicroscopic infectious particle composed of a protein coat (capsid) and a nucleic acid core (either DNA or RNA).
Viruses are similar in size to a large protein macromolecule, generally smaller than 200 nm in diameter.

3. Discovery of Viruses
Search for cause of tobacco mosaic disease led to viruses
Beijerinck proved that the disease was caused by a virus.
The elusive virus was crystallized in 1935 by Wendell Stanley.

5. Beijerinck’s experiment
Fig. 19-2
Beijerinck’s experiment
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

6. Viral Capsids
Capsids are built from protein subunits called capsomeres
May be rod-shaped (helical viruses), polyhedral (icosahedral viruses) or more complex
Some viruses have membranous envelopes that help them infect hosts (flu virus)
Bacteriophages, also called phages, infect bacteria

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. Viruses are obligate 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

9. Viral Reproduction
Once a viral genome has entered a cell, the cell begins to manufacture viral proteins using the host cell’s materials (enzymes, ribosomes, tRNAs, amino acids, ATP, etc.)
** RNA viruses may have codes for their own enzymes however.

10. 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

11. Phages are the best understood of all viruses
Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

12. The Lytic Cycle
The lytic cycle culminates in the death of the host cell by producing new phages and digests the host’s cell wall, releasing the progeny viruses
A phage that reproduces only by the lytic cycle is called a virulent phage
Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

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 Lysogenic Cycle
The lysogenic cycle replicates the phage genome without destroying the host
The viral DNA molecule is incorporated into the host cell’s chromosome and is called a prophage.
Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
Viruses that can be lysogenic or lytic are called temperate phages.

15. 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.

16. Animal Viruses
Classified as DNA or RNA viruses, single or double-stranded
Many have envelopes with glycoproteins that are specific for receptors.
The glycoproteins are made by the ER and added to the host cell’s membrane which envelopes the emerging viruses.

17. Notice the viral mRNA codes for Glycoproteins that are added to
Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Notice the viral
mRNA codes for
Glycoproteins that
are added to
the cell membrane.
RNA viruses often have
Codes for their own
enzymes unlike DNA
Viruses.
Capsid
proteins ER
Copy of
genome (RNA)
Glyco-
proteins
Figure 19.7 The reproductive cycle of an enveloped RNA virus
New virus

18. Table 19-1a
Table 1

19. Table 19-1b
Table 1

20. RNA Viruses
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 is ex.)
The viral DNA that is integrated into the host genome is called a provirus
Unlike a prophage, a provirus remains a permanent resident of the host cell
Replication

21. Fig. 19-8a
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
RNA
DNA
Reverse
transcriptase
HOST CELL
HIV
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Replication
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

22. 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

23. Evolution of Viruses
Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid
Candidates for the source of viral genomes are plasmids and transposons (small mobile DNA segments)
Mimivirus, a double-stranded DNA virus, is the largest virus yet discovered…. not any more…. Mega Virus

24. Mimivirus and megavirus
Mimivirus was first isolated in 1992 from amoeba growing in a water tower. Megavirus was isolated from infecting amoeba with mimiviruses.
Which came first,
the cell or the mimivirus?

26. How fast can viruses evolve?
When viruses face an obstacle to infecting the cells they normally infect, how long does it take for them to evolve to successfully invade them again? A new study has a frightening answer: just a little more than two weeks.
how fast viruses evolve – lambda virus

27. Viral diseases in animals
Symptoms caused by
- toxins
- body’s defense mechanisms
Vaccines – weakened or derivatives of viral particles capable of causing an immune response
Antibiotics not effective
Some antiviral medications interfere with viral nucleic acid synthesis

28. Why are antibiotics ineffective against viruses?
They target 70s ribosomes, cell walls, or bacteria-specific enzymes
High rates of mutation in viral protein coats and enzymes make it difficult to develop vaccines and drugs against viruses

29. Where do new viruses come from?
Mutations of existing viruses
The dissemination of an existing virus to a more widespread population
Or spread between species
Epidemic – general outbreak of a disease
Pandemic – global epidemic

30. (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

31. Plant viruses
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
Most plant viruses have an RNA genome
Plant viral disease can spread by vertical transmission from parent plant or by horizontal transmission from an external source.

32. Fig
Figure Viral infection of plants

33. 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 misfolded 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

34. Viroids in Plants

35. Misfolding of proteins to form prions
Fig
Misfolding of proteins to form prions
Remember: Prion - Protein
Original
prion
Prion
Aggregates
of prions
New
prion
Normal
protein
Figure Model for how prions propagate

36. Scrapie in sheep

37. How Prions Arise

38. Why is it hard to treat viroid and prion infections?
Due to their simple structure, it is difficult to attack them without attaching native cell proteins or RNA

39. Hybrid Viruses
hybrid viruses
avian-human flu
viral replication