1. 17
Viruses

2. Overview: A Borrowed Life
A virus: also called a virion
An infectious, acellular particle consisting of little more than genes packaged into a protein coat
Viruses lead “a kind of borrowed life,” existing in a shady area between life-forms and chemicals
Obligate intracellular parasite of all other living things
Must have a host to survive; cannot independently fulfill characteristics of life
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3. Concept 17.1: A virus consists of a nucleic acid surrounded by a protein coat
Ultramicroscopic size
Basic structure: only 2 pieces
Genome: nucleic acid core; either SS-DNA, DS-DNA,SS-RNA, or DS-RNA
Capsid: protein coat
Built from protein subunits called capsomeres
A capsid can have various structures
Some viruses have membranous envelopes that help them infect hosts
These viral envelopes are made from host cell PM and viral glycoproteins
Evade immune system and helps virus enter cell; e.g. influenza virus
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4. Viral Structures Membranous envelope RNA Capsomere RNA DNA Capsid Head
Figure 17.2
Viral Structures
Membranous
envelope
RNA
Capsomere
RNA
DNA
Capsid
Head
DNA
Capsomere
of capsid
Tail
sheath
Tail
fiber
Glycoprotein
Glycoproteins
18  250 nm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
Figure 17.2 Viral structure
80 nm
50 nm
50 nm
50 nm
(a)Tobacco mosaic
virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4
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5. A Viral Replicative Cycle
Figure 17.3
VIRUS
DNA
Capsid
Transcription and
manufacture of
capsid proteins
Entry and
uncoating
Replication
HOST
CELL
Viral DNA
A Viral Replicative Cycle
mRNA
Viral DNA
Capsid
proteins
Figure 17.3 A simplified viral replicative cycle
Self-assembly of
new virus particles
and their exit from
the cell
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6. Concept 17.2: Viruses replicate only in host cells
Viruses are obligate intracellular parasites, because they can replicate only within a host cell
Each virus has a host range
Limited number of host cells that it can infect
Cell has to have specific receptor on its PM surface for viral attachment
May be 1 host species or many
HIV (only humans) vs rabies (many animals)
May be 1 tissue or many within host
Hepatitis (liver)
HIV (specific WBC)
Cold virus (lining of RT)
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7. General Features of Viral Replicative Cycles
2 Reproductive Cycles: lytic and lysogenic cycles
Lytic Cycle
A viral replicative cycle that results in death or “lysis” of host cell
Releases new phage particles
Called virulent phages: phages that lyse their host cells
Host either gets better or dies
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8. Lytic versus Lysogenic Replicative Cycles
Figure 17.UN02
Lytic versus Lysogenic Replicative Cycles
The phage attaches to a
host cell and injects its DNA.
Phage DNA
Prophage
Bacterial
chromosome
Lytic cycle
• Virulent or temperate phage
• Destruction of host DNA
• Production of new phages
• Lysis of host cell causes release
of progeny phages
Lysogenic cycle •
Temperate phage only
Genome integrates into bacterial
chromosome as prophage, which
(1) is replicated and passed on to
daughter cells and
(2) can be induced to leave the chromo-
some and initiate a lytic cycle
Figure 17.UN02 Summary of key concepts: lytic and lysogenic cycles
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9. Lytic Cycle of Phage T4 Attachment Entry of phage DNA and degradation
Figure 17.4-s5
Attachment
Lytic Cycle of Phage T4
Entry of phage
DNA and
degradation
of host DNA
Release
Phage assembly
Figure 17.4-s5 The lytic cycle of phage T4, a virulent phage (step 5)
Synthesis of
viral genomes
and proteins
Self-assembly
Head
Tail
Tail
fibers
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10. The Lysogenic Cycle
A viral replication cycle that replicates the phage genome without destroying the host
Incorporates viral genome into host genome without killing host and then remains inactive until reactivated
Viral DNA integrated into host bacteria’s chromosome known as a prophage
Temperate phages: viruses that use both the lytic and lysogenic cycles
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11. The Lytic and Lysogenic Cycles of a Phage
Figure 17.5
The Lytic and Lysogenic Cycles of a Phage
Daughter cell
with prophage
Phage
DNA
The phage injects its DNA.
Many cell
Divisions
create many
Infected
bacteria.
Tail fiber
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Prophage exits
chromosome.
Lytic cycle
Lysogenic cycle
Prophage is copied
with bacterial
chromosome.
The cell lyses, releasing phages.
Prophage
Figure 17.5 The lytic and lysogenic cycles of phage λ, a temperate phage
Phage DNA and proteins are
synthesized and assembled.
Phage DNA integrates into
bacterial chromosome.
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12. RNA as Viral Genetic Material
The broadest variety of RNA genomes is found in viruses that infect animals
Retroviruses: “backward” viruses; RNA to DNA using reverse transcriptase
Viral RNA  viral DNA  another viral RNA  protein
HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)
Viral DNA that is integrated into the host genome is called a provirus
A provirus is a permanent resident of the host cell
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13. Replicative Cycle of HIV Virus
Figure 17.8
Membrane of
white blood cell
Glycoprotein
Viral envelope
HIV
Capsid
RNA (two
identical
strands)
HOST
CELL
HIV
Reverse
transcriptase
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
Replicative Cycle of HIV Virus
0.25 𝛍m
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
progeny
viruses
Figure 17.8 The replicative cycle of HIV, the retrovirus that causes AIDS
mRNA
New virus
New HIV leaving a cell
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14. Evolution of Viruses
Viruses do not fit our definition of living organisms
Since viruses can replicate only within cells, they probably evolved after the first cells appeared
Candidates for the source of viral genomes:
Plasmids (circular DNA in bacteria and yeasts)
Transposons (small mobile DNA segments)
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15. Concept 17.3: Viruses and prions are formidable pathogens in animals and plants
Diseases caused by viral infections afflict humans, agricultural crops, and livestock worldwide
Prions: infectious proteins; lack nucleic acid (pure protein!)
Identified in 1982; misfolded versions of normal brain proteins
Cause other brain proteins to misfold….causing a chain reaction
Transmitted in food
Cause spongiform encephalopathies: holes in brain
Scrapie: sheep and goats
Bovine spongiform encephalopathy: Mad Cow Disease
Called Creutzfeldt-Jacob Disease (CJD) in humans
10 year incubation period; no cure….fatal
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16. Prions Original prion New Prion prion Normal protein Aggregates
Figure 17.13
Original
prion
New
prion
Prion
Normal
protein
Prions
Figure Model for how prions propagate
Aggregates
of prions
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17. Vaccines
Harmless variation of a pathogenic (disease-causing) microbe to induce host’s immune system to produce antibodies against the harmful pathogen
Often use dead or severely diluted virus
Major medical tool for preventing viral infections
Antibiotics are powerless against viruses
Antiviral drugs: can help to treat, though not cure, viral infections
Interferes with viral nucleic acid synthesis
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18. Receptor Proteins
Receptor proteins on the surfaces of host cells: important in treatment or prevention of viral infection
Some people have been found to be resistant to HIV infection
They have an unusual form of the CCR5 protein, one of the proteins to which HIV must bind in order to infect most cells
A drug that masks the CCR5 protein is being tested currently
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19. HIV Resistance Due to Differences in Cell Surface Proteins
Figure 17.9
HIV Resistance Due to Differences in Cell Surface Proteins
HIV
Receptor
(CD4)
Receptor (CD4)
but no CCR5
Figure 17.9 HIV resistance due to differences in cell-surface proteins
Co-receptor
(CCR5)
Plasma
membrane
(a)
HIV can infect people with CCR5 on its surface, as in most people
(b)
HIV cannot infect a cell lacking CCR5, as in resistant people
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20. Emerging Viruses Viruses that suddenly become obvious
New tropical rain forest viruses that recently emerged from cross-species jump from their natural hosts to humans
E.g. HIV, dengue, Lassa, hanta, West
The Ebola virus, recognized initially in 1976 in central Africa, causes hemorrhagic fever, an often fatal syndrome
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21. H1N1 spread rapidly, causing a pandemic, or global epidemic
In 2009 an epidemic or general outbreak of a flu-like illness occurred in Mexico and the United States; the virus responsible was named H1N1 (swine flu)
H1N1 spread rapidly, causing a pandemic, or global epidemic
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22. 3 processes contribute to the emergence of viral diseases
Mutation of existing viruses; especially high in RNA viruses
Spread of a viral disease from a small, isolated human population; allows disease to go unnoticed before it begins to spread
Spread of existing viruses from animal populations (called cross-species jumping); about 75% of new human diseases originate this way
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23. Strains of influenza A: infect a wide range of animals and have caused 4 major flu epidemics in humans in last 100 years
H1N1 identifies 2 viral surface proteins: hemagglutinin (H) and neuraminidase (N)
There are numerous types of hemagglutinin and neuraminidase, identified by numbers
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24. Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known
They have enormous impacts on the agricultural and horticultural industries
Same basic structure and mode of replication as animal viruses
Most plant viruses known thus far:
RNA genome
Helical capsid
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25. Plant viral diseases spread by 2 major routes:
Horizontal transmission: Infection from an external source of virus
Vertical transmission: Inheritance of the virus from a parent
Herbivores, especially insects, pose a double threat because they can both carry a virus and help it get past the plant’s outer layer of cells
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26. Viral Evolution Figure 17.UN01-1 A/California/07/2009 Group 1
A/Taiwan/1164/2010
Group 3
A/Taiwan/T1773/2009
Group 6
A/Taiwan/T1338/2009
A/Taiwan/T0724/2009
A/Taiwan/T1821/2009
A/Taiwan/937/2009
A/Taiwan/T1339/2009
Group 7
A/Taiwan/940/2009
A/Taiwan/7418/2009
A/Taiwan/8575/2009
A/Taiwan/4909/2009
A/Taiwan/8542/2009
A/Taiwan/1018/2011
Group 9
A/Taiwan/552/2011
A/Taiwan/2826/2009
A/Taiwan/T0826/2009
A/Taiwan/1017/2009
A/Taiwan/7873/2009
A/Taiwan/11706/2009
Group 8
A/Taiwan/6078/2009
A/Taiwan/6341/2009
A/Taiwan/6200/2009
A/Taiwan/5270/2010
Figure 17.UN01-1 Skills exercise: analyzing a sequence-based phylogenetic tree to understand viral evolution (part 1)
Group 8-1
A/Taiwan/3994/2010
A/Taiwan/2649/2011
Group 10
A/Taiwan/1102/2011
A/Taiwan/4501/2011
A/Taiwan/67/2011
A/Taiwan/1749/2011
A/Taiwan/4611/2011
A/Taiwan/5506/2011
Group 11
A/Taiwan/1150/2011
A/Taiwan/2883/2011
Viral Evolution
A/Taiwan/842/2010
A/Taiwan/3697/2011
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