1. 19 Viruses Chapter 19 - Viruses
Lecture Presentation by Nicole Tunbridge and
Kathleen Fitzpatrick
Unit5: Molecular Basis of Inheritance

2. Chapter 19 - Viruses
Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat
Virus: A Borrowed Life
Viruses are not cells
Infectious very small infectious particle consisting of nucleic acid enclosed in a protein coat
Some have membranous envelope
Much simpler structure than even prokaryotic cells!
Cannot reproduce outside of a host cell
Carry out metabolism outside of a host cell
Viruses were detected indirectly long before they were actually seen!
Unit5: Molecular Basis of Inheritance

3. Are the viruses (budding from this cell) alive??
Chapter 19 - Viruses
Are the viruses (budding from this cell) alive??
Figure 19.1 Are the viruses (red) budding from this cell alive?
Unit5: Molecular Basis of Inheritance

4. Viral Genomes Viral genomes may consist of either Genome:
Chapter 19 - Viruses
Viral Genomes
Viral genomes may consist of either
Double- or single-stranded DNA or RNA
Type of nucleic acid  DNA virus or RNA virus
Genome:
Contains 3-several thousand genes
Single linear molecule of nucleic acid
Circular molecule of nucleic acid
Unit5: Molecular Basis of Inheritance

5. Capsids and Envelopes Capsid: Protein shell enclosing viral genome
Chapter 19 - Viruses
Capsids and Envelopes
Capsid: Protein shell enclosing viral genome
Built from capsomeres (protein subunits)
Has a variety of structures
Some viruses have accessory structures that help them infect hosts
Viral Envelopes
Derived from membranes of host cells
Surrounds capsids of influenza viruses and many other viruses found in animals
Contains combination of viral and host cell molecules
Unit5: Molecular Basis of Inheritance

6. Membranous envelope RNA RNA Capsomere Head DNA Capsid DNA Capsomere
Figure 19.3
Chapter 19 - Viruses
Membranous
envelope
RNA
RNA
Capsomere
Head
DNA
Capsid
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 19.3 Viral structure
20 nm
50 nm
50 nm
50 nm
(a)
Tobacco mosaic
virus
(b)
Adenoviruses
(c)
Influenza viruses
(d)
Bacteriophage T4
Unit5: Molecular Basis of Inheritance

7. Bacteriophages (Phages)
Chapter 19 - Viruses
Bacteriophages (Phages)
Viruses that infect bacteria
Have most complex capsids found in viruses
Elongated capsid head: Encloses DNA
Protein tail: Attaches phage to host cell
Injects phage DNA into host cell
Unit5: Molecular Basis of Inheritance

8. Concept 19.2: Viruses replicate only in host cells
Chapter 19 - Viruses
Concept 19.2: Viruses replicate only in host cells
Viruses: Obligate intracellular parasites
Require a host cell in order to replicate
Each has host range: Limited # host cells it can infect
General Features of Viral Replicative Cycles
Genome enters cell  Cell makes viral proteins
Virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, & other molecules
Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
Unit5: Molecular Basis of Inheritance

9. Phages have 2 alternative reproductive mechanisms:
Lytic cycle
Lysogenic cycle
Environmental signal can trigger virus genome to exit the bacterial chromosome and switch to the lytic mode
Temperate Phages
Phages that use both lytic and lysogenic cycles

10. Animation: Simplified Viral Reproductive Cycle

11. VIRUS DNA Transcription and manufacture of capsid proteins Capsid
Figure 19.4
Chapter 19 - Viruses
VIRUS
DNA 3
Transcription and
manufacture of
capsid proteins
Capsid 1
Entry and
uncoating
HOST
CELL 2
Replication
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Figure 19.4 A simplified viral replicative cycle 4
Self-assembly of
new virus particles
and their exit from
the cell
Unit5: Molecular Basis of Inheritance

12. The Lytic Cycle Phage replicative cycle
Chapter 19 - Viruses
The Lytic Cycle
Phage replicative cycle
Culminates in death of host cell
Produces new phages  Lyses (breaks open) host’s cell wall  Releases progeny viruses
Virulent Phage:
Phage that reproduces only by lytic cycle
Bacteria have defenses against phages
Restriction enzymes:
Recognize and cut up phage DNA
Unit5: Molecular Basis of Inheritance

13. Animation: Phage T4 Lytic Cycle

14. Phage T4 Lytic Cycle Attachment Entry of phage DNA and degradation
Chapter 19 - Viruses
Phage T4 Lytic Cycle 1
Attachment 2
Entry of phage
DNA and
degradation
of host DNA 5
Release
Phage assembly
Figure The lytic cycle of phage T4, a virulent phage (step 5) 3
Synthesis of viral
genomes and
proteins 4
Head
Tail
Tail
fibers
Self-assembly
Unit5: Molecular Basis of Inheritance

15. The Lysogenic Cycle Replicates phage genome without destroying host
Chapter 19 - Viruses
The Lysogenic Cycle
Replicates phage genome without destroying host
Viral DNA molecule incorporated into host cell’s chromosome
Prophage: Integrated viral DNA
Host copies phage DNA  Host cell divides  Passes copies to daughter cells
Unit5: Molecular Basis of Inheritance

16. Animation: Phage Lambda Lysogenic and Lytic Cycles

17. Phage DNA circularizes. Lytic cycle Lysogenic cycle Prophage
Figure 19.6
Chapter 19 - Viruses
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Many cell
divisions
create many
infected
bacteria.
Phage DNA
circularizes.
Tail fiber
Phage
Bacterial
chromosome
Prophage exits
chromosome.
Lytic
cycle
Lysogenic
cycle
Prophage is copied
with bacterial
chromosome.
The cell lyses,
releasing
phages.
Prophage
Figure 19.6 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.
Unit5: Molecular Basis of Inheritance

18. Lytic cycle Lysogenic cycle The phage attaches to a
Figure 19.UN03
Chapter 19 - Viruses
The phage attaches to a
host cell and injects its DNA.
Phage DNA
Prophage
Bacterial
chromosome
Lytic cycle
Lysogenic cycle
• Virulent or temperate phage
• Destruction of host DNA
• Production of new phages
• Lysis of host cell causes
releases of progeny phages
• Temperate phage only
• Genome integrates into bacterial
Figure 19.UN03 Summary of key concepts: lytic and lysogenic cycles
chromosome as prophage, which
(1) is replicated and passed on to
daughter cells and
(2) can be induced to leave the
chromosome and initiate a lytic
cycle
Unit5: Molecular Basis of Inheritance

19. Replicative Cycles of Animal Viruses
Chapter 19 - Viruses
Replicative Cycles of Animal Viruses
Classification Variables for Viruses that Infect Animals
An RNA or DNA genome
A single-stranded or double-stranded genome
Many animal viruses have envelope & RNA genome
Few bacteriophages have either
Unit5: Molecular Basis of Inheritance

20. Table 19.1
Chapter 19 - Viruses
Table 19.1 Classes of animal viruses
Unit5: Molecular Basis of Inheritance

21. Viral Envelopes Membranous Envelopes Viral glycoproteins on envelope
Chapter 19 - Viruses
Viral Envelopes
Membranous Envelopes
Viral glycoproteins on envelope
Bind to specific receptor molecules on host’s cell surface
Some envelopes derived from host cell’s plasma membrane as the viral capsids exit
Other viral membranes form from the host’s nuclear envelope  Replaced by an envelope made from Golgi apparatus membrane
Unit5: Molecular Basis of Inheritance

22. Capsid RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA)
Figure 19.7
Chapter 19 - Viruses
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome
(RNA)
Template
mRNA
Capsid
proteins
Copy of
genome
(RNA) ER
Figure 19.7 The replicative cycle of an enveloped RNA virus
Glyco-
proteins
New virus
Unit5: Molecular Basis of Inheritance

23. RNA as Viral Genetic Material
Chapter 19 - Viruses
RNA as Viral Genetic Material
Viruses that infect animals: Broadest variety of RNA genomes
Retroviruses: Use reverse transcriptase to copy their RNA genome into DNA
HIV: Retrovirus that causes AIDS
Provirus: Viral DNA integrated into host genome
Remains a permanent resident of the host cell
Unlike prophage
RNA Polymerase: Transcribes proviral DNA  RNA
RNA molecules function as both:
mRNA for synthesis of viral proteins
Genomes for new virus particles  Invade new cells
Unit5: Molecular Basis of Inheritance

24. Animation: HIV Reproductive Cycle

25. Viral envelope Membrane of white blood cell Glycoprotein HIV Capsid
Chapter 19 - Viruses
Figure 19.8
Viral envelope
Membrane of
white blood cell
Glycoprotein
HIV
Capsid
RNA (two
identical
strands)
HOST
CELL
HIV
Reverse
transcriptase
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 µm
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the progeny
viruses
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS
mRNA
New virus
New HIV leaving
a cell
Unit5: Molecular Basis of Inheritance

26. Evolution of Viruses Do not fit definition of living organisms
Chapter 19 - Viruses
Evolution of Viruses
Do not fit definition of living organisms
Cannot replicate without host cells
Hypothesis: Evolved as bits of cellular nucleic acid
Sources of viral genomes: Plasmids & transposons
Plasmids, transposons, and viruses are all mobile genetic elements
Controversy: Virus evolution before or after cells?
Largest virus discovered = size of small bacterium
Genome encodes proteins involved in translation, DNA repair, protein folding, and polysaccharide synthesis
Unit5: Molecular Basis of Inheritance

27. Unit5: Molecular Basis of Inheritance
Chapter 19 - Viruses
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
Group 8-1
Figure 19.UN01a Skills exercise: analyzing a sequence-based phylogenetic tree to understand viral evolution (part 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
A/Taiwan/842/2010
A/Taiwan/3697/2011
Unit5: Molecular Basis of Inheritance

28. Chapter 19 - Viruses
Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants
Viral Infections
Diseases affect humans, agricultural crops, and livestock worldwide
Viroids and Prions
Smaller, less complex entities
Viroids: Cause disease in plants
Prions: Cause disease in animals
Epidemic: General outbreak
Pandemics: Global epidemics
Unit5: Molecular Basis of Inheritance

29. Viral Diseases in Animals
Chapter 19 - Viruses
Viral Diseases in Animals
Viruses may damage or kill cells
Hydrolytic enzymes released from lysosomes
Some cause infected cells to produce toxins
Lead to disease symptoms
Others have molecular components that are toxic
Ex: Envelope proteins
Unit5: Molecular Basis of Inheritance

30. Vaccines Harmless derivatives of pathogenic microbes
Chapter 19 - Viruses
Vaccines
Harmless derivatives of pathogenic microbes
Stimulate immune system to mount defenses against the pathogen
Can prevent certain viral illnesses
Viral infections cannot be treated by antibiotics
Antiviral drugs can help to treat viral infections
Not a cure
Unit5: Molecular Basis of Inheritance

31. Emerging Viruses Viruses that arise suddenly within populations
Chapter 19 - Viruses
Emerging Viruses
Viruses that arise suddenly within populations
Influenza H1N1, 2009:
Epidemic of flu-like illness appeared in US & Mexico
Flu Epidemics: Caused by new strains of virus to which people have little immunity
New viral diseases can emerge when viruses spread from animals to humans
Changing species: Viruses exchange genetic information  Humans have no immunity
Can cause pandemics
Ex: “Swine Flu” 2009: Pandemic was likely passed from pigs to humans
Unit5: Molecular Basis of Inheritance

32. 1 µm (a) 2009 pandemic H1N1 influenza A virus (b)
Figure 19.9
Chapter 19 - Viruses
Figure 19.9 Influenza in humans
1 µm
(a)
2009 pandemic H1N1
influenza A virus
(b)
2009 pandemic screening
Unit5: Molecular Basis of Inheritance

33. Viral Diseases in Plants
Chapter 19 - Viruses
Viral Diseases in Plants
>2,000 viral diseases of plants are known
Cause spots on leaves and fruits, stunted growth, and damaged flowers or roots
Most plant viruses have an RNA genome
Many have a helical capsid
Some have an icosahedral capsid
Plant Viruses: Modes of Transmission
Horizontal Transmission
Enter via damaged cell walls
Vertical Transmission
Inherit virus from a parent plant
Unit5: Molecular Basis of Inheritance

34. Viroids & Prions: Simplest Infectious Agents
Chapter 19 - Viruses
Viroids & Prions: Simplest Infectious Agents
Viroids: Small circular RNA molecules
Infect plants  Disrupt growth
Prions: Slow-acting, virtually indestructible infectious proteins
Cause brain diseases in mammals
Propagation: Convert normal protein  Prion protein
Examples of Prion Diseases:
Scrapie (Sheep)
Mad cow disease (Cattle)
Creutzfeldt-Jakob disease (Humans)
Kuru (Humans)
Unit5: Molecular Basis of Inheritance

35. Model Prion Propagation
Chapter 19 - Viruses
Model Prion Propagation
Prion
Original
prion
Aggregates
of prions
New
prion
Normal
protein
Figure 19.10
Unit5: Molecular Basis of Inheritance

36. Post-Notes Activities!
1. Flu Epidemic:
Can you stop the flu spreading world-wide?
2. Making Vaccines:
Read the short introduction, then click on the activity link.
3. Viruses Exploration:
Gather information to complete the chart for the three viruses listed and one of your choosing!