1. Lesson 7 Viruses, Viroids, and Prions
February 19, 2015

2. General Characteristics of Viruses
Obligatory intracellular parasites—require living host cells in order to multiply
Some features of a virus are:
Contain DNA or RNA
Do not contain both
Contains an encapsulating protein coat
Some viruses have spikes

3. General Characteristics of Viruses
Uses host cell machinery to replicate
Why is this problematic for humans????
Have a varied host range (cells that it infects)
determined by specific host attachment sites and cellular factors
Viruses that infect bacteria are called bacteriophages

4. Chlamydia elementary body
Figure 13.1 Virus sizes.
225 nm
Human red blood cell
10,000 nm in diameter
Bacteriophage T4
Rabies virus
170 × 70 nm
Bacteriophage M13
800 × 10 nm
Adenovirus
90 nm
Rhinovirus
30 nm
Chlamydia elementary body
300 nm
Tobacco mosaic virus
250 × 18 nm
Bacteriophages
f2, MS2
24 nm
Viroid
300×10 nm
Prion
200 × 20 nm
Poliovirus
30 nm
Vaccinia virus
300 × 200 × 100 nm
Ebola virus
970 nm
E. coli
(a bacterium)
3000 × 1000 nm
Plasma membrane
of red blood cell
10 nm thick

5. Virion Structure Virion—complete, fully developed viral particle
Nucleic Acid
DNA or RNA (double stranded or single stranded)
Positive-strand RNA(uses host’s RNA polymerase) vs Negative stranded RNA (encodes own RNA polymerase)
Capsid—protein coat that protects nucleic acid
Capsomeres—small protein subunits of the capsid
Arrangement of capsomeres is virus-specific

6. Nucleic Acid Capsomere Capsid A polyhedral virus Mastadenovirus
Figure 13.2 Morphology of a nonenveloped polyhedral virus.
Nucleic Acid
Capsomere
Capsid
A polyhedral virus
Mastadenovirus

7. Virion Structure
Envelope—surrounds the capsid
Comprised of lipids, proteins, and carbohydrates
Envelopes are comprised of the host cell’s plasma membrane (extrusion)
Some viruses are non-enveloped
Capsid protects genetic material from nucleases
Spikes—carbohydrate-protein complexes that protrude from the envelope.
Viruses can escape the immune system by mutating the surface proteins!!!!
Influenza (flu-vaccine not 100% effective due to mutations)

8. Taxonomy of Viruses Family names end in -viridae
Genus names end in -virus
Family Herpesviridae, genus Simplexvirus
Viral species: a group of viruses sharing the same genetic information and ecological niche (host)
Common names are used for species
Subspecies are designated by a number
Human Immunodeficiency Virus subspecies I (HIV-1)

9. Growing Viruses Viruses MUST be grown in living cells Why is this?
Bacteriophages form plaques on lawn of bacteria
Plaques—clearing of bacteria. Concentration of viral suspension measured by plaque-forming units (PFUs)
Animal viruses may be grown in living animals or in embryonated eggs or in cell cultures
Continuous cell lines (HeLa cells)
Influenza for flu vaccine are grown in embryonic eggs (primary cell lines)

10. Figure 13.6 Viral plaques formed by bacteriophages.

11. Chlorioallantoic membrane inoculation Air sac
Figure 13.7 Inoculation of an embryonated egg.
Amniotic cavity
Chlorioallantoic
membrane
Shell
Chlorioallantoic membrane inoculation
Air sac
Amniotic inoculation
Yolk sac
Allantoic inoculation
Shell membrane
Yolk sac inoculation
Albumin
Allantoic cavity

12. Virus Identification
Cytopathic Effects—visual effects seen in viral infected cell
Cell rounding
Cell lysis
Cell clumping (multiple nuclei in one cell)
Serological Tests
Detect antibodies against viruses in a patient (ELISA)
Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot
Nucleic Acids
RFLPs
PCR

13. Viral Multiplication
In order for a virus to multiply it must invade a host cell and use the host’s metabolic machinery
Host cell can be ruptured to release virus (Lytic cycle) or survive to continue producing viruses indefinitely (Lysogenic Cycle)
Unlike bacteria, viruses have a one-step growth curve

14. Figure 13.10 A viral one-step growth curve.
Acute infection
Virions released from host cell
Number of virions
Eclipse
period
Time (days)

15. Results of Multiplication of Bacteriophages
Viral infection can be categorized into two different processes
Lytic cycle
Phage causes lysis and death of host cell
Lysogenic cycle
Prophage DNA incorporated in host DNA
Phage conversion—host cell exhibiting new properties
Some bacteria are only pathogenic when they contain prophage DNA
Shiga toxin in E. coli and exotoxin of S. aureus that causes Toxic Shock Syndrome

16. The Lytic Cycle Attachment: phage attaches by tail fibers to host cell
Penetration: phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell
Biosynthesis: production of phage DNA and proteins
Maturation: assembly of phage particles
Release: phage lysozyme breaks cell wall

17. Figure 13.11 The lytic cycle of a T-even bacteriophage.
Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid (head)
Sheath
Attachment: Phage attaches to host cell. 1
Tail fiber
Tail
Baseplate
Pin
Cell wall
Plasma membrane
Penetration: Phage penetrates host cell and injects its DNA. 2
Sheath contracted
Tail core
Biosynthesis: Phage DNA directs synthesis of viral components by the host cell. 3
Tail
DNA
Maturation: Viral components are assembled into virions. 4
Capsid
Tail fibers
Release: Host cell lyses, and new virions are released. 5

18. Lytic cycle Lysogenic cycle
Figure The lysogenic cycle of bacteriophage λ in E. coli. 1
Phage attaches
to host cell and
injects DNA. 5
Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle.
Phage DNA
(double-stranded)
Bacterial
chromosome
Many cell
divisions
Lytic cycle
Lysogenic
cycle 4A
Cell lyses, releasing
phage virions. 2
Phage DNA circularizes and enters
lytic cycle or lysogenic cycle. 4B
Lysogenic bacterium
reproduces normally.
Prophage OR 3A
New phage DNA and
proteins are synthesized
and assembled into virions. 3B
Phage DNA integrates within the
bacterial chromosome by recombination, becoming a prophage.

19. Enveloped vs. Non-Enveloped Virus
Figure Replication of a DNA-Containing Animal Virus. 1
ATTACHMENT
Virion attaches to host cell.
A papovavirus is a typical
DNA-containing virus that
attacks animal cells. 7
RELEASE
Virions are
released.
Papovavirus
DNA 2
ENTRY and
UNCOATING
Virion enters cell,
and its DNA is uncoated.
Host cell
Capsid 6
MATURATION
Virions
mature.
Nucleus
Cytoplasm
Capsid proteins
Viral DNA
Capsid proteins 4
BIOSYNTHESIS Viral DNA is replicated, and some viral proteins are made.
mRNA 5
Late translation;
capsid proteins are
synthesized. 3
A portion of viral DNA is
transcribed, producing
mRNA that encodes
“early” viral proteins.
Enveloped vs. Non-Enveloped Virus

20. Differences between Animal Virus and Bacteriophage Infection

21. Latent vs. Persistent Infections
Latent viral infections are infections where the viral DNA is integrated into the host
May not produce disease for a long period of time
Reactivated via immunosupression or exogenous stimulus
Herpesviruses
Reactivated by fever or sunburn (fever blister)
Varicellovirus (chickenpox)
Acquired in childhood but remains dormant in nerve cells
Changes in T-cell response activate the virus (Shingles)

22. Latent vs. Persistent Infections
Persistent viral infections occurs gradually over a long period
Also called chronic viral infections
Viruses gradually are produced over a long period of time
Usually fatal
HIV and measles virus are examples of persistent infections

23. Latent and Persistent Viral Infections

24. Prions Proteinaceous infectious particle
Initially discovered in sheep and called scrapie
Infectivity in sheep’s brain reduced by proteases and not radiation
Inherited and transmissible by ingestion, transplanted nerve tissue, and surgical instruments
Spongiform encephalopathies: Creutzfeldt-Jakob disease, kuru, mad cow disease
Caused by the conversion of a normal host glycoprotein PrPC into an infectious form PrPSC

25. Figure 13.22 How a protein can be infectious.
PrPc
PrPSc 1
PrPc produced by cells is secreted to the cell surface. 2
PrPSc may be acquired or produced by an
altered PrPc gene. 3
PrPSc reacts with PrPc
on the cell surface. 4
PrPSc converts the PrPc to PrPSc. 5
The new PrPSc converts more PrPc. 6
The new PrPSc is taken in, possibly by receptor-mediated endocytosis.
Lysosome 7 8
PrPSc continues to accumulate as the endosome contents are transferred to lysosomes. The result is cell death.
PrPSc accumulates in
endosomes.
Endosome

26. Creutzfeldt-Jakob disease

27. Plant Viruses and Viroids
Plant cells are normally protected via their cell walls therefore viruses enter through wounds or via insects
Insects can serve as reservoirs for some plant viruses
Causes many diseases to economically important crops

28. Viroids Short pieces of infectious RNA Lacks a protein coat
nucleotides long
Do not encode proteins
Possibly derived from introns
Lacks a protein coat
Internally paired (forms stem-loops) that protect it from degradation
Example: Potato spindle tuber disease

29. Figure 13.23 Linear and circular potato spindle tuber viroid (PSTV).