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Viruses, viroids and prions
What are viruses?
• Very small
• Obligatory intracellular parasites
– Difficult to isolate, detect, cultivate
– Somewhat like Rickettsia…
What are viruses?
• Contain genetic material
– DNA or RNA
• Protein coat
– Sometimes encased in lipids,
carbs and proteins
• Reproduction inside living cells
• No metabolic enzymes
Helical viruses
– Use host enzyme
– Problem for drug creators!
Enveloped viruses
Polyhedral Viruses
Complex viruses
What is a host range?
• Host cells a virus can infect
– Very narrow
– Useful for treating diseases?
• Viral therapy
• Oncolytic viruses
• Range determined by cell receptor sites
What is a virion?
• One, complete,
infectious viral
particle
– Contains
• Nucleic acid
– DNA or RNA
– Both can be
double- or
single-stranded
• Protein coat
(capsid)
– Classification
based on type
of capsid
– Capsomeres:
protein subunits
What is a virion?
– Contains envelope (not all)
• Covers capsid
• Lipids, carbs, proteins
• Used to fuse with host PM
• May have spikes
What types of viruses are there?
• Helical
– Rabies, ebola
What types of viruses are there?
• Polyhedral
– Animal, plant, phages
What types of viruses are there?
• Enveloped
– Influenzavirus
What types of viruses are there?
• Complex
– Many
phages
– Additiona
l
structures
What are some viruses I should know?
• No specific epithet
• DNA viruses
– Adenoviridae
– Poxviridae
– Herpesviridae
• Human herpes
virus 1, HHV 2,
HHV 3
– Papovaviridae
– Hepadnaviridae
What are some viruses I should know?
• RNA viruses
• Picornaviridae
• Retroviridae
– Lentivirus
– Human Immunodeficiency Virus 1, HIV 2
Virus Identification
• Cytopathic effects
• Serological tests
– Detect antibodies against viruses in a patient.
– Use antibodies to identify viruses in neutralization tests,
viral hemagglutination, and Western blot.
• Nucleic acids
– RFLPs
– PCR
Viral Replication
How do viruses multiply?
• Virus only has
a few genes
and proteins
– All other
proteins come
from host cell
• E.g.
ribosomes,
tRNA, etc.
– Must take
over host
metabolism
How do phages multiply?
• Two possibilities
– Lytic cycle
– Lysogenic cycle
• T-4
– About 100 genes
– Multiplication in 5 stages
•
•
•
•
•
Attachment
Penetration
Biosynthesis
Maturation
Release
– animation
What’s the difference between lytic and
lysogenic cycles?
Lytic cycle: Phage causes lysis and death of host cell.
Lysogenic cycle: Prophage DNA incorporated in host
DNA.
What else should I know about the
lysogenic phase?
• When latent
(temperate phase)
– Can’t be infected
with the same virus
again
– Phage conversion
can happen
• C. diptheriae:
produces toxin only
in latent phase
• C. botulinum, C.
cholerae and some
streptococci also
What else should I know about the
lysogenic phase?
• When latent
(temperate phase)
– Specialized
transduction is
possible
How do viruses multiply in animals like us?
•
•
•
•
•
•
Attachment: Viruses attach to cell membrane.
Penetration by endocytosis or fusion.
Uncoating by viral or host enzymes.
Biosynthesis: Production of nucleic acid and proteins.
Maturation: Nucleic acid and capsid proteins assemble.
Release by budding (enveloped viruses) or rupture.
What’s different about entry?
• Attachment first
– Sites vary from person to
person
• Penetration
– Q: how does it happen in
phages?
– Pinocytosis OR
– Fusion (animation)
• HIV
• Uncoating
– by viral or host enzymes
Figure 13.14a
What are the final stages for multiplication
in animal viruses?
• Maturation
– Nucleic acid
– Capsid proteins
• Release
– budding (enveloped viruses) or
• animation
– Rupture
Multiplication of DNA Virus
Figure 13.15
Pathways of Multiplication
for RNA-Containing Viruses
Figure 13.17
Multiplication of a Retrovirus
PLAY
Animation: Viral Replication
Figure 13.19
DNA and RNA transcriptase
• DNA, reverse transcriptase: Cellular enzyme
transcribes viral DNA in nucleus; reverse
transcriptase copies mRNA to make viral
DNA.
• RNA, reverse transcriptase: Viral enzyme
copes viral RNA to make DNA in cytoplasm.
Viruses and disease
Is there a connection between viruses and
cancer?
• Yes!
• Oncogenic viruses
– 10% of all cancers
– DNA integrates
into host cell
– Tumor-specific
transplantation
antigen (TSTA)
– Cytopathic changes
Can you give me some examples?
• HPV—cervical cancer
• Adenoviridae—adenocarcinoma
• Herpesviridae
– Epstein-Barr (EB) virus—Burkitt’s lymphoma
– HHV-8—Kaposi’s sarcoma
• Hep-B—liver cancer
Do all viral infections causes symptoms
immediately?
• No
• Latent period for many
– EB may be latently carried in 9 out 10 people
– Herpesviruses can stay for lifetime
• Cold sores
– Chickenpox—shingles (zoster)
• Persistent infections for some
– Usually fatal
– Gradual appearance and worsening of symptoms
– Subacute sclerosing panecephalitis (from measles)
What are prions?
• Not viruses
• Infectious proteins
• Inherited and transmissible
by
– Ingestion
– Transplant
– surgical instruments
• Mad Cow Disease
– Creutzfeldt-Jacob disease