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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
– Use host enzyme– Problem for drug creators!
Helical viruses
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– Addition
al 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
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