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Viral Structure & Functions - Coggle Diagram
Viral Structure & Functions
Viruses
Obligatory intracellular parasites
Require host cell to replicate
Contain nucleic acid + protein coat ± envelope
Structure
Envelopes (Sometimes Present)
Derived from host membrane
→ Helps with fusion/entry
→ Some viruses are non-enveloped
Spikes
→ Glycoproteins on envelope
→ Used for attachment
→ Determine host specificity
→ Examples: Influenza H & N spikes
Genetic material
→ DNA or RNA (never both)
→ Types:
ssDNA (e.g., Parvovirus)
dsDNA (e.g., Herpesvirus)
+ssRNA (e.g., Rhinovirus, Polio)
−ssRNA (e.g., Ebola)
dsRNA (e.g., Rotavirus)
Capsids
Made of capsomeres
Protects nucleic acid
Viral Morphology
Polyhedral
→ Icosahedral (20 triangle faces)
→ Example: Adenovirus
Helical
→ Rod-shaped
→ Example: Ebola, Rabies
Enveloped Icosahedral
Example: Herpesviridae
Complex Viruses
Example: T-bacteriophages (heads + tails)
Host range and attachement
→ Viruses infect specific cells
→ Determined by receptor sites
Bacteria: cell wall, fimbriae, flagella
Animals: membrane proteins/glycoproteins
→ Narrow vs. broad host range
Phage therapy
Bacteriophages used to kill bacteria
Viral replication cycles
Bacteriophages
Lytic Cycle
→ Attachment → Penetration → Biosynthesis → Assembly → Release
→ Results in host cell lysis
Lysogenic Cycle
→ Viral DNA integrates as prophage
→ Host cell survives
→ Can cause phage conversion
(e.g., diphtheria toxin produced only when phage is present)
Animal Viruses
B. Animal Viruses
→ Attachment
→ Entry by fusion or endocytosis
→ Uncoating
→ Biosynthesis
→ Assembly
→ Release by budding (enveloped) or lysis (non-enveloped)
Special Viral topics
Latent infections
→ Virus remains inactive
→ Reactivated by stress
→ Examples: Herpes simplex, Varicella-Zoster (shingles)
Retroviruses (e.g., HIV)
→ Carry reverse transcriptase
→ Convert RNA → DNA
→ Integrate into host genome as provirus
→ Allows long-term latency
Influenza
Structure
→ Orthomyxoviridae
→ Enveloped
→ 8 RNA segments
→ Spikes:
H (hemagglutinin): attachment
N (neuraminidase): release
Antigenic Drift
→ Small, yearly mutations
→ Minor spike changes
→ Causes seasonal flu variation
Antigenic Shift
→ Major genetic reassortment
→ Occurs when human + avian strains mix in swine
→ Can cause pandemics (e.g., 1918 H1N1)
Transmission
→ Aerosols
→ High mortality in elderly
→ Prevention: annual vaccine
Historical Discoveries
→ 1886: Mayer – TMV transmission
→ 1892: Iwanoski – “filterable agent”
→ 1930s: Stanley isolated TMV
→ First human viral disease: Yellow Fever
Yellow Fever
Yellow Fever
→ Flavivirus (ssRNA, enveloped)
→ Transmitted by Aedes mosquitoes
→ Sylvatic cycle (monkeys) & Urban cycle (humans)