Virus Sizes
• Range: 20-100 nm
• Ebola virus is one of the largest viruses (~970 nm).
• Viruses are obligatory intracellular parasites, meaning they require a host cell to reproduce.
• Contain either DNA or RNA (never both).
Host Range
• Most viruses infect only specific types of cells in one host.
• Host range is determined by:
• Specific attachment sites on host cells.
• Cellular factors necessary for viral replication.
Virion Structure
Nucleic Acid
• Can be DNA or RNA (but not both).
• Can be:
• Single-stranded (SS) or double-stranded (DS)
• Linear or circular
• Segmented (e.g., influenza virus has a segmented RNA genome).
Capsid (Protein Coat)
• Made of capsomeres (protein subunits).
• Functions:
• Protects nucleic acid from degradation by host nucleases.
• Serves as a vehicle for transmission.
• Assists in attachment to host cells.
• Immunogenic – triggers an immune response (antibody production).
• Some viruses evade immunity by mutating and changing capsid proteins, such as:
• Influenza virus – changes its spikes (antigenic variation).
• HIV – alters its antigens to escape immune detection.
Envelope
• Composed of lipids, proteins, and carbohydrates.
• Some viruses acquire an envelope by budding from the host cell, incorporating the host’s plasma membrane into their envelope.
Virus Morphology
1. Polyhedral Viruses
• Icosahedral capsid (20-sided structure).
2. Helical Viruses
• Capsid is rod-shaped or cylindrical.
3. Complex Viruses
• Have a complicated structure.
• Bacteriophages have:
• Head (contains nucleic acid).
• Icosahedral capsid.
• Helical tail with tail fibers for attachment.
4. Poxviruses
• Do not have an identified capsid but have a complex structure.
Virus Classification
• Family names end in -viridae.
• Genus names end in -virus.
• Species are often named after the host they infect (e.g., Human Immunodeficiency Virus).
• Subspecies are designated by numbers (e.g., HIV-1, HIV-2).
Virus Cultivation
• Viruses must be grown in living cells.
• Bacteriophages (viruses that infect bacteria) form plaques on a lawn of bacteria.
• Each plaque corresponds to a single virus.
• Plaques are counted using PFU (Plaque-Forming Units).
• Animal and plant viruses can be grown in:
1. Cell cultures
• Continuous cell lines can be maintained indefinitely.
• Primary cell cultures grow in a monolayer (single layer of cells).
• Transformed (cancerous) cells grow uncontrollably and do not form a monolayer.
2. Embryonated eggs (commonly used for vaccine production).
3. Living animals (sometimes necessary for certain viruses).
Viral Detection Methods
1. Cytopathic Effects (CPE)
• Observable changes in host cells due to viral infection.
2. Serological Tests
• Detect antibodies against viruses in a patient’s blood.
• Use antibodies to identify viruses via:
• Neutralization tests.
• Hemagglutination assays.
• Western blotting.
3. Nucleic Acid Tests
• Polymerase Chain Reaction (PCR) detects viral DNA/RNA.
Viral Replication
General Characteristics
• Viruses have only a few genes, which encode:
• Structural components (e.g., capsid proteins).
• Enzymes for replication (e.g., RNA polymerase for RNA viruses).
• Viruses hijack the host’s metabolic machinery for replication.
Virus Growth Curve
• One-step growth curve – rapid infection, replication, and release.
1. Eclipse Period
• No complete virions are present; virus is replicating inside the host.
2. Lytic Cycle (used by virulent phages)
• Results in host cell destruction.
Steps of Lytic Cycle:
• Attachment: Virus attaches to host cell via specific receptors.
• Penetration:
• Bacteriophages inject their DNA into the host.
• Animal viruses enter via endocytosis or membrane fusion.
• Biosynthesis: Viral genome directs host machinery to make viral components.
• Maturation: Viral particles are assembled.
• Release:
• Bacteriophages: Host cell lyses, releasing new virions.
• Animal viruses: Released by budding (enveloped viruses) or lysis (non-enveloped viruses).
3. Lysogenic Cycle (used by temperate phages)
• Viral DNA integrates into host genome as a prophage.
• Host cell survives and continues to reproduce with viral DNA.
• External triggers (e.g., stress, UV light) may cause the virus to enter the lytic cycle.