Chapter 13

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.