Viruses: Introduction and Historical Contributions

Early Discoveries in Virology

Virology, the study of viruses, began with the work of pioneering scientists who identified infectious agents smaller than bacteria.

• Dimitri Ivanowski: Demonstrated in 1892 that extracts from diseased tobacco plants could transmit disease even after filtration, suggesting the presence of a non-bacterial infectious agent.

• Martinus Beijerinck: In 1898, confirmed Ivanowski's findings and coined the term "virus" (Latin for poison), recognizing viruses as unique infectious entities.

Defining Viruses: Alive or Not?

Better Terms for Viruses

Viruses are not classified as "alive" or "dead" because they do not exhibit all characteristics of living organisms. Instead, they are described as active (capable of infection and replication) or inactive (unable to infect or replicate).

• Reason: Viruses lack cellular structure, metabolism, and independent reproduction; they require a host cell for replication.

Viral Classification and Nomenclature

Suffixes in Viral Taxonomy

• Family: Names end with -viridae (e.g., Herpesviridae).

• Genus: Names end with -virus (e.g., Herpesvirus).

Host Range and Tropism

Definitions and Differences

• Host Range: The spectrum of host species a virus can infect (e.g., humans, animals, plants).

• Tropism: The specificity of a virus for a particular cell type or tissue within a host (e.g., neurotropism for nerve cells).

Characteristics and Structure of Viruses

Four Key Characteristics

• 1. Obligate intracellular parasites: Require host cells for replication.

• 2. Genetic material: Contain either DNA or RNA, never both.

• 3. Protein coat (capsid): Surrounds and protects the genetic material.

• 4. Lack of metabolic machinery: Do not carry out metabolism or energy production.

Common Structures

• Nucleic acid core (DNA or RNA)

• Capsid (protein shell)

Key Terms Defined

• Virion: A complete, infectious virus particle outside a host cell.

• Nucleocapsid: The combination of viral nucleic acid and its capsid.

• Capsid: The protein shell enclosing the viral genome.

• Capsomere: Protein subunits that assemble to form the capsid.

Enveloped vs. Naked Viruses

• Enveloped viruses: Possess a lipid membrane (envelope) derived from the host cell membrane, surrounding the capsid.

• Naked viruses: Lack an envelope; only have a nucleocapsid.

Viral Envelope Structure and Function

• Composed of a lipid bilayer with embedded proteins (including glycoprotein spikes).

• Functions in host cell recognition, attachment, and entry.

Viral Covering and Central Core

• Covering: Capsid and, if present, envelope.

• Central core: Nucleic acid (DNA or RNA) and sometimes enzymes.

Types of Genetic Material in Virions

• DNA (single-stranded or double-stranded)

• RNA (single-stranded or double-stranded; positive-sense or negative-sense)

Viral Enzymes and Their Functions

• Polymerases: Synthesize DNA or RNA from nucleotides.

• Replicases: Copy RNA from an RNA template.

• Reverse transcriptase: Synthesizes DNA from an RNA template (e.g., in retroviruses).

Viral Shapes

• Helical: Rod-shaped, with nucleic acid coiled inside a cylindrical capsid.

• Icosahedral: Spherical, with 20 triangular faces.

• Complex: Irregular shapes, often with additional structures (e.g., bacteriophages).

Animal Virus Life Cycle

Five-Step Life Cycle

1. Adsorption: Virus attaches to host cell via specific receptors.

2. Penetration: Virus or its genetic material enters the host cell.

3. Uncoating: Viral capsid is removed, releasing nucleic acid.

4. Synthesis: Viral genome is replicated and proteins are produced.

5. Assembly and Release: New virions are assembled and exit the host cell.

Penetration Mechanisms

• Endocytosis: Virus is engulfed by the host cell membrane.

• Fusion: Viral envelope fuses with the host cell membrane, releasing contents.

Receptor-Binding Proteins and Specificity

• Viral proteins (spikes) bind to specific host cell receptors, determining which cells and species can be infected.

Penetration of Naked vs. Enveloped Viruses

• Naked viruses: Enter by endocytosis or direct penetration.

• Enveloped viruses: Enter by fusion or endocytosis.

Site of Synthesis

• DNA viruses: Usually replicate in the host cell nucleus.

• RNA viruses: Usually replicate in the host cell cytoplasm.

Viral Exit Strategies

• Lysis: Host cell bursts, releasing virions (common for naked viruses).

• Budding (exocytosis): Virions exit individually, acquiring an envelope from the host membrane (common for enveloped viruses).

Bacteriophages: Structure and Infection

Bacteriophage Structure

• Head (capsid): Contains viral DNA.

• Tail sheath: Contracts to inject DNA into the host.

• Tail fibers: Attach to bacterial surface.

• Base plate: Anchors the phage to the host cell.

Bacteriophage Infection Process

1. Attachment to bacterial cell wall.

2. Penetration: Injection of viral DNA into the host.

3. Synthesis: Host machinery produces viral components.

4. Assembly: New phages are assembled.

5. Release: Host cell lyses, releasing new phages.

Bacteriophage Exit

• Phages typically exit by lysing (breaking open) the bacterial cell.

Lytic vs. Lysogenic Cycles

• Lytic cycle: Phage replicates and lyses the host cell.

• Lysogenic cycle: Phage DNA integrates into the host genome as a prophage and replicates with the host cell.

Key Terms in Bacteriophage Biology

• Virulent phage: Only undergoes the lytic cycle.

• Temperate phage: Can undergo both lytic and lysogenic cycles.

• Lysogeny: State in which phage DNA is integrated into the host genome.

• Prophage: Phage DNA within the bacterial chromosome.

• Lysogenic conversion: Host cell acquires new traits due to prophage genes.

Induction

• Process by which a prophage is excised from the host genome and enters the lytic cycle, often triggered by stress.

Bacterial Defenses Against Phages

• Restriction-modification systems: Enzymes degrade foreign DNA.

• CRISPR-Cas systems: Adaptive immunity using stored viral sequences to target and destroy phage DNA.

Viruses in Research and Medicine

• Bacteriophage therapy: Use of phages to treat bacterial infections.

• Gene therapy: Viruses as vectors to deliver therapeutic genes.

• Vaccine development: Use of viral components to stimulate immunity.

Prions and Prion Diseases

Definition and Disease Forms

• Prion: Infectious protein particle lacking nucleic acid, causing neurodegenerative diseases.

• Cattle form: Bovine spongiform encephalopathy (BSE, "mad cow disease").

• Human forms: Creutzfeldt-Jakob disease (CJD), variant CJD, kuru.

Pathogenesis of Prion Diseases

• Prions induce abnormal folding of normal proteins in the brain, leading to transmissible spongiform encephalopathies (TSEs), characterized by sponge-like brain tissue and neurodegeneration.

Vaccines and Immunization

Key Definitions

• Vaccine: A preparation containing antigens that stimulate an immune response to confer protection against specific diseases.

• Immunization: The process by which an individual becomes protected against a disease through vaccination or natural exposure.