Virology: Structure, Classification, and Multiplication of Viruses

General Characteristics of Viruses

Definition and Nature

Viruses are unique infectious agents that lack the characteristics of living organisms outside a host cell. They are considered non-living and are obligate intracellular parasites, meaning they require a host cell to replicate and propagate.

• Ultramicroscopic size: Range from 20 nm to 450 nm, requiring electron microscopy for detection.

• Non-cellular structure: Composed of a protein shell (capsid) surrounding a nucleic acid core.

• Nucleic acid: Either DNA or RNA, never both; can be single or double-stranded.

• Inactive outside host: Viruses are inert macromolecules outside the host cell and become active only inside.

• Replication: Viruses hijack the host cell's genetic machinery to synthesize and assemble new virions.

• Lack metabolic enzymes: Cannot synthesize proteins or generate ATP independently.

Virion

A virion is a fully formed virus capable of establishing an infection in a host.

Viral Classification

Criteria for Classification

Viruses are classified based on:

• Type of nucleic acid: DNA or RNA, single or double-stranded.

• Morphology: Shape and structure of the capsid and presence or absence of an envelope.

• Replication methods: How the virus replicates its genome and assembles new virions.

Structure of Viruses

Basic Components

The structure of a virus can be divided into two main parts:

• Covering: All viruses have a capsid (protein shell); some have an envelope (membrane) derived from the host cell.

• Central Core: Contains the nucleic acid (DNA or RNA); some viruses also have matrix proteins and enzymes.

Naked vs. Enveloped Viruses

• Naked viruses: Lack an envelope, contain only capsid and nucleic acid, and are more resistant to antimicrobial chemicals.

• Enveloped viruses: Have a phospholipid membrane envelope derived from the host cell; more sensitive to lipid-soluble antimicrobial chemicals.

• Spikes: Glycoprotein projections on the envelope essential for attachment to host cells.

Capsid and Capsomeres

• Capsid: Protein coat that encloses and protects the viral nucleic acid.

• Capsomeres: Identical protein subunits that make up the capsid.

Envelope

• Envelope: Found in some viruses, surrounds the capsid, composed of host cell membrane.

• Spikes: Carbohydrate-protein complexes essential for attachment to host cells.

• Pleomorphism: Enveloped viruses can have variable shapes due to the fluid nature of the membrane.

Function of Capsid/Envelope

• Protects the nucleic acid when the virion is outside the host cell.

• Helps bind the virion to a cell surface and assists penetration of viral DNA or RNA into a suitable host cell.

Viral Enzymes

Some viruses contain enzymes necessary for their replication:

• Polymerases: Synthesize new strands of DNA or RNA.

• Replicase: Copies RNA.

• Reverse transcriptase: Synthesizes DNA from RNA (e.g., HIV).

• Lysozyme: Degrades bacterial cell walls (found in bacteriophages).

Morphology of Viruses

Types of Viral Shapes

Viruses exhibit four main morphological types:

• Helical: Hollow cylinder with nucleic acids inside (e.g., Ebola, Tobacco mosaic virus).

• Polyhedral: Many-sided, often icosahedral (e.g., adenovirus).

• Complex: Bacteriophage with icosahedral head and helical tail sheath.

• Enveloped: Roughly spherical but pleomorphic (e.g., influenza, HIV).

Multiplication of Animal Viruses

General Phases

The multiplication cycle of animal viruses consists of several distinct phases:

1. Adsorption (Attachment): Virus binds to specific molecules on the host cell surface.

2. Penetration: Genome enters the host cell, often via receptor-mediated endocytosis or fusion.

3. Uncoating: Viral nucleic acid is released from the capsid.

4. Synthesis: Viral components are produced using host cell machinery.

5. Assembly: New viral particles are constructed.

6. Release: Viruses are released by budding (exocytosis) or cell lysis.

Adsorption and Host Range

• Attachment occurs via specific receptors on the host cell membrane.

• Host range is determined by the availability of appropriate attachment sites.

• Viruses can have restricted, intermediate, or broad host ranges.

Penetration and Uncoating

• Enveloped viruses: Enter via endocytosis or direct fusion with the host cell membrane.

• Naked viruses: Enter via receptor-mediated endocytosis, direct penetration, or endosomal escape.

• Uncoating: Release of viral nucleic acid into the cytoplasm, facilitated by host or viral enzymes.

Synthesis and Maturation

• DNA viruses replicate in the nucleus; RNA viruses replicate in the cytoplasm.

• Positive-sense RNA viruses can be directly translated; negative-sense RNA viruses require conversion to mRNA.

• Maturation involves assembly of viral components into complete virions.

Mechanisms of Release

• Budding: Enveloped viruses exit the cell by budding, acquiring a portion of the host membrane.

• Lysis: Non-enveloped and complex viruses are released when the host cell ruptures.

Growing Animal Viruses in the Laboratory

Methods

• Injection into living animals.

• Inoculation into embryonated eggs.

• Cell cultures: Detection via cytopathic effects (CPE).

Cytopathic Effects and Viral-Induced Cancer

Cytopathic Effects (CPE)

• Changes in cell size and shape.

• Cytoplasmic and nuclear inclusion bodies.

• Cell fusion, lysis, DNA alterations, and transformation into cancerous cells.

Viral-Induced Cancer

• Some viruses permanently alter host genetic material, leading to cancer (oncoviruses).

• Examples: Papillomavirus (cervical cancer), Epstein-Barr virus (Burkitt’s lymphoma).

• Approximately 10% of cancers are virally induced.

Antiviral Drugs and Interferons

Modes of Action

• Entry inhibitors: Block attachment, fusion, and uncoating (e.g., Tamiflu).

• Nucleic acid synthesis inhibitors: Inhibit viral polymerase, reverse transcriptase, integrase (e.g., Acyclovir, AZT).

• Assembly and release inhibitors: Target protease enzymes (e.g., Saquinovir, Tamiflu).

Interferons

• Produced naturally by virally infected cells; signal neighboring cells to inhibit viral replication.

• Used therapeutically to enhance immune response and inhibit cancer cells.

Types of Viral Infections

Acute, Latent, and Persistent Infections

• Acute infection: Rapid onset, brief symptoms, resolution within days.

• Latent infection: Virus remains dormant in host cells, may reactivate (e.g., herpes virus).

• Persistent infection: Gradual increase in virions over time, often fatal (e.g., measles, HPV).

Bacteriophages

Structure and Multiplication

Bacteriophages are viruses that infect bacteria, exhibiting unique structural and replication features.

• Only the nucleic acid enters the bacterial cytoplasm; no uncoating required.

• Release occurs via cell lysis, not budding.

• Bacteriophages are grown in bacteria and detected by plaque formation.

Lytic Cycle

The lytic cycle consists of:

1. Attachment (adsorption): Phage attaches to host cell via tail fibers.

2. Penetration: Lysozyme degrades cell wall; DNA injected into cell.

3. Biosynthesis: Production of phage DNA and proteins.

4. Maturation: Assembly of phage particles.

5. Release: Cell wall is broken, phages released.

Lysogenic Cycle

• Phage DNA incorporates into host DNA as a prophage.

• Host cell replicates prophage DNA; can later enter lytic cycle.

• Lysogenic conversion can confer new properties, such as toxin production.

Prions, Viroids, and Satellite Viruses

Prions

• Proteinaceous infectious particles; misfolded proteins with no nucleic acid.

• Cause fatal neurodegenerative diseases (e.g., scrapie, BSE, CJS).

• Extremely resistant to sterilization techniques.

Viroids and Satellite Viruses

• Viroids: Short pieces of RNA without a protein coat; infect plants.

• Satellite viruses: Require other viruses for replication (e.g., adeno-associated virus).

Additional info: Academic context and explanations have been expanded for clarity and completeness.