Viruses, Viroids, and Prions: Structure, Classification, and Multiplication

Viruses, Viroids, and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ fundamentally from bacteria and other microorganisms. They are obligatory intracellular parasites, meaning they require living host cells to multiply. Viruses contain either DNA or RNA as their genetic material, but never both, and are surrounded by a protein coat. Unlike bacteria, viruses lack ribosomes and an ATP-generating mechanism, making them dependent on host cells for replication and metabolism.

• Obligatory Intracellular Parasites: Viruses cannot reproduce outside a host cell.

• Genetic Material: DNA or RNA, single- or double-stranded, linear or circular.

• Protein Coat: Called a capsid, made of subunits called capsomeres.

• No Ribosomes or ATP Generation: Viruses rely entirely on host cell machinery.

Host Range and Size

The host range of a virus refers to the spectrum of host cells it can infect. Most viruses are highly specific, infecting only certain cell types within a host, determined by attachment sites and cellular factors. Bacteriophages are viruses that infect bacteria. Viruses are much smaller than most bacteria, ranging from 20 nm to 1000 nm in length.

• Host Range: Determined by specific host attachment sites and cellular factors.

• Bacteriophages: Infect bacteria.

• Size Comparison: Viruses are generally much smaller than bacteria.

Viral Structure

A virion is a complete, fully developed viral particle. The structure of viruses can be enveloped or nonenveloped, and consists of nucleic acid, capsid, envelope, and spikes. The capsid is a protein coat, the envelope is a lipid, protein, and carbohydrate layer present in some viruses, and spikes are projections from the outer surface.

• Virion: Complete viral particle.

• Nucleic Acid: DNA or RNA.

• Capsid: Protein coat made of capsomeres.

• Envelope: Lipid, protein, and carbohydrate coating (in some viruses).

• Spikes: Projections for attachment.

Nonenveloped Polyhedral Virus

Polyhedral viruses have many-sided shapes. The capsid surrounds the nucleic acid, and spikes may be present for attachment.

Enveloped Helical Virus

Enveloped viruses have a lipid envelope surrounding the capsid. Helical viruses have a hollow, cylindrical capsid.

Comparison of Viruses and Bacteria

Viruses and bacteria differ in several key aspects, including their structure, metabolism, and sensitivity to antibiotics and interferon.

	Typical Bacteria	Rickettsias/Chlamydias	Viruses
Intracellular Parasite	No	Yes	Yes
Plasma Membrane	Yes	Yes	No
Binary Fission	Yes	Yes	No
Pass through Bacteriological Filters	No	No/Yes	Yes
Possess Both DNA and RNA	Yes	Yes	No
ATP-Generating Metabolism	Yes	Yes	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Viral Morphology

Viruses are classified based on their morphology: polyhedral, enveloped, helical, and complex. Each type has distinct structural features.

• Polyhedral: Many-sided, often icosahedral.

• Enveloped: Surrounded by a lipid envelope.

• Helical: Hollow, cylindrical capsid.

• Complex: Complicated structures, such as bacteriophages.

Viral Taxonomy

Viruses are classified into families, genera, and species based on genetic information and ecological niche. Genus names end in -virus, family names end in -viridae, and order names end in -ales. Viral species share genetic information and host range.

• Genus: Ends in -virus (e.g., Simplexvirus).

• Family: Ends in -viridae (e.g., Herpesviridae).

• Order: Ends in -ales.

• Species: Group sharing genetic information and ecological niche.

Isolation, Cultivation, and Identification of Viruses

Viruses must be grown in living cells. Bacteriophages are cultured in bacteria and form plaques on agar. Animal viruses are grown in living animals, embryonated eggs, or cell cultures. Identification methods include cytopathic effects, serological tests, and nucleic acid analysis.

• Plaque Method: Used for bacteriophages; plaques correspond to single viruses.

• Animal Virus Cultivation: In animals, eggs, or cell cultures.

• Identification: Cytopathic effects, serological tests (e.g., Western blotting), PCR.

Viral Multiplication

Viruses multiply by invading host cells and taking over their metabolic machinery. The multiplication cycle includes attachment, penetration, biosynthesis, maturation, and release. Bacteriophages can undergo lytic or lysogenic cycles.

• Lytic Cycle: Phage causes lysis and death of the host cell.

• Lysogenic Cycle: Phage DNA is incorporated into host DNA as a prophage.

• Virulent Phages: Replicate only through the lytic cycle.

• Temperate Phages: Replicate through both lytic and lysogenic cycles.

Multiplication of Animal Viruses

Animal viruses attach to the cell membrane, enter by endocytosis or fusion, uncoat, undergo biosynthesis, mature, and are released by budding or rupture. DNA viruses replicate in the nucleus, while RNA viruses replicate in the cytoplasm.

• Attachment: To cell membrane.

• Entry: By endocytosis or fusion.

• Uncoating: Separation of nucleic acid from protein coat.

• Biosynthesis: Production of nucleic acid and proteins.

• Maturation: Assembly of viral particles.

• Release: By budding (enveloped viruses) or rupture.

Viruses and Cancer

Some viruses can cause cancer by transforming normal cells into tumor cells. Oncogenic viruses integrate into host DNA and induce tumors. Both DNA and RNA viruses can be oncogenic.

• Oncogenes: Transform normal cells into cancerous cells.

• DNA Oncogenic Viruses: Herpesviridae, Papovaviridae, Hepadnaviridae.

• RNA Oncogenic Viruses: Retroviridae (e.g., HIV).

Latent and Persistent Viral Infections

Latent viral infections remain dormant in host cells and may reactivate. Persistent infections progress slowly and are often fatal.

• Latent Infections: Cold sores, shingles.

• Persistent Infections: Subacute sclerosing panencephalitis (measles virus).

Plant Viruses and Viroids

Plant viruses enter through wounds or insects. Viroids are short pieces of naked RNA that cause plant diseases. Virusoids are viroids enclosed in a protein coat and require coinfection with a virus.

• Plant Viruses: Enter through wounds or insects.

• Viroids: Naked RNA, cause diseases like potato spindle tuber disease.

• Virusoids: Viroids with protein coat, require coinfection.

Prions

Prions are infectious proteins that cause neurodegenerative diseases. They are inherited and transmissible by ingestion, transplant, and surgical instruments. Prions convert normal cellular prion protein (PrPC) into the disease-causing form (PrPSc), which accumulates in brain cells and leads to cell death.

• Prions: Proteinaceous infectious particles.

• Diseases: Mad cow disease, Creutzfeldt-Jakob disease, sheep scrapie.

• Mechanism: PrPSc converts PrPC to PrPSc, leading to accumulation and cell death.

Example:

Potato spindle tuber disease is caused by viroids; mad cow disease is caused by prions.

Additional info:

Knowledge of viral replication phases is important for drug development strategies and understanding disease pathology.

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