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

Ch. 13 - Viruses, Viroids, and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ fundamentally from bacteria and other microorganisms. They are characterized by their simple structure and dependence on host cells for replication.

• Obligatory intracellular parasites: Viruses require living host cells to multiply; they cannot reproduce independently.

• Genetic material: Contain a single type of nucleic acid, either DNA or RNA, which may be single- or double-stranded, linear or circular.

• Protein coat: The nucleic acid is surrounded by a protein coat called a capsid.

• No ribosomes: Viruses lack the machinery for protein synthesis.

• No ATP-generating mechanism: They do not generate or store energy.

Viruses vs. Bacteria

Viruses and bacteria differ in several key aspects, including structure, replication, and sensitivity to treatments.

Feature	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	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Host Range

The host range of a virus is the spectrum of host cells it can infect. Most viruses infect only specific types of cells in one host, determined by specific attachment sites and cellular factors.

Virus Sizes

Viruses are much smaller than bacteria, typically ranging from 20 to 1000 nanometers in length. Their small size allowed researchers to detect their presence before the invention of the electron microscope, as they could pass through filters that retained bacteria.

Viral Structure and Morphology

Virion Structure

A virion is a complete, fully developed viral particle. Its main components include:

• Nucleic acid: DNA or RNA, single- or double-stranded.

• Capsid: Protein coat made of subunits called capsomeres.

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

• Spikes: Projections from the outer surface, often glycoproteins, that aid in attachment to host cells.

General Morphology

Viruses exhibit several morphological types:

• Helical viruses: Hollow, cylindrical capsid (e.g., tobacco mosaic virus).

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

• Enveloped viruses: Surrounded by a lipid envelope (e.g., influenza virus).

• Complex viruses: Complicated structures, such as bacteriophages with a head, tail, and tail fibers.

Taxonomy of Viruses

Classification and Nomenclature

Viruses are classified based on their genetic material, structure, and host range. The taxonomy includes:

• Genus names: End in -virus

• Family names: End in -viridae

• Order names: End in -ales

• Viral species: A group of viruses sharing the same genetic information and ecological niche (host)

• Subspecies: Designated by a number

Isolation, Cultivation, and Identification of Viruses

Growing Bacteriophages

Bacteriophages are grown in bacteria. They form plaques (clearings) on a lawn of bacteria on agar, each corresponding to a single virus. The number of plaques can be expressed as plaque-forming units (PFU).

Growing Animal Viruses

• In living animals: Used to study immune responses.

• In embryonated eggs: Virus is injected into the egg; growth is signaled by changes or embryo death.

• In cell cultures: Tissues are treated with enzymes to separate cells; virally infected cells show cytopathic effects (CPE). Continuous cell lines are often used for their longevity.

Viral Identification

• Cytopathic effects: Observable changes in cell morphology due to viral infection.

• Serological tests: Detection of viral proteins using antibodies (e.g., Western blotting).

• Nucleic acid tests: RFLPs (Restriction Fragment Length Polymorphism) and PCR (Polymerase Chain Reaction) for genome analysis.

Viral Multiplication

Lytic and Lysogenic Cycles in Bacteriophages

Bacteriophages can multiply via two main cycles:

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

• Lysogenic cycle: Phage DNA integrates into the host genome as a prophage and can remain latent.

T-Even Bacteriophages: The Lytic Cycle

1. Attachment: Phage attaches to host cell.

2. Penetration: Phage injects DNA into host.

3. Biosynthesis: Phage DNA and proteins are produced.

4. Maturation: Assembly of phage particles.

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

Bacteriophage Lambda (λ): The Lysogenic Cycle

• Phage DNA integrates into host DNA as a prophage.

• Prophage is replicated with host chromosome.

• Phage conversion can give host new properties (e.g., toxin production).

• Specialized transduction: Only specific bacterial genes near the integration site are transferred by the phage.

Multiplication of Animal Viruses

The multiplication cycle of animal viruses involves several steps:

1. Attachment: Virus attaches to cell membrane.

2. Entry: By receptor-mediated endocytosis or fusion.

3. Uncoating: Viral or host enzymes remove the capsid.

4. Biosynthesis: Production of nucleic acid and proteins.

5. Maturation: Assembly of viral components.

6. Release: By budding (enveloped viruses) or rupture (nonenveloped viruses).

Viral Genomes and Replication Strategies

Biosynthesis of DNA Viruses

• DNA viruses replicate their DNA in the nucleus using viral enzymes.

• Capsid proteins are synthesized in the cytoplasm using host enzymes.

Biosynthesis of RNA Viruses

• RNA viruses replicate in the cytoplasm using RNA-dependent RNA polymerase.

• +ssRNA (sense): Viral RNA serves as mRNA.

• -ssRNA (antisense): Viral RNA is transcribed to + strand to serve as mRNA.

• dsRNA: Both strands are present; replication involves synthesis of mRNA from the template.

Retroviruses

• Single-stranded RNA viruses that produce DNA using reverse transcriptase.

• Viral DNA integrates into the host chromosome as a provirus.

Viruses and Cancer

Oncogenic Viruses

Some viruses can cause cancer by integrating their genetic material into the host genome, leading to transformation of normal cells into tumor cells.

• Oncogenes: Genes that can transform a normal cell into a cancerous cell.

• DNA oncogenic viruses: Adenoviridae, Herpesviridae (e.g., Epstein-Barr virus), Papovaviridae (e.g., human papillomavirus), Hepadnaviridae (e.g., hepatitis B virus).

• RNA oncogenic viruses: Retroviridae (e.g., HTLV-1, HTLV-2).

Latent and Persistent Viral Infections

Latent Viral Infections

• Virus remains in host cell without producing disease for long periods.

• May reactivate due to changes in immunity (e.g., cold sores, shingles).

Persistent Viral Infections

• Virus is continuously released over a long period, often leading to fatal outcomes (e.g., HIV/AIDS, liver cancer from hepatitis B virus).

Plant Viruses and Viroids

Plant Viruses

• Enter plant cells through wounds or via insects.

• Plant cells are generally protected by an impermeable cell wall.

Viroids and Virusoids

• Viroids: Short pieces of naked RNA that cause plant diseases (e.g., potato spindle tuber disease).

• Virusoids: Viroids enclosed in a protein coat; require coinfection with a virus to cause disease.

Prions

Prion Diseases

Prions are infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of specific normal cellular proteins found most abundantly in the brain.

• Inherited and transmissible by ingestion, transplant, and surgical instruments.

• Diseases include spongiform encephalopathies (e.g., mad cow disease, Creutzfeldt-Jakob disease, Kuru, sheep scrapie).

• PrPC: Normal cellular prion protein.

• PrPSc: Misfolded, infectious form that accumulates in brain cells, forming plaques.