13 Viruses, Viroids, and Prions: Structure, Classification, and Replication

Viruses, Viroids, and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ significantly from bacteria and other microorganisms. They are obligate intracellular parasites, meaning they require living host cells to multiply. Viruses contain a single type of nucleic acid (either DNA or RNA), a protein coat (capsid), and sometimes an envelope derived from the host cell membrane. They lack ribosomes and ATP-generating mechanisms, making them entirely dependent on host cellular machinery for replication.

• Obligate intracellular parasites: Cannot reproduce outside a living cell.

• Genome: Contains either DNA or RNA, never both.

• Capsid: Protein coat that protects the nucleic acid.

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

• No ribosomes or ATP-generating mechanisms: Cannot synthesize proteins or generate energy independently.

Viruses and Bacteria Compared

Viruses differ from bacteria in several fundamental ways. The following table summarizes key differences:

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.

• Attachment sites: Viral surface proteins interact with specific host cell receptors.

• Cellular factors: Host cell machinery must support viral replication.

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, linear or circular.

• Capsid: Protein coat made of capsomeres (subunits).

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

• Spikes: Glycoprotein projections from the outer surface, important for attachment.

General Morphology

Viruses exhibit several morphological types:

• Helical viruses: Hollow, cylindrical capsid (e.g., Ebola 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 uses the following conventions:

• Genus names: End in -virus

• Family names: End in -viridae

• Order names: End in -ales

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

• Subspecies: Designated by a number

Examples of Virus Families

Family	Genus	Genome Type	Clinical Features
Adenoviridae	Mastadenovirus	dsDNA, nonenveloped	Respiratory infections, tumors in animals
Poxviridae	Orthopoxvirus	dsDNA, enveloped	Smallpox, cowpox
Herpesviridae	Simplexvirus, Varicellovirus	dsDNA, enveloped	Cold sores, chickenpox, mononucleosis
Picornaviridae	Enterovirus, Rhinovirus	ssRNA (+), nonenveloped	Polio, common cold, hepatitis A
Retroviridae	Lentivirus (HIV)	ssRNA-RT, enveloped	AIDS

Isolation, Cultivation, and Identification of Viruses

Growing Bacteriophages

Bacteriophages are grown in bacteria. They form plaques (clearings) on a lawn of bacteria on agar plates. Each plaque corresponds to a single virus and can be quantified as plaque-forming units (PFU).

Growing Animal Viruses

• In living animals: Used to study immune responses.

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

• In cell cultures: Tissues treated with enzymes to separate cells; virally infected cells show cytopathic effect (CPE). Continuous cell lines are immortal and preferred for routine culture.

Viral Identification

• Cytopathic effects: Observable changes in host cells due to viral infection.

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

• Nucleic acid tests: RFLPs and PCR for genome analysis and amplification.

Viral Multiplication

Lytic and Lysogenic Cycles in Bacteriophages

Bacteriophages can undergo two types of replication cycles:

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

• Lysogenic cycle: Phage DNA integrates into host DNA as a prophage and replicates with the host cell, potentially conferring new properties (phage conversion).

Steps of the Lytic Cycle (T-Even Bacteriophages)

1. Attachment: Phage attaches to host cell.

2. Penetration: Phage injects DNA into host.

3. Biosynthesis: Phage DNA and proteins are synthesized.

4. Maturation: Assembly of phage particles.

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

Lysogenic Cycle (Bacteriophage Lambda λ)

• Phage DNA integrates into host genome as a prophage.

• Prophage is replicated with host DNA and can confer new traits (e.g., toxin production).

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

Multiplication of Animal Viruses

Animal viruses follow a similar but distinct replication process:

1. Attachment: Virus binds to cell membrane receptors.

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

3. Uncoating: Viral or host enzymes remove capsid.

4. Biosynthesis: Synthesis of viral nucleic acid and proteins.

5. Maturation: Assembly of viral components.

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

Biosynthesis of DNA and RNA Viruses

DNA Viruses

• Replicate DNA in the nucleus using viral enzymes.

• Synthesize capsid proteins in the cytoplasm using host enzymes.

RNA Viruses

• Replicate in the cytoplasm using RNA-dependent RNA polymerase.

• +ssRNA (sense): Serves directly as mRNA.

• -ssRNA (antisense): Must be transcribed to +ssRNA before translation.

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

Retroviruses

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

• Viral DNA integrates into host genome as a provirus.

Viruses and Cancer

Oncogenes and Transformation

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

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

• Transformed cells: Exhibit uncontrolled growth and express tumor-specific antigens.

Examples of Oncogenic Viruses

• DNA oncogenic viruses: Adenoviridae, Herpesviridae (Epstein-Barr virus), Papovaviridae (HPV), Hepadnaviridae (Hepatitis B virus).

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

Latent and Persistent Viral Infections

Latent Viral Infections

• Virus remains dormant in host cells 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).

Plant Viruses and Viroids

Plant Viruses

• Enter plant cells through wounds or via insect vectors.

• Protected by the plant cell wall, which limits infection.

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. They are inherited or transmissible by ingestion, transplant, or surgical instruments. Prion diseases include:

• Creutzfeldt-Jakob disease (CJD)

• Kuru

• Gerstmann-Sträussler-Scheinker syndrome

• Fatal familial insomnia

• Sheep scrapie

Prions exist in two forms: PrPC (normal) and PrPSc (misfolded, infectious). PrPSc accumulates in brain cells, forming plaques and causing disease.