Chapter 13: Viruses, Viroids, and Prions – Structure, Taxonomy, and Multiplication

Distinctive Features of Viruses

Obligatory Intracellular Parasites

Viruses are unique infectious agents that require living host cells to multiply. Unlike cellular organisms, they lack the machinery for independent life and reproduction.

• Obligatory intracellular parasites: Must infect living cells to reproduce.

• Genetic material: Contain either DNA or RNA (single- or double-stranded), but never both.

• Protein coat: All viruses possess a protein coat called a capsid.

• Host cell machinery: Viruses use the host cell's synthesizing machinery for replication.

• No ribosomes: Cannot synthesize proteins independently.

• No ATP-generating mechanism: Rely on host cell for energy.

Comparison: Viruses vs. Bacteria

Key Differences in Structure and Function

Viruses differ fundamentally from bacteria in several aspects, as summarized below.

Feature	Bacteria (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	Yes/No	Yes
Possess Both DNA and RNA	Yes	Yes	No
ATP-Generating Metabolism	Yes	No	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Host Range

Specificity of Viral Infection

The host range refers to the spectrum of host cells a virus can infect. Most viruses are highly specific, infecting only certain cell types within a host species.

• Attachment sites: Specific viral proteins interact with host cell receptors.

• Bacteriophages: Infect bacteria; receptor sites may be on cell wall, fimbriae, or flagella.

• Animal viruses: Receptor sites are typically on the plasma membrane.

Viral Structure

Components of a Virion

A virion is a complete, fully developed viral particle capable of infection. Its structure includes:

• 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: Projections from the outer surface, often involved in attachment.

Nucleic Acid

Genetic Material of Viruses

Viral genomes are diverse in structure and size.

• Contain either DNA or RNA, never both.

• May be single-stranded or double-stranded.

• Can be linear, circular, or segmented.

• Genome size ranges from a few thousand to 250,000 nucleotides.

Capsid and Envelope

Protective and Functional Layers

• Capsid: Protein coat composed of capsomeres.

• Envelope: Present in some viruses, external to the capsid; composed of lipid, protein, and carbohydrate; often derived from host cell membrane during budding.

• Spikes: Carbohydrate and protein structures on some enveloped viruses, used for attachment.

General Morphology of Viruses

Shapes and Structural Types

Viruses exhibit several morphological types:

• Helical viruses: Hollow, cylindrical capsid (e.g., rabies virus, Ebola virus).

• Polyhedral viruses: Many-sided, most commonly icosahedral (e.g., adenoviruses, poliovirus).

• Enveloped viruses: Usually roughly spherical.

• Complex viruses: Complicated structures (e.g., bacteriophage).

Taxonomy of Viruses

Classification Systems

Viruses are classified based on their genetic material and replication strategy, notably by the Baltimore classification system.

• Baltimore classification: Based on nucleic acid type and mRNA production.

• Seven groups called "realms".

• Genus names: End in -virus.

• Family names: End in -viridae.

• Order names: End in -ales.

• Viral species: Group sharing genetic information and ecological niche (host).

Isolation, Cultivation, and Identification of Viruses

Laboratory Techniques

Viruses must be grown in living cells. Several methods are used for cultivation and identification:

• Bacteriophage cultivation: Grown in bacteria; form plaques (clearings) on agar, each representing a single virus and measured as plaque-forming units (PFU).

• Animal virus cultivation:

  • In living animals (e.g., mice, rabbits, guinea pigs).

  • In embryonated eggs (used for vaccine production).

  • In cell cultures:

    • Primary cell lines: Derived from tissues, form monolayers, but die out after a few generations.

    • Diploid cell lines: From human embryos, maintained for ~100 generations.

    • Continuous cell lines: From cancerous cells, maintained indefinitely (e.g., HeLa cells).

• Identification techniques:

  • Cytopathic effects (CPE): Visible changes in infected cell cultures.

  • Serological tests: ELISA (antibody-based detection).

  • Nucleic acid tests: PCR (polymerase chain reaction).

Viral Multiplication

General Principles

Viruses multiply by invading host cells and commandeering their metabolic machinery. A single virion can produce thousands of progeny in one cell. The one-step growth curve illustrates the rapid increase in viral particles following infection.

Multiplication of Bacteriophages

Lytic and Lysogenic Cycles

Bacteriophages (viruses that infect bacteria) can multiply via two main mechanisms:

• Lytic cycle: Phage causes lysis and death of the host cell (e.g., Tequatrovirus infecting E. coli).

• Lysogenic cycle: Phage DNA integrates into host DNA as a prophage, remaining latent and replicating with the host genome.

T-Even (Tequatrovirus) Bacteriophages: The Lytic Cycle

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

2. Penetration: Phage lysozyme opens cell wall; tail sheath contracts to inject DNA.

3. Biosynthesis: Phage DNA and proteins are produced; host protein synthesis halts.

4. Maturation: Assembly of new phage particles.

5. Release: Phage lysozyme breaks cell wall, releasing new virions.

Bacteriophage Lambdavirus (λ): The Lysogenic Cycle

• Lysogeny: Phage DNA integrates into host DNA as a prophage; no lysis occurs.

• Prophage DNA replicates with host chromosome.

• Three outcomes:

  1. Lysogenic host cells are immune to reinfection by the same phage.

  2. Phage conversion: Host cell acquires new properties (e.g., diphtheria toxin production).

  3. Specialized transduction: Specific bacterial genes are transferred to another bacterium via a phage, altering genetic properties.

Comparison: Bacteriophage vs. Animal Virus Multiplication

Stages of Viral Replication

Stage	Bacteriophages	Animal Viruses
Attachment	Tail fibers attach to cell wall proteins	Receptor sites on plasma membrane proteins and glycoproteins
Entry	Viral DNA injected into host cell	Capsid enters by receptor-mediated endocytosis or fusion
Uncoating	Not required	Enzymatic removal of capsid proteins
Biosynthesis	In cytoplasm	In nucleus (DNA viruses) or cytoplasm (RNA viruses)
Chronic infection	Lysogeny	Latency; slow viral infections; cancer
Release	Host cell is lysed	Enveloped viruses bud out; nonenveloped viruses rupture plasma membrane

Key Terms and Concepts

• Virion: Complete viral particle.

• Capsid: Protein shell of a virus.

• Capsomere: Subunit of a capsid.

• Envelope: Lipid membrane surrounding some viruses.

• Prophage: Phage DNA integrated into bacterial genome.

• Cytopathic effect (CPE): Observable changes in host cells due to viral infection.

• Plaque-forming unit (PFU): Measurement of virus quantity based on plaques formed.

Example

Example: The HeLa cell line is a continuous cell line derived from human cervical cancer cells, widely used for culturing animal viruses in research and vaccine production.

Additional info: The Baltimore classification system divides viruses into seven groups based on their genome type and replication strategy, which is essential for understanding viral diversity and pathogenesis.