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

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 either DNA or RNA as their genetic material, but never both. They lack ribosomes and an ATP-generating mechanism, making them entirely dependent on host cellular machinery for replication.

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

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

• Protein coat (capsid): Protects the genetic material.

• No ribosomes or ATP generation: Cannot synthesize proteins or generate energy independently.

Table Purpose: This table compares the fundamental properties of typical bacteria, rickettsias/chlamydias, and viruses, highlighting differences in structure, metabolism, and sensitivity to antibiotics and interferon.

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 single host species. This specificity is determined by the presence of compatible attachment sites and cellular factors. Bacteriophages are viruses that infect bacteria. Viruses are much smaller than most cells, typically ranging from 20 nm to 1000 nm in length.

Image Purpose: This image visually compares the sizes of various viruses, bacteria, and eukaryotic cells, emphasizing the small size of viruses relative to other biological entities.

Viral Structure

Virion Structure

A virion is a complete, fully developed viral particle capable of causing infection. The main components of a virion include:

• Nucleic acid: DNA or RNA, which carries the genetic information.

• Capsid: Protein coat made of subunits called capsomeres.

• Envelope (in some viruses): Lipid, protein, and carbohydrate layer derived from the host cell membrane.

• Spikes: Protein projections that aid in attachment to host cells.

Types of Viral Morphology

• Helical viruses: Hollow, cylindrical capsid.

• Polyhedral viruses: Many-sided, often icosahedral.

• Enveloped viruses: Surrounded by a lipid envelope.

• Complex viruses: Complicated structures, such as bacteriophages.

Image Purpose: These images illustrate the structural diversity of viruses, including polyhedral, helical, enveloped, and complex forms.

Taxonomy of Viruses

Classification and Nomenclature

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

• Genus names: End in -virus (e.g., Lentivirus).

• Family names: End in -viridae (e.g., Retroviridae).

• Order names: End in -ales.

• Viral species: Group of viruses sharing genetic information and ecological niche.

• Subspecies: Designated by numbers (e.g., HIV-1, HIV-2).

Example: Herpesviridae (family), Herpesvirus (genus), Human herpes virus (species), HHV-1, HHV-2, HHV-3 (subspecies).

Isolation, Cultivation, and Identification of Viruses

Growing Bacteriophages

Bacteriophages are grown in bacterial cultures. When bacteriophages infect bacteria on an agar plate, they form clear zones called plaques, each representing a single virus. The number of plaques can be quantified as plaque-forming units (PFU).

Image Purpose: This image shows plaques on a bacterial lawn, which are used to quantify bacteriophage concentration.

Growing Animal Viruses

• In living animals: Used for viruses that do not grow well in vitro.

• In embryonated eggs: Virus is injected into various egg compartments; growth is detected by embryo changes or death.

• In cell cultures: Tissues are enzymatically treated to separate cells, which are then grown in culture. Viral infection is detected by the cytopathic effect (CPE), or cell deterioration.

Image Purpose: This image demonstrates the cytopathic effect, a key method for identifying viral infection in cultured cells.

Viral Identification

• Cytopathic effects: Observable changes in host cells.

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

• Nucleic acid analysis: Techniques such as RFLPs and PCR.

Viral Multiplication

One-Step Growth Curve

The replication of viruses follows a one-step growth curve, with a latent (eclipse) period followed by a sharp increase in virion numbers as host cells are lysed and new virions are released.

Image Purpose: This graph illustrates the stages of viral replication, including the eclipse period and acute infection phase.

Multiplication of Bacteriophages

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

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

Image Purpose: This diagram shows the steps of the lysogenic and lytic cycles in bacteriophage lambda.

Comparison: Bacteriophage vs. Animal Virus Multiplication

Stage	Bacteriophages	Animal Viruses
Attachment	Tail fibers attach to cell wall proteins	Attachment sites are plasma membrane proteins/glycoproteins
Entry	Viral DNA injected into host cell	Capsid enters by 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

Multiplication of Animal Viruses

• Attachment: Virus binds to specific receptors on the cell surface.

• Entry: By receptor-mediated endocytosis or fusion.

• Uncoating: Separation of viral nucleic acid from the capsid.

• Biosynthesis: Synthesis of viral nucleic acids and proteins.

• Maturation: Assembly of viral components into new virions.

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

Image Purpose: These images illustrate the mechanisms of viral entry (pinocytosis and fusion) and release (budding) in animal cells.

Biosynthesis of DNA and RNA Viruses

• DNA viruses: Replicate DNA in the nucleus using viral enzymes; synthesize capsid proteins in the cytoplasm.

• RNA viruses: Replicate in the cytoplasm; mechanisms vary by virus type.

• Retroviruses: Use reverse transcriptase to synthesize DNA from RNA, which integrates into the host genome as a provirus.

Image Purpose: These images show the replication cycles of DNA viruses and retroviruses, highlighting key steps such as reverse transcription and integration.

Viruses and Cancer

Oncogenes and Transformation

Oncogenes are genes that can transform normal cells into cancerous cells. Oncogenic viruses integrate their genetic material into the host genome, potentially activating oncogenes and inducing tumor formation. Transformed cells often display unique antigens and altered growth properties.

• 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

Definitions and Examples

• Latent viral infections: Virus remains dormant in host cells and may reactivate (e.g., cold sores, shingles).

• Persistent viral infections: Virus is continuously present and often fatal (e.g., subacute sclerosing panencephalitis caused by measles virus).

Image Purpose: The graph and table illustrate the differences between acute, latent, and persistent infections, and provide examples of diseases caused by each type.

Summary Table: Key Differences Between Viruses and Bacteria

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

Table Purpose: This table summarizes the distinguishing features of viruses compared to typical bacteria and rickettsias/chlamydias.