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

Viruses, Viroids, and Prions

Characteristics of Viruses

Viruses are unique infectious agents that differ significantly from cellular life forms. They are obligate intracellular parasites, meaning they require living host cells to reproduce and cannot carry out metabolic processes independently.

• Parasitism: Viruses must invade host cells to multiply, as they lack the cellular machinery for independent life.

• Genome: Each virus contains either DNA or RNA as its genetic material, but never both.

• Protein Coat: The viral genome is protected by a protein shell called the capsid.

• Deficiencies: Viruses lack ribosomes and enzymes for energy production, making them dependent on host cell machinery.

• Host Specificity: Most viruses infect only specific types of cells in one or a few host species, a phenomenon known as host range.

Structure and Morphology of Viruses

The structure of viruses is adapted for protection, transmission, and host cell recognition. The complete, infectious form of a virus is called a virion.

• Capsid: The protein shell surrounding the nucleic acid, composed of subunits called capsomeres.

• Envelope: Some viruses possess an outer lipid membrane derived from the host cell, known as the envelope.

• Spikes: Glycoprotein projections on the envelope or capsid that facilitate attachment to host cells.

• Morphology: Viruses are classified by shape:

  • Helical: Rod-shaped, with nucleic acid inside a helical capsid (e.g., Tobacco mosaic virus).

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

  • Complex: Complicated structures, such as bacteriophages with heads and tails.

Multiplication and Growth of Viruses

Viruses replicate by hijacking host cell machinery. Their multiplication can be studied in the laboratory using various methods.

• Bacteriophages: Viruses that specifically infect bacteria.

• Lytic Cycle: The virus replicates rapidly, causing the host cell to lyse and release new virions.

• Lysogenic Cycle: Viral DNA integrates into the host genome and replicates along with it, remaining dormant until triggered.

• Laboratory Growth: Viruses are grown in living animals, embryonated eggs, or cell cultures, as they require living cells for replication.

• Detection: Viral infections are detected by observing cytopathic effects (cell damage), or by molecular methods such as PCR (Polymerase Chain Reaction).

Example: The influenza virus is grown in embryonated chicken eggs for vaccine production.

Cancer and Viral Infections

Some viruses can cause cancer or persist in the host for extended periods.

• Oncogenes: Genes that can transform normal cells into cancerous cells.

• Oncogenic Viruses: Viruses that integrate their genetic material into the host chromosome, potentially leading to tumor formation (e.g., Human papillomavirus).

• Latent Infections: Viral infections that remain dormant within the host and can reactivate later (e.g., Herpes simplex virus).

• Persistent Infections: Infections where the virus is continuously present and often fatal (e.g., HIV).

Viroids and Prions

In addition to viruses, other infectious agents include viroids and prions, which are even simpler in structure.

• Viroids: Small, circular pieces of naked RNA that infect plants and cause disease without encoding proteins.

• Prions: Infectious proteins that cause degenerative diseases of the nervous system by inducing abnormal folding of normal cellular proteins (e.g., Creutzfeldt-Jakob disease).

Agent	Genetic Material	Protein Coat	Envelope	Host Range	Example Disease
Virus	DNA or RNA	Yes (capsid)	Sometimes	Animals, plants, bacteria	Influenza, HIV
Viroid	RNA	No	No	Plants	Potato spindle tuber disease
Prion	None	No	No	Animals	Creutzfeldt-Jakob disease

Additional info: Prions are unique in that they contain no nucleic acids and propagate by inducing misfolding of normal proteins. Viroids are the smallest known infectious agents and are significant in plant pathology.