Viruses, Viroids, and Prions

Viral General Information

Viruses are unique infectious agents that differ fundamentally from cellular life forms. They are composed of genetic material encased in a protein shell and require host cells for replication.

• Virus: A non-cellular infectious particle containing either DNA or RNA, enclosed in a protein capsid. Viruses are obligate intracellular parasites, meaning they can only replicate inside living host cells and are inert outside them.

• Genome: The genetic material of a virus, which may be DNA or RNA, and can be single-stranded or double-stranded.

• Capsid: The protein coat that surrounds and protects the viral genome. It is composed of subunits called capsomeres.

• Host Range: The spectrum of host cells a virus can infect. This range is usually limited to specific species or cell types.

• Bacteriophages: Viruses that specifically infect bacteria.

• Transmission: The process by which viruses spread to and infect new hosts.

• Tissue Tropism: The specificity of a virus for particular host tissues or cell types.

• Uncoating: The process by which the viral genome is released from the capsid inside the host cell.

Key Concepts in Viral Biology

• Viruses typically have a limited host range due to specific interactions with host cell receptors.

• Viral reproduction involves several steps: attachment to the host cell, entry, hijacking of host cellular machinery, assembly of new virions, and exit from the host cell.

• A virus refers to the intracellular form, while a virion is the complete, infectious extracellular particle.

The Capsid and Envelope

The structure of viruses includes a protein shell and, in some cases, a lipid envelope derived from the host cell.

• Capsomeres: Protein subunits that make up the capsid.

• Viral Envelope: A lipid membrane surrounding some viruses, derived from the host cell membrane during viral exit.

• Spike Proteins: Glycoproteins embedded in the envelope that facilitate attachment to host cells.

Viral Classification

Viruses are classified based on genetic material, structure, and host range.

• Viral Species: A group of viruses sharing genetic information and host range.

• Viruses are often named using descriptive common names.

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

Viroids and Prions

Viroids and prions are infectious agents distinct from viruses, lacking typical viral structures.

• Viroids: Infectious RNA molecules without a protein capsid, primarily infecting plants.

• Prions: Infectious misfolded proteins that cause neurodegenerative diseases.

• PrPc: The normal, cellular form of the prion protein.

• PrPsc: The abnormal, infectious form of the prion protein.

• Prions cause protein aggregation and neural damage (e.g., Creutzfeldt-Jakob disease).

Capsid Structures

Viruses exhibit diverse capsid morphologies, which influence their classification and infection mechanisms.

• Helical: Rod-shaped capsid structure.

• Icosahedral: 20-sided, symmetrical structure.

• Polyhedral: Many-sided structure.

• Enveloped: Surrounded by a lipid membrane.

• Complex: More intricate structures, such as those found in bacteriophages.

Viral Replication

Viruses replicate through distinct cycles, often visualized and quantified in laboratory settings.

• Plaque Method: A technique to visualize and quantify viruses by the formation of clear zones (plaques) on a cell layer.

• Plaques: Clear zones indicating cell lysis due to viral infection.

• PFU (Plaque-Forming Units): A measure of viral quantity based on the number of plaques formed.

Lytic Cycle

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

• Entry: Viral genome enters the host cell.

• Synthesis: Host machinery is hijacked to produce viral components.

• Assembly: New virions are assembled from synthesized components.

• Lysis: Host cell bursts, releasing new virions.

Lysogenic Cycle

• Integration: Viral genome integrates into the host genome.

• Dormancy: Viral genome remains latent within the host.

• Replication: Host cell replicates, copying the viral genome.

• Induction: Environmental triggers cause the virus to enter the lytic cycle.

Examples of Important Viruses

• HPV (Human Papillomavirus): Icosahedral, non-enveloped, double-stranded DNA virus; replicates in the nucleus.

• HIV (Human Immunodeficiency Virus): Causes AIDS, infects T cells, uses reverse transcriptase to convert RNA to DNA, integrates as a provirus, and buds from host cells.

• Influenza A: Enveloped virus with segmented RNA genome. Hemagglutinin (HA) mediates attachment; neuraminidase (NA) facilitates release. The 1918 Spanish flu pandemic caused massive mortality.

Summary Table: Virus Types and Features

Additional info: The table above summarizes key structural and genetic features of representative viruses discussed in this chapter.