Chapter 13: Viruses, Viroids, and Prions – Structure, Replication, and Pathogenesis

Chapter 13: Viruses, Viroids, and Prions

Overview of Viruses

Viruses are acellular infectious agents that require a host cell to replicate. They infect all forms of life, including animals, plants, bacteria (bacteriophages), and archaea. Understanding their structure, replication cycles, and effects on hosts is fundamental in microbiology.

• Definition: A virus is a small infectious agent composed of genetic material (DNA or RNA) surrounded by a protein coat, and sometimes a lipid envelope.

• Host Range: Viruses are specific to their hosts due to interactions between viral surface proteins and host cell receptors.

• Examples: Influenza virus (animal), Tobacco mosaic virus (plant), Bacteriophage T4 (bacteria).

General Characteristics of Viruses

• Obligate intracellular parasites – cannot reproduce outside a host cell.

• Contain either DNA or RNA, never both.

• Lack ribosomes and metabolic machinery.

• Viral structure includes a capsid (protein coat) and, in some cases, a lipid envelope.

• Viral classification is based on nucleic acid type, capsid symmetry, presence of envelope, and host range.

Viral Structure

• Capsid: Protein shell composed of subunits called capsomeres.

• Nucleocapsid: The combination of viral nucleic acid and capsid.

• Envelope: Lipid membrane derived from host cell, present in some viruses; contains viral glycoproteins.

• Spikes: Surface proteins involved in host cell attachment.

Example: The influenza virus has an envelope with hemagglutinin and neuraminidase spikes.

Viral Replication Cycles

Viruses follow specific steps to infect and replicate within host cells. The general cycle includes:

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

2. Entry: Virus or its genetic material enters the host cell (via fusion, endocytosis, or injection).

3. Uncoating: Viral capsid is removed, releasing nucleic acid.

4. Synthesis: Host machinery synthesizes viral components (nucleic acids and proteins).

5. Assembly: New viral particles are assembled from synthesized components.

6. Release: New virions exit the host cell (by lysis or budding).

Animal Virus Life Cycles: May include latent or persistent infections.

Bacteriophage Life Cycles: Include lytic and lysogenic cycles.

Lytic vs. Lysogenic Cycles (Bacteriophages)

Feature	Lytic Cycle	Lysogenic Cycle
Outcome	Host cell is lysed, releasing new phages	Phage DNA integrates into host genome (prophage)
Viral Genome	Replicates independently	Replicates with host DNA
Example	T4 phage	Lambda phage

Viral Latency and Cancer

• Latency: Some animal viruses can remain dormant within host cells, reactivating later (e.g., herpesviruses).

• Oncogenic Viruses: Certain viruses can cause cancer by integrating into host DNA and disrupting normal cell regulation (e.g., Human papillomavirus, Epstein-Barr virus).

• Relationship: Viral genes may encode proteins that inactivate tumor suppressor genes or activate oncogenes.

Viroids and Prions

• Viroids: Small, circular RNA molecules that infect plants; lack a protein coat.

• Prions: Infectious proteins that cause neurodegenerative diseases in animals and humans (e.g., Creutzfeldt-Jakob disease, mad cow disease).

• Diseases: Prion diseases are caused by misfolded proteins that induce normal proteins to misfold.

Comparison of Infectious Agents

Agent	Genetic Material	Protein Coat	Envelope	Cellular Structure	Example Disease
Virus	DNA or RNA	Yes	Sometimes	No	Influenza, HIV
Viroid	RNA	No	No	No	Potato spindle tuber disease
Prion	No	No	No	No	Creutzfeldt-Jakob disease
Bacterium	DNA	No	No	Yes	Tuberculosis
Fungus	DNA	No	No	Yes	Candidiasis
Protozoan	DNA	No	No	Yes	Malaria
Helminth	DNA	No	No	Yes	Schistosomiasis

Summary of Key Concepts

• Viruses are unique infectious agents with diverse structures and replication strategies.

• Bacteriophages can undergo lytic or lysogenic cycles, affecting bacterial populations and gene transfer.

• Some viruses are associated with cancer development in humans.

• Viroids and prions are non-viral infectious agents with distinct mechanisms of pathogenesis.

• Understanding the differences among infectious agents is crucial for diagnosis and treatment.