Chapter 13: Viruses, Viroids, and Prions - Structured Study Notes

Chapter 13: Viruses, Viroids, and Prions

Viruses: General Characteristics

Viruses are acellular infectious agents that require a host cell for replication. They are distinct from living cells and possess unique structural and functional properties.

• NOT a Cell: Viruses lack cellular structure and metabolic machinery. They do not carry out cellular processes independently.

• Obligate Intracellular Parasites: Viruses must infect a host cell to reproduce, relying on the host's metabolic systems.

• Structure: Viruses consist of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. Some viruses have an additional lipid envelope derived from the host cell membrane.

• Size: Viruses are much smaller than prokaryotic and eukaryotic cells, typically measured in nanometers.

• Host Specificity: Viruses are highly specific to their host species and cell types, determined by the presence of specific receptor molecules.

Example: Influenza virus infects respiratory epithelial cells in humans.

Viral Structure

Viruses display a variety of shapes and structural features, which influence their mode of infection and replication.

• Capsid: The protein shell that encases the viral genome. Capsids are composed of subunits called capsomeres.

• Envelope: Some viruses possess a lipid envelope surrounding the capsid, acquired from the host cell during viral release.

• Genome: Viral genomes may be DNA or RNA, single-stranded or double-stranded, linear or circular.

• Shapes: Viruses can be helical, polyhedral (icosahedral), or complex in structure.

Example: Bacteriophage T4 has a complex structure with a head, tail, and tail fibers.

Viral Life Cycle

The viral life cycle involves several distinct stages, each critical for successful infection and replication within the host cell.

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

• Entry: Viral genome enters the host cell, either by fusion with the membrane or endocytosis.

• Replication: Viral genome is replicated using host cell machinery. DNA viruses typically replicate in the nucleus, while RNA viruses often replicate in the cytoplasm.

• Synthesis: Viral proteins and nucleic acids are synthesized.

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

• Release: Viruses exit the host cell, often destroying it in the process (lysis) or budding off (in enveloped viruses).

Example: HIV enters T cells via CD4 receptors, replicates its RNA genome, and buds from the cell membrane.

Viroids and Prions

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

• Viroids: Small, circular RNA molecules that infect plants. They do not encode proteins and cause disease by interfering with host gene expression.

• Prions: Infectious proteins that cause neurodegenerative diseases in animals and humans. Prions induce abnormal folding of normal cellular proteins, leading to cell damage.

Example: Prion diseases include Creutzfeldt-Jakob disease in humans and mad cow disease in cattle.

Table: Comparison of Viruses, Viroids, and Prions

Agent	Genetic Material	Structure	Host	Diseases
Virus	DNA or RNA	Capsid, sometimes envelope	Animals, plants, bacteria	Influenza, HIV, rabies
Viroid	RNA	No capsid or envelope	Plants	Potato spindle tuber disease
Prion	None	Protein only	Animals	Mad cow disease, CJD

Key Terms and Definitions

• Capsid: Protein shell surrounding viral genetic material.

• Envelope: Lipid membrane acquired from host cell.

• Capsomere: Subunit of the capsid.

• Obligate Intracellular Parasite: Organism that can only reproduce inside a host cell.

• Prion: Infectious protein causing neurodegenerative disease.

• Viroid: Infectious RNA molecule affecting plants.

Additional info: Expanded explanations and examples were added for clarity and completeness. The table was inferred from the notes' comparison points.