Characterizing and Classifying Viruses, Viroids, and Prions: Study Guide

Chapter 13 – Characterizing and Classifying Viruses, Viroids, and Prions

Horizontal Gene Transfer (HGT) in Microbiology

Horizontal gene transfer is the movement of genetic material between organisms other than by vertical transmission (parent to offspring). It plays a crucial role in microbial evolution and adaptation.

• Three Processes of HGT:

  • Transformation: Uptake of naked DNA from the environment by a bacterial cell.

  • Transduction: Transfer of DNA from one bacterium to another via bacteriophages (viruses that infect bacteria).

  • Conjugation: Direct transfer of DNA between two bacterial cells through cell-to-cell contact, often via a pilus.

• Generalized vs Specialized Transduction:

  • Generalized Transduction: Any bacterial gene can be transferred. Occurs during the lytic cycle when a phage accidentally packages host DNA.

  • Specialized Transduction: Only specific bacterial genes near the prophage insertion site are transferred. Occurs during the lysogenic cycle when a prophage excises incorrectly.

• Effects of HGT on Human Health: HGT can spread antibiotic resistance, virulence factors, and new metabolic capabilities among pathogens, impacting disease treatment and epidemiology.

Acellular Agents Infecting Eukaryotes

Acellular agents are infectious entities lacking cellular structure. The main types are viruses, viroids, and prions.

• Viruses: Infectious particles composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid), sometimes with a lipid envelope.

• Viroids: Small, circular RNA molecules without a protein coat, infecting plants.

• Prions: Infectious proteins causing neurodegenerative diseases in animals and humans.

Key Viral Structures and Definitions

• Virion: The complete, infectious form of a virus outside a host cell.

• Capsid: The protein shell that encases the viral genome.

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

Nucleic Acid Diversity in Viruses

Viruses exhibit remarkable diversity in their genetic material.

• Can be DNA or RNA

• Single-stranded (ss) or double-stranded (ds)

• Linear or circular

• Positive-sense or negative-sense (for RNA viruses)

Animal Virus Replication: Key Steps

Animal viruses follow a series of steps to infect host cells and replicate.

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

2. Entry/Penetration: Virus enters the cell via endocytosis or membrane fusion.

3. Uncoating: Viral capsid is removed, releasing the genome into the host cell.

4. Synthesis: Viral genome is replicated and viral proteins are synthesized using host machinery.

5. Assembly: New virions are assembled from synthesized components.

6. Release: Virions exit the cell by lysis or budding.

Example: Influenza virus enters via endocytosis, uncoats, replicates its RNA, assembles, and buds from the cell membrane.

Bacteriophage Life Cycles: Lytic vs Lysogenic

Bacteriophages (viruses that infect bacteria) can follow two main replication cycles.

• Lytic Cycle: Phage infects, replicates, and lyses the host cell, releasing new phages.

• Lysogenic Cycle: Phage genome integrates into the host genome as a prophage, replicating with the host until induction triggers the lytic cycle.

Prophage and Lysogenic Conversion

• Prophage: The phage genome integrated into the bacterial chromosome during lysogeny.

• Lysogenic Conversion: The acquisition of new traits by bacteria due to prophage genes, such as toxin production (e.g., diphtheria toxin).

• Impact on Human Health: Lysogenic conversion can make bacteria more pathogenic.

Latency and Proviruses

Some animal viruses can remain dormant within host cells.

• Latency: The state in which a virus remains inactive within a host cell.

• Provirus: Viral DNA integrated into the host genome, persisting without producing new virions.

• Examples: Varicella-zoster virus (chicken pox), Hepatitis B virus, Herpes simplex virus.

Viruses and Cancer

Some viruses are associated with cancer development (oncogenic viruses).

• Examples: Human papillomavirus (HPV) causes cervical cancer; Hepatitis B and C viruses can cause liver cancer.

Retroviruses

• Definition: Viruses with RNA genomes that use reverse transcriptase to synthesize DNA from RNA.

• Example: Human Immunodeficiency Virus (HIV).

• Key Enzyme: Reverse transcriptase.

Viroids

• Definition: Infectious, small, circular RNA molecules lacking a protein coat.

• Host: Plants.

• Example: Potato spindle tuber viroid.

Prions

• Definition: Infectious proteins that cause misfolding of normal cellular proteins, leading to neurodegenerative diseases.

• Etiology: Prion diseases result from the conversion of normal prion protein (PrPC) to the misfolded, disease-causing form (PrPSc).

• Molecular Mechanism: PrPSc induces misfolding of PrPC, leading to aggregation and cell death.

Prion Disease Example

• Scientific Name: Bovine Spongiform Encephalopathy (BSE)

• Host Animal: Cattle

• Signs and Symptoms: Behavioral changes, loss of coordination, neurodegeneration, and ultimately death.

Summary Table: Acellular Agents

Agent	Genetic Material	Protein Coat	Envelope	Host	Example
Virus	DNA or RNA	Yes (capsid)	Sometimes	Animals, plants, bacteria	Influenza virus
Viroid	RNA (circular)	No	No	Plants	Potato spindle tuber viroid
Prion	None	No	No	Animals	Bovine Spongiform Encephalopathy

Additional info:

• Reverse transcriptase catalyzes the reaction: $RNA \rightarrow DNA$

• Prophages can confer new traits to bacteria, such as toxin production.

• Prion diseases are collectively known as transmissible spongiform encephalopathies (TSEs).