Study Notes: Viruses and Prions (Microbiology Chapter 13)

Viruses: Fundamental Concepts

Definition and Nature of Viruses

Viruses are unique infectious agents that exist at the boundary between living and non-living entities. They are obligate intracellular parasites, meaning they require a host cell to replicate and cannot carry out metabolic processes independently.

• Living vs. Non-living: Viruses are considered non-living outside a host cell because they lack cellular structure and metabolism.

• Cellular Hijackers: Once inside a host cell, viruses hijack the cellular machinery to reproduce.

• Host Specificity: Each virus infects specific cell types, determined by the presence of compatible receptors.

Basic Structure of Viruses

Viruses are composed of genetic material encased in a protein shell, and sometimes surrounded by a lipid envelope.

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

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

• Capsomere: Subunits that make up the capsid.

• Envelope: A lipid membrane derived from the host cell, containing viral proteins (including spike proteins).

• Spike Proteins: Glycoproteins embedded in the envelope, crucial for host cell recognition and entry.

Virus Classification and Nomenclature

Viruses are classified based on their genetic material, structure, and replication strategy.

• Nucleic Acid Type: Viruses may contain DNA or RNA, but never both. This distinguishes them from living cells.

• Shapes of Viruses: The three basic shapes are:

  • Helical

  • Icosahedral

  • Complex (e.g., bacteriophages)

• Naming: Virus names do not follow the same binomial system as living organisms.

Bacteriophages and Viral Infection Cycles

Bacteriophages

Bacteriophages are viruses that infect bacteria. Their infection cycles differ from those of animal viruses.

• Lytic Cycle: The phage replicates and lyses the host cell.

• Lysogenic Cycle: The phage genome integrates into the host DNA and replicates passively.

• Temperate Phage: A phage capable of both lytic and lysogenic cycles.

Comparison of Lytic and Lysogenic Cycles

Feature	Lytic Cycle	Lysogenic Cycle
Viral Replication	Active, rapid	Passive, integrated
Host Cell Fate	Lysis (death)	Survival, possible later lysis
Genetic Integration	No	Yes (prophage)

Viral Replication Steps

Viral replication involves several key steps:

1. Attachment

2. Penetration

3. Uncoating (in animal viruses)

4. Replication and synthesis

5. Assembly

6. Release

Viral Genomes and Replication Strategies

Types of Viral Nucleic Acids

Viruses may possess different types of nucleic acids, which determine their replication and protein synthesis strategies.

• DNA Viruses: Replicate using host DNA polymerases.

• RNA Viruses: Require viral RNA-dependent RNA polymerase for replication.

• Antigenic Drift: RNA viruses are prone to mutations due to error-prone replication.

Viral Enzymes

Some viruses encode unique enzymes essential for their replication.

• Reverse Transcriptase: Enzyme encoded by retroviruses (e.g., HIV) that synthesizes DNA from RNA.

Lysogenic Conversion and Medical Importance

Lysogenic Conversion

Some bacteria acquire new traits when infected by lysogenic phages, a process called lysogenic conversion.

• Example: Corynebacterium diphtheriae produces diphtheria toxin only when infected by a lysogenic phage.

Persistent and Latent Infections

Definitions

• Persistent Infection: The virus remains in the host for long periods, often with ongoing replication (e.g., HIV, hepatitis B).

• Latent Infection: The virus remains dormant in the host, with periodic reactivation (e.g., herpes simplex virus).

Comparison of Latent and Persistent Infections

Feature	Latent Infection	Persistent Infection
Viral Activity	Dormant, occasional reactivation	Continuous, low-level replication
Examples	Herpes simplex, Varicella-zoster	HIV, Hepatitis B

Viruses and Cancer

Oncoviruses and Oncogenes

Some viruses are associated with cancer in humans. These are called oncoviruses.

• Oncogenes: Genes that can transform a normal cell into a cancerous cell.

• Examples: Human papillomavirus (HPV) and hepatitis B virus (HBV).

Prions

Definition and Comparison to Viruses

Prions are infectious proteins that cause neurodegenerative diseases. Unlike viruses, prions lack nucleic acids and are composed solely of misfolded protein.

• Prion Diseases: Creutzfeldt-Jakob disease, mad cow disease.

• Difference from Viruses: Prions do not contain DNA or RNA and do not elicit an immune response.

Laboratory Cultivation of Viruses

Virus Culture Requirements

Viruses cannot be grown on standard agar plates because they require living cells for replication.

• Cell Culture: Viruses are cultured using living cells (e.g., animal cell lines, embryonated eggs).

• Vaccine Production: Some vaccines are produced in eggs, which is relevant for individuals with egg allergies.

Key Equations and Concepts

• Viral Replication Rate:

  • $\text{Rate} = k[\text{Virus}][\text{Host Cell}]$

• Reverse Transcription (Retroviruses):

  • $\text{RNA} \xrightarrow{\text{Reverse Transcriptase}} \text{DNA}$

Additional info: Academic context and definitions have been expanded for clarity and completeness. Tables have been inferred and formatted for comparison purposes.