CHap 6 B

Viral Replication Pathways

Introduction

Viruses are obligate intracellular pathogens that rely on host cells for replication. Understanding the replication pathways of bacteriophages and animal viruses is essential for microbiology students, as these processes underpin viral pathogenesis and inform strategies for detection and treatment.

Bacteriophage Replication Pathways

Bacteriophages, viruses that infect bacteria, can replicate via two main pathways: lytic and lysogenic replication.

• Lytic Replication: The phage immediately hijacks the host cell machinery to produce new virions, resulting in host cell lysis and release of progeny phages.

• Lysogenic Replication: The phage genome integrates into the host cell genome, becoming a prophage. The host cell survives and divides, copying the prophage DNA until certain conditions trigger entry into the lytic cycle.

Lytic Replication Steps

1. Attachment (Adsorption): Phage binds to specific receptors on the bacterial cell surface.

2. Penetration (Entry): Phage injects its genetic material into the host cell.

3. Replication (Synthesis): Phage commandeers host cell factors to transcribe and translate viral genes.

4. Assembly (Maturation): Viral genome is packaged into capsids and new phage particles are assembled.

5. Release: Host cell lyses, releasing new phages.

Lysogenic Replication Steps

1. Attachment

2. Penetration

3. Integration: Phage genome incorporates into host genome as a prophage.

4. Cell Division: Host cell divides, copying the prophage.

5. Induction: Under stress, prophage may excise and enter the lytic cycle.

Phage Conversion

Phage conversion occurs when prophages confer new pathogenic properties to bacterial cells, such as toxin production. This is medically important because it can transform harmless bacteria into pathogens.

• Examples:

  • Corynebacterium diphtheriae

  • Clostridium botulinum

Animal Virus Replication

Generalized Animal Virus Replication Steps

Animal viruses follow a six-step replication pathway, with variations depending on the virus family.

1. Attachment: Viral envelope or capsid proteins bind to host cell receptors.

2. Penetration: Enveloped viruses enter via endocytosis or membrane fusion; naked viruses enter by endocytosis.

3. Uncoating: Viral genome is released from the capsid.

4. Replication (Synthesis): Viral genome is replicated and viral proteins are synthesized.

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

6. Release: Enveloped viruses are released by budding; naked viruses typically cause cell lysis.

Key Points

• Attachment: Specificity is determined by interactions between viral proteins and host cell receptors.

• Penetration: Enveloped viruses can fuse with the host membrane; naked viruses rely on endocytosis.

• Uncoating: Necessary for the viral genome to access host cell machinery.

• Release: Budding does not lyse the cell, but the cell may still be damaged or die over time.

Mechanisms that Cause Persistent Infections

Types of Viral Infections

Viruses can cause different types of infections based on their replication strategies and interactions with the host immune system.

• Acute Infections: Rapid viral replication, followed by immune clearance. Symptoms are typically short-lived.

• Persistent Infections: Virus evades immune clearance and remains in the host. Includes chronic and latent infections.

• Chronic Infections: Continuous release of virions over months or years, with slow disease progression.

• Latent Infections: Periods of dormancy interrupted by bursts of viral replication and symptoms.

Comparison of Infection Types

Type	Viral Replication	Symptoms	Duration
Acute	High, then cleared	Rapid onset	Short-term
Chronic	Continuous, low-level	Slow progression	Long-term
Latent	Burst during reactivation	Intermittent	Long-term

Key Terms and Concepts

• Bacteriophage: A virus that infects bacteria.

• Prophage: Phage DNA integrated into the bacterial genome.

• Virion: A complete, infectious virus particle.

• Phage Conversion: Acquisition of new traits by bacteria due to prophage genes.

• Oncogenic Virus: Virus capable of causing cancer by disrupting cell cycle regulation.

Examples and Applications

• Phage Conversion: Corynebacterium diphtheriae produces diphtheria toxin only when infected by a specific prophage.

• Oncogenic Viruses: Human papillomavirus (HPV) is associated with cervical cancer.

Formulas and Equations

• Viral Replication Rate: $\text{Rate} = k[\text{Virus}][\text{Host Cell}]$ Where $k$ is the rate constant, and the brackets denote concentrations.

Summary

• Bacteriophages and animal viruses have distinct replication pathways that impact host cell fate and disease progression.

• Persistent infections can be chronic or latent, with important implications for disease management and treatment.

• Phage conversion and oncogenic viruses illustrate the medical significance of viral genetics and host interactions.