📌Definition Table

Term	Definition
Lytic Cycle	Viral replication process resulting in destruction of the host cell and release of new viruses.
Lysogenic Cycle	Viral replication strategy where viral DNA integrates into the host genome and remains dormant before activation.
Provirus	Viral genome integrated into the host DNA.
Latency	Period where a virus remains dormant in host cells without producing new viruses.
Reverse Transcriptase	Enzyme that synthesises DNA from an RNA template (in retroviruses).
Lysozyme	Enzyme used by bacteriophages to break bacterial cell walls during infection.

📌Introduction

Viral replication requires host cell machinery because viruses lack the enzymes and structures necessary for independent reproduction. Depending on the virus, replication may follow a lytic pathway, where new viruses are made immediately, or a lysogenic pathway, where the viral genome integrates into the host cell and remains inactive until triggered.

❤️ CAS Link: Develop a school science outreach activity where students model viral replication using simple props to show lytic vs lysogenic cycles.

📌 General Process of Viral Replication

• Attachment: Virus recognises and binds to specific host cell receptors.
• Entry: Virus injects its genome or enters the host via endocytosis/membrane fusion.
• Replication: Host cell machinery is hijacked to replicate viral genetic material.
• Assembly: Viral components self-assemble into new virions.
• Release: Viruses exit the cell via lysis (bursting) or budding (enveloped viruses).
• Cycle repeats upon infection of new host cells.

🧠 Examiner Tip: Always include attachment → entry → replication → assembly → release in sequence when describing viral replication for full marks.

📌 Lytic Cycle

• Immediate replication upon infection.
• Host cell is destroyed at the end of the cycle.
• Steps:
  • Attachment to host cell surface.
  • Injection or entry of viral genome.
  • Replication of genome and synthesis of viral proteins.
  • Assembly of new viral particles.
  • Host cell lysis via enzymes like lysozyme, releasing new viruses.
• Example: T4 bacteriophage.
• Rapidly produces many new viruses, often causing acute infections.

🌍 Real-World Connection: The lytic cycle explains why some viral diseases (e.g., influenza) cause sudden, severe symptoms.

📌 Lysogenic Cycle

• Viral DNA integrates into the host genome as a provirus (in eukaryotes) or prophage (in bacteria).
• Host cell continues normal functions while replicating viral DNA along with its own.
• The virus can remain dormant for years (latency).
• Environmental triggers (e.g., UV light, stress) can activate the lytic cycle.
• Lysogeny allows viruses to persist without killing the host.
• Example: Lambda bacteriophage, herpesviruses.

📝 Paper 2: Data Response Tip: In questions with viral life cycle diagrams, always identify key triggers for switching from lysogenic to lytic — these are often part of the mark scheme.