📌Definition Table

Term	Definition
Lytic Cycle	Viral replication process in which the host cell is lysed to release new virus particles.
Lysogenic Cycle	Viral replication strategy in which the viral genome integrates into the host genome and remains dormant before activation.
Reverse Transcriptase	Enzyme used by retroviruses to convert RNA into DNA inside the host cell.
Provirus	Viral DNA integrated into the host’s genome during lysogeny.
Host Range	The variety of organisms or cell types a virus can infect, determined by receptor compatibility.

📌Introduction

Viruses cannot reproduce independently and must use the machinery of a host cell to replicate. The replication process varies depending on the virus type, genome composition, and host. Two main replication pathways are observed in bacteriophages: the lytic cycle, which rapidly destroys the host cell, and the lysogenic cycle, which allows the virus to persist in a dormant state before activation. Animal viruses may enter via membrane fusion or endocytosis, while bacteriophages inject their DNA directly. Retroviruses like HIV have unique replication mechanisms involving reverse transcription.

📌 The Lytic Cycle

• Virus attaches to host cell via specific receptor proteins.
• Viral genome enters the cell (injection for bacteriophages, fusion/endocytosis for animal viruses).
• Host cell machinery is hijacked to replicate viral nucleic acids and synthesise viral proteins.
• New viral particles are assembled in the cytoplasm.
• Host cell bursts (lysis), releasing many new viruses to infect other cells.
• This cycle results in rapid spread but also rapid destruction of host cells.

🧠 Examiner Tip: For IB diagrams, ensure the lytic cycle steps are in the correct order and labelled clearly with terms like “assembly” and “lysis”.

📌 The Lysogenic Cycle

• Viral genome integrates into host DNA, forming a provirus (or prophage in bacteria).
• The provirus is replicated along with the host’s DNA during cell division.
• No immediate damage occurs to the host cell.
• Environmental triggers (e.g., UV light, stress) can activate the virus.
• The provirus then enters the lytic cycle, producing new viruses.
• Lysogeny allows long-term persistence within the host population.

🧬 IA Tips & Guidance: A safe simulation IA could model virus spread using computer software, comparing lytic and lysogenic replication rates.

📌 Retrovirus Replication (HIV Example)

• Virus binds to specific receptors (e.g., CD4 and CCR5 on helper T cells).
• Viral RNA and enzymes enter the host cell.
• Reverse transcriptase converts viral RNA into DNA.
• Viral DNA integrates into the host genome as a provirus.
• Host cell transcribes and translates viral genes to make new viral proteins.
• New virions assemble and bud from the host cell, often without immediate lysis.
• Antiretroviral drugs target reverse transcriptase or other replication steps.

🌐 EE Focus: An EE could investigate the effectiveness of different antiviral drugs in inhibiting specific stages of viral replication.

📌 Factors Affecting Replication Speed

• Type of genome: RNA viruses generally replicate faster than DNA viruses.
• Host cell type: actively dividing cells often support faster replication.
• Immune system status: weakened immunity can allow faster viral spread.
• Presence of antiviral drugs: can slow or block replication steps.
• Environmental triggers: can activate dormant lysogenic viruses.
• Viral load: higher initial dose can lead to faster onset of symptoms.

❤️ CAS Link: A CAS project could involve designing an educational game that simulates the spread of lytic vs lysogenic viruses.

📌 Microscopy and Molecular Tools in Studying Replication

• TEM visualises viral assembly stages inside host cells.
• Fluorescent tagging tracks viral protein movement in real time.
• PCR detects and quantifies viral genomes in infected cells.
• Western blotting identifies viral proteins during replication.
• Cell culture allows controlled study of infection cycles.
• Data from these methods support antiviral drug development.

🔍 TOK Perspective: The study of viral replication shows how models and simulations help scientists understand processes too small or rapid to observe directly.

📝 Paper 2:
Be able to diagram the lytic and lysogenic cycles, explain retrovirus replication, and discuss how replication strategies influence disease progression.