1.5.2Inquiry Process
Replication in Viruses — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Lytic Cycle
In the lytic cycle the virus replicates at once and destroys the host.
1
Attachment: the phage binds to specific receptor proteins on the surface of a bacterium.
2
Entry: the phage injects its DNA into the host while the empty capsid remains outside.
3
Takeover: host enzymes copy the viral DNA and host ribosomes make viral proteins, shutting down normal bacterial activity.
4
Assembly: new capsids form and are filled with copies of the viral genome to make complete phages.
5
Release: the host cell bursts open (lysis), freeing many new virus particles to infect other cells.
Part 2 of 5Lysogenic Cycle
Lysogenic Cycle
Integration
- Integration The viral DNA inserts into the host chromosome and is then known as a prophage.
Silent copying
- Silent copying Each time the host divides, the prophage is copied with its DNA and passed to daughter cells.
Induction
- Induction Stress such as UV light can trigger the prophage to leave the chromosome and enter the lytic cycle.
Advantage
- Advantage The virus spreads through many generations of bacteria without killing them or showing symptoms.
| Feature | Lytic | Lysogenic |
|---|---|---|
| Host cell | Destroyed quickly | Survives |
| Viral DNA | Free in cell | Integrated |
| Symptoms | Immediate | Delayed |
Part 3 of 5Retrovirus Replication
Binding
- Binding HIV glycoproteins bind to receptors on helper T cells, allowing the envelope to fuse with the cell membrane.
Reverse transcription
- Reverse transcription Reverse transcriptase makes a DNA copy from the viral RNA genome, reversing the usual flow of information.
Integration step
- Integration step The viral DNA enters the nucleus and is spliced into a host chromosome by integrase.
Transcription
- Transcription Host RNA polymerase makes viral RNA, which acts as new genomes and as mRNA for viral proteins.
Budding
- Budding New virus particles assemble and leave by budding through the plasma membrane, taking an envelope with them.
Part 4 of 5Worked Example
Question
- A culture of bacteria infected with a phage stays clear for several generations, then suddenly clears as the cells burst after UV treatment.
- Explain these observations.
- At first the phage DNA was integrated into the bacterial chromosome as a prophage, the lysogenic cycle.
- The bacteria divided normally and copied the prophage with their own DNA, so no cells were killed.
- UV light damaged the cells and induced the prophage to leave the chromosome and start the lytic cycle.
- New phages were assembled and the cells burst by lysis, so the culture cleared.
Answer The culture was lysogenic until UV induced the lytic cycle, which destroyed the host cells.
Part 5 of 5Exam Checklist & Tips
1.5.2 – Key Points for the Exam
- Lysis Bursting of a cell, releasing its contents.
- Prophage Viral DNA integrated into a bacterial chromosome.
- Reverse transcriptase An enzyme making DNA from an RNA template.
- Retrovirus An RNA virus that makes DNA copies of its genome.
Common Mistakes
- Saying viruses replicate by binary fission; they are assembled using host machinery.
- Thinking the lysogenic cycle kills the host immediately.
- Stating that reverse transcriptase makes RNA from DNA; it does the opposite.
- Forgetting that virus proteins are made on host ribosomes.
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