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1.5.2Inquiry Process

Replication in Viruses — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 13 of 160

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Part 1 of 5Lytic Cycle

1.5.2 Replication in Viruses

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
  • The viral DNA inserts into the host chromosome and is then known as a prophage.
Silent copying
  • Each time the host divides, the prophage is copied with its DNA and passed to daughter cells.
Induction
  • Stress such as UV light can trigger the prophage to leave the chromosome and enter the lytic cycle.
Advantage
  • The virus spreads through many generations of bacteria without killing them or showing symptoms.
FeatureLyticLysogenic
Host cellDestroyed quicklySurvives
Viral DNAFree in cellIntegrated
SymptomsImmediateDelayed

Part 3 of 5Retrovirus Replication

Binding
  • HIV glycoproteins bind to receptors on helper T cells, allowing the envelope to fuse with the cell membrane.
Reverse transcription
  • Reverse transcriptase makes a DNA copy from the viral RNA genome, reversing the usual flow of information.
Integration step
  • The viral DNA enters the nucleus and is spliced into a host chromosome by integrase.
Transcription
  • Host RNA polymerase makes viral RNA, which acts as new genomes and as mRNA for viral proteins.
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.

  1. At first the phage DNA was integrated into the bacterial chromosome as a prophage, the lysogenic cycle.
  2. The bacteria divided normally and copied the prophage with their own DNA, so no cells were killed.
  3. UV light damaged the cells and induced the prophage to leave the chromosome and start the lytic cycle.
  4. 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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