A2.3.4 Lysogenic cycle of a virus (HL) Notes

The Lysogenic Cycle: Viral Dormancy and Integration

Bacteriophage lambda can switch between two distinct life cycles: the lytic cycle and the lysogenic cycle.

Step-By-Step Breakdown Of The Lysogenic Cycle

1. Step 1: Attachment
   1. Bacteriophage λ attaches to E. coli using tail proteins that bind specifically to maltoporin, a protein in the bacterial outer membrane.
2. Step 2: DNA Entry (Penetration)
   1. The phage contracts its tail sheath like a syringe, injecting viral DNA into the bacterial cytoplasm.
   2. The viral DNA enters as a linear molecule with sticky ends, which rapidly circularizes inside the host.
   3. Only the DNA enters, the capsid remains outside.
3. Step 3: Integration into Host Genome
   1. The enzyme integrase (encoded by the phage) inserts viral DNA into a specific site in the bacterial chromosome.
   2. Once integrated, the viral DNA is called a prophage.
4. Step 4: Host Cell Division (Prophage Replication)
   1. The bacterial cell continues normal growth and reproduction.
   2. The prophage DNA is replicated passively alongside host DNA.
   3. Each daughter cell inherits both bacterial and viral DNA.
5. Step 5: Latency and Maintenance
   1. Viral repressor proteins keep most viral genes switched off, preventing entry into the lytic cycle.
   2. The host cell remains healthy, continuing normal metabolism and reproduction.
   3. This dormant period is called lysogeny, and the virus is described as temperate.
6. Step 6: Induction (Switch to Lytic Cycle)
   1. Environmental stressors (UV light, DNA damage, chemicals) inactivate the viral repressor.
   2. The prophage is excised from the host genome.
   3. The virus re-enters the lytic cycle, producing viral proteins and eventually lysing the cell.

The Decision: Lytic or Lysogenic?