Question

Describe the lysogenic cycle.

Answer 1

The lysogenic cycle is one of the two replication strategies used by temperate bacteriophages (viruses that infect bacteria). Instead of immediately producing many new virions and lysing the host (the lytic cycle), a phage in the lysogenic cycle integrates its genome into the bacterial chromosome or maintains it as a stable plasmid and remains latent. Key points and steps:

- Attachment and entry: A phage attaches to a susceptible bacterium and injects its DNA.
- Decision point: For temperate phages, regulatory proteins determine whether the phage enters lytic or lysogenic mode.
- Integration (establishment of lysogeny): The phage genome becomes a prophage by integrating into a specific site in the host chromosome (site-specific recombination) or by persisting as a plasmid. The integrated prophage is largely transcriptionally silent for lytic genes.
- Maintenance: A phage-encoded repressor protein (e.g., lambda phage cI repressor) blocks expression of genes required for the lytic cycle, so the prophage is replicated passively along with the host DNA as the bacterium divides. The infected bacterium is called a lysogen.
- Effects on the host: Lysogens are often immune to superinfection by the same phage (superinfection immunity). Prophages can carry genes that alter host phenotype (lysogenic conversion), such as toxin genes that increase pathogenicity.
- Induction: Environmental stresses (UV light, DNA damage, certain chemicals) or specific signals can inactivate the repressor, triggering excision of the prophage and entry into the lytic cycle. The phage genome then replicates, new virions are assembled, and the host is lysed.

Biological significance: Lysogeny allows phages to persist through unfavorable conditions, promotes horizontal gene transfer, and can change bacterial traits (including virulence). The classic example is bacteriophage lambda; analogous latency occurs with some animal viruses (e.g., retroviral proviruses), though molecular details differ.