What can trigger the switch from lysogenic to lytic cycle?

• A. A trigger from the environment
• B. Spontaneous autoreset
• C. Antibiotics
• D. Viral replication always continuous

Answer: (A)

Environmental stress triggers the switch from lysogenic to lytic growth. When a temperate phage is in the lysogenic state, its genome sits as a prophage and is kept silent by phage repressors. If the host experiences damage or stress—such as DNA damage from environmental factors—the bacterial SOS response is activated. This leads to RecA-mediated autoproteolysis of the phage repressor, releasing the brakes on the lytic gene cluster. Once the repressor is inactivated, the phage begins replicating, assembles new virions, and lyses the cell. Spontaneous induction can occur but is relatively rare, so the typical and biologically significant trigger is environmental stress that tips the balance toward the lytic cycle. Antibiotics that damage DNA can contribute to induction, but the overarching idea is that external stress signals prompt the switch. The notion that replication would always be continuous in this context is incorrect because lysogeny is a deliberate latent state that is halted until such triggers push it into the lytic program.