What triggers a lysogenic virus to enter the lytic cycle?

• A. Random genetic mutation
• B. Nutrient depletion
• C. Exposure to ultraviolet light is required
• D. Stress

Answer: (D)

When a temperate phage is in lysogeny, its genome sits quiet in the bacterial chromosome, kept silent by phage repressor proteins. If the host experiences stress, especially when DNA is damaged, the bacterial SOS response is activated. This activates RecA, which promotes cleavage of the phage repressor, lifting the block and allowing the phage genes for the lytic cycle to be expressed. The prophage then excises from the host genome, replicates, and new viral particles are produced, leading to cell lysis.

Stress is the key trigger because it signals that the cellular environment is unfavorable, prompting the phage to switch strategies from dormancy to reproduction and escape. Examples of stress include UV-induced DNA damage or general nutrient limitation. Random genetic mutations aren’t a reliable, directed trigger, and while UV exposure is a stressor, the important concept is that the host’s stress response initiates prophage induction.