The lysogenic cycle is a form of viral replication where a virus incorporates its genetic material into the host cell’s DNA without immediately destroying the cell. This allows the virus to exist in a “silent” or “dormant” state within the host. The integrated viral genome replicates along with the host’s DNA, spreading the viral genetic information to daughter cells without causing an active infection. This cycle provides a way for viruses to persist within a population of host cells over time.
Establishing the Latent State
The lysogenic cycle begins when a virus, such as a bacteriophage that infects bacteria, attaches to a host cell and injects its genetic material. For bacteriophages, this involves the phage injecting its DNA into the bacterial cytoplasm. Once inside the host cell, the viral DNA does not immediately initiate the production of new viral particles. Instead, the viral DNA integrates into the host cell’s chromosome.
This integration is often facilitated by viral enzymes, such as integrases, which help insert the viral genetic material into specific regions of the host’s DNA. Once integrated, the viral genome is referred to as a “prophage” if it’s a bacteriophage in a bacterial cell, or a “provirus” if it’s a virus in a eukaryotic cell. The host cell continues to function normally, carrying the viral blueprint within its own genome.
Dormant Replication and Spread
Following integration, the viral genetic material enters a phase of dormant replication. Every time the host cell divides, it replicates its own DNA, and in doing so, also replicates the integrated viral DNA. The viral DNA is passively replicated as part of the host’s chromosome, effectively “hitching a ride” on the host’s cellular machinery.
During this period, the host cell remains healthy and continues its normal functions, showing no immediate signs of viral infection. The integrated prophage or provirus does not produce new viral particles or cause harm to the host cell. This silent replication allows the virus to spread its genetic information through generations of host cells, ensuring a long-term presence.
Activation and Departure
The lysogenic state is not always permanent; the integrated viral genome can become active and transition into the lytic cycle. This transition, known as induction, is often triggered by various environmental stressors. Factors such as ultraviolet (UV) radiation, certain chemicals, or starvation can induce the prophage or provirus to excise itself from the host genome.
Once excised, the viral DNA initiates the lytic cycle, taking over the host cell’s machinery to produce new viral particles. The viral DNA replicates independently, and viral proteins are synthesized to assemble new virions. This process leads to the lysis, or bursting, of the host cell, releasing numerous new viral particles that can then infect other cells. This dynamic switch allows the virus to balance long-term persistence with rapid progeny production when conditions become unfavorable.