The lysogenic cycle, a distinct viral life strategy, typically does not lead to the immediate death of its host cell. Instead, a virus integrates its genetic material into the host cell’s genome, allowing the host to survive and continue its normal functions. This contrasts sharply with other viral life cycles that result in rapid host cell destruction.
Understanding Viral Replication Cycles
Viruses, as obligate intracellular parasites, rely entirely on host cells to multiply, employing diverse strategies. The two main approaches are the lytic and lysogenic cycles, each with a different outcome for the infected cell. In the lytic cycle, the virus takes over the host cell’s machinery to produce new viral particles, which then burst from and destroy the host cell. This process is often rapid and culminates in cell lysis.
Conversely, the lysogenic cycle involves a more subtle interaction where the viral genetic material becomes part of the host’s own genetic information. This integration allows the virus to replicate passively alongside the host cell’s DNA. The host cell continues to function and divide, passing the integrated viral DNA to its daughter cells.
The Lysogenic Cycle Explained
The lysogenic cycle begins when a virus, often a bacteriophage that infects bacteria, attaches to a susceptible host cell. Following attachment, the virus injects its genetic material, either DNA or RNA, into the host’s cytoplasm. This viral genetic material then integrates into the host cell’s chromosome, becoming what is known as a prophage in bacteria or a provirus in eukaryotic cells.
At this stage, the viral genes are largely inactive, and the host cell continues its regular cellular activities. As the host cell divides, it replicates its own genome, also replicating the integrated viral DNA. This passes the viral genetic information down to all subsequent daughter cells.
Why Lysogeny Spares the Host Cell
The survival of the host cell during the lysogenic cycle is a direct result of precise molecular mechanisms that prevent the expression of viral genes associated with replication and cell destruction. Key among these is the integration of the viral DNA directly into the host’s genome, allowing it to be replicated passively as part of the host’s own genetic material.
Crucially, specific viral proteins, such as repressor proteins, are produced to inhibit the expression of other viral genes that would otherwise initiate the lytic cycle. These repressor proteins bind to regulatory regions on the viral DNA, effectively turning off the genes responsible for producing new viral particles and enzymes that would lyse the cell. For instance, in bacteriophage lambda, the cI repressor protein maintains the lysogenic state by blocking the transcription of lytic genes. This ensures that the genes necessary for active viral replication and assembly remain unexpressed.
Consequences for the Host Cell
While the lysogenic cycle does not immediately kill the host cell, the integrated viral DNA can have significant consequences. One effect is lysogenic immunity, where the host cell becomes resistant to superinfection by the same or closely related viruses. This occurs because repressor proteins from the integrated prophage inhibit the lytic genes of new, incoming viruses.
The integrated viral genes can also confer new traits to the host cell, a phenomenon known as phage conversion or lysogenic conversion. These new traits can alter the host cell’s phenotype, sometimes increasing its virulence. For example, non-toxic bacteria like Corynebacterium diphtheriae and Vibrio cholerae can become pathogenic and produce toxins only when they acquire specific phage genes through lysogenic conversion.
When Lysogeny Becomes Lytic
The lysogenic state, while stable, is not permanent. Under certain environmental conditions, the integrated viral DNA can be “induced” to excise itself from the host genome and switch to the lytic cycle. This transition, known as prophage induction, can be triggered by various stressors.
Common inducing factors include ultraviolet (UV) radiation, certain chemicals, or nutrient deprivation. When induced, the viral DNA detaches from the host chromosome, begins active replication, and initiates the production of new viral particles. This leads to the lysis and death of the host cell.