The lysogenic cycle describes how certain viruses, called bacteriophages, interact with their bacterial hosts. In this process, the viral genetic material integrates into the host bacterium’s chromosome, rather than immediately producing new viruses. This allows the viral DNA to reside within the host without immediate harm, becoming a stable part of its genome.
Stages of the Lysogenic Cycle
The lysogenic cycle begins when a bacteriophage attaches to a bacterial cell and injects its genetic material into the bacterial cytoplasm. Following injection, this viral genetic material integrates into the host bacterium’s chromosome. An enzyme called integrase facilitates this process, and the integrated viral DNA is then referred to as a prophage.
The prophage then enters a dormant state. As the host bacterium undergoes normal cell division, the integrated prophage DNA is replicated along with the bacterial chromosome, passing the viral material to daughter cells.
Under certain environmental stressors, such as UV radiation or specific chemicals, the prophage can become active. This activation, known as induction, causes the prophage to excise itself from the host chromosome and initiate the lytic cycle.
How the Lysogenic Cycle Differs from the Lytic Cycle
The lysogenic cycle and the lytic cycle are two distinct strategies viruses employ for replication, primarily differing in their immediate impact on the host cell. In the lytic cycle, the virus takes over the host cell’s machinery to rapidly produce new viral particles, which then cause the host cell to burst and die. This results in the immediate destruction of the infected cell.
Conversely, the lysogenic cycle allows the host cell to survive and continue to replicate. The viral genetic material integrates into the host’s genome, becoming a prophage, and replicates passively along with the bacterial chromosome. This means the host cell remains intact and functional, with the viral DNA silently propagated through generations.
In the lytic cycle, the viral genetic material remains separate from the host genome and directly commands the cell to produce new viruses. The lytic pathway involves active replication of many new viral particles, leading to a rapid increase in viral numbers. The lysogenic cycle, however, is characterized by a latent phase where the viral DNA is dormant within the host. The viral DNA is only replicated when the host cell divides, leading to a more gradual spread of the viral genetic material without immediate new virion production.
Broader Implications of the Lysogenic Cycle
The lysogenic cycle has significant consequences beyond simply replicating viral genetic material, particularly impacting bacterial evolution and virulence. Integrated prophages can introduce new genes into bacteria through a process called lysogenic conversion. These new genes can alter the characteristics of the bacterium, sometimes making non-virulent strains pathogenic. For example, Vibrio cholerae, the bacterium causing cholera, becomes virulent only when infected by a specific bacteriophage that carries the gene for the cholera toxin. Similarly, Corynebacterium diphtheriae produces the diphtheria toxin only when it carries a prophage encoding this toxin.
Bacteriophages also play a role in horizontal gene transfer, which is the movement of genetic material between bacteria. This process contributes to bacterial diversity and adaptation, including the spread of traits like antibiotic resistance genes. While transduction, a type of horizontal gene transfer mediated by phages, can occur during the lytic cycle, lysogenic phages facilitate the stable integration and transfer of such genes across bacterial generations.
Understanding the lysogenic cycle is relevant for developing new antibacterial treatments, particularly in the field of phage therapy. While phage therapy primarily uses phages that undergo the lytic cycle to destroy bacteria, knowledge of lysogeny helps in selecting appropriate phages and engineering them to prevent the transfer of undesirable genes, such as those for antibiotic resistance or virulence factors, to target bacteria. This understanding also contributes to biotechnology, where phages can be modified as tools for genetic engineering.