Lysogenic Replication/Lysogenic Cycle

Lysogenic replication, also known as the lysogenic cycle or lysogeny, is a type of viral replication cycle observed in certain temperate bacteriophages, which are viruses that infect bacteria. Unlike the lytic cycle, where the virus immediately replicates and destroys the host cell, the lysogenic cycle allows the virus to integrate its genetic material into the host cell’s genome and replicate along with it, remaining dormant for an extended period. Here’s a detailed explanation of the steps involved in lysogenic replication:

  1. Adsorption: The first step of lysogenic replication is the adsorption or attachment of the bacteriophage to the surface of the host bacterium. The phage recognizes specific receptors on the bacterial cell wall and attaches to it.
  2. Penetration: After adsorption, the viral genetic material is injected into the host cell. The viral genetic material consists of DNA in the case of most bacteriophages, although some bacteriophages may have RNA genomes.
  3. Integration: Once inside the host cell, the viral DNA integrates into the bacterial chromosome. This integration occurs through a process called recombination, where specific viral proteins facilitate the insertion of the viral DNA into the bacterial genome. The integrated viral DNA is known as a prophage.
  4. Lysogeny: After integration, the infected bacterial cell is called a lysogen. During lysogeny, the viral genes are present in the bacterial genome but remain dormant and do not actively replicate. The bacterial cell continues to divide and replicate, passing the integrated viral DNA to daughter cells.
  5. Maintenance: The viral genes within the bacterial genome are maintained through multiple generations of bacterial replication. The lysogen remains in a stable state, coexisting with the bacterial host.
  6. Induction: Under certain conditions, such as exposure to stress or environmental cues, the lysogenic bacteriophage can be induced to enter the lytic cycle. Induction involves the activation of viral genes that initiate the lytic replication process.
  7. Replication and Release: Once the bacteriophage is induced, the viral genes are expressed, leading to the synthesis of viral components, including new viral DNA and structural proteins. The viral components assemble to form new viral particles. The bacteriophage enzymes then lyse (rupture) the host bacterial cell, releasing the newly formed bacteriophages, which can go on to infect new bacterial cells and initiate the lytic cycle.

Lysogenic replication provides several advantages to both the bacteriophage and the host bacterium. For the phage, lysogeny allows it to persist within the host without immediately killing it, increasing its chances of survival. For the bacterial host, the incorporation of viral DNA into its genome can confer new genetic traits or even provide immunity against related phages through a phenomenon called superinfection exclusion.

The lysogenic cycle plays a critical role in the horizontal transfer of genetic material between bacteria. If the bacterial host carrying a prophage is infected by another bacteriophage, the newly infecting phage may undergo recombination with the integrated prophage, leading to the transfer of additional genetic elements between different bacterial strains.

In summary, lysogenic replication is a viral replication cycle in which the viral genetic material integrates into the host bacterium’s genome, remaining dormant and replicating along with the host during cell division. Lysogeny provides benefits to both the phage and the host, and under certain conditions, the viral genes can be induced to enter the lytic cycle, leading to replication and release of new viral particles.

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