What is Lysogenic Cycle?
Lysogeny, or the lysogenic cycle, is one of two cycles of viral reproduction (the lytic cycle being the other). Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium’s genome or formation of a circular replicon in the bacterial cytoplasm.
In this condition the bacterium continues to live and reproduce normally, while the bacteriophage lies in a dormant state in the host cell.
The genetic material of the bacteriophage, called a prophage, can be transmitted to daughter cells at each subsequent cell division, and later events (such as UV radiation or the presence of certain chemicals) can release it, causing proliferation of new phages via the lytic cycle.
Lysogenic cycles can also occur in eukaryotes, although the method of DNA incorporation is not fully understood.
For instance, the HIV viruses can either infect humans lytically, or lay dormant (lysogenic) as part of the infected cells’ genome, keeping the ability to return to lysis at a later time. The rest of this article is about lysogeny in bacterial hosts.
The difference between lysogenic and lytic cycles is that, in lysogenic cycles, the spread of the viral DNA occurs through the usual prokaryotic reproduction, whereas a lytic cycle is more immediate in that it results in many copies of the virus being created very quickly and the cell is destroyed.
One key difference between the lytic cycle and the lysogenic cycle is that the latter does not lyse the host cell straight away. Phages that replicate only via the lytic cycle are known as virulent phages while phages that replicate using both lytic and lysogenic cycles are known as temperate phages.
In the lysogenic cycle, the phage DNA first integrates into the bacterial chromosome to produce the prophage. When the bacterium reproduces, the prophage is also copied and is present in each of the daughter cells.
The daughter cells can continue to replicate with the prophage present or the prophage can exit the bacterial chromosome to initiate the lytic cycle. In the lysogenic cycle the host DNA is not hydrolyzed but in the lytic cycle the host DNA is hydrolyzed in the lytic phase.
Steps of Lysogenic Cycle
The steps in a lysogenic cycle depend solely on the virus’s ability to find a host. They are as follows:
1. Adsorption
This step is similar to the lytic cycle in bacteriophage, where the virus attaches explicitly to the receptors on the bacterial cell surface.
2. Penetration
After successful attachment, the phage injects its DNA into the host bacterial cell. This injection is often facilitated by viral enzymes or proteins that penetrate the cell membrane, allowing the viral DNA to enter the cell’s cytoplasm. On entering the host cell, the phage DNA circularizes.
3. Integration
The circular phage DNA then integrates into the host chromosome using site-specific recombination. It is the hallmark of the lysogenic cycle.
Instead of taking immediate control, the viral DNA harmoniously coexists with the host DNA. This integrated state is often referred to as prophage, a key feature distinguishing the lysogenic cycle from the lytic cycle.
While integrated into the host genome, the prophage remains inactive and does not produce viral particles.
4. Replication
As the bacterium replicates its DNA, it makes numerous copies of the prophage. When the host cell undergoes cell division, the viral genes are subsequently passed on to the daughter cells and their DNA.
5. Induction
The lysogenic state of the virus may persist for an extended period. Under certain conditions, the phage is triggered to exit the bacterial chromosome and initiate the lytic cycle. It occurs in the presence of environmental stress like ultraviolet radiation or the presence of specific chemical signals.
6. Synthesis, Assembly, and Release
Upon entry into the lytic cycle, the virus utilizes the host cell machinery to synthesize its DNA and proteins required to produce new viruses (virions).
The newly synthesized lysogenic phages destroy the host cell. They are released only to participate in a fresh infective cycle, and the process continues. The method of virion release in a lysogenized host is known as budding.
At the end of all these stages, the lysogenic host cell can either remain in the inactive latent form until further induction by factors such as UV radiation or may enter the lytic cycle, producing more virus particles.
Transition from lysogenic to lytic
If a bacterium containing prophage is exposed to stressors, such as UV light, low nutrient conditions, or chemicals like mitomycin C, prophage may spontaneously extract themselves from the host genome and enter the lytic cycle in a process called induction.
This process, however, is not perfect and prophage may sometimes leave portions of their DNA behind or take portions of host DNA with them when they re-circularize.
If they then infect a new host cell, they may transport bacterial genes from one strain to another in a process called transduction.
This is one method by which antibiotic resistance genes, toxin and superantigen-encoding genes and other virulence traits may spread through a bacterial population.
Recent work has shown that transition between lytic and lysogenic infection is also dependent on the abundance of phage in an area as they are able to produce and sense small peptides in a process akin to quorum sensing.