Lytic and Lysogenic Cycle
The lytic cycle and the lysogenic cycle are means of viral replication. This takes place within the host cell and the virus takes control of the host cell and controls its cellular mechanism to reproduce itself. The lytic and lysogenic cycles are well-studied in bacteriophages as they are an ideal model to study the virus's life cycle. Temperate phages become a part of the host chromosome and replicate this part with the cell genomes until the time comes when it can induce the cell to create new viruses. A brief explanation of the lytic and lysogenic cycle of virus is given below.
What is the Lytic Cycle?
The lytic cycle starts with a virulent phage and as it takes over the host cell it starts producing new phage particles, and eventually destroys the cell. The T-phage can be taken as a good example of how the stage of the lytic cycle is carried out.
The first stage is the attachment in which the phage interacts with the bacterial surface receptors which are certain lipopolysaccharides and OmpC protein on host surfaces. Most phages have a narrow host range and may infect one species of bacteria or one strain with a species. This recognition can then be exploited for targeted treatment of bacterial infection by phage therapy.
The second stage of the lytic cycle is entry or penetration. This occurs via contraction of the tail sheath, which acts like a hypodermic needle and injects the viral genome through the cell wall and membrane. The phage head and other remaining components remain outside the bacteria.
The third stage of the infection is the biosynthesis of viral components. This takes place after the phage enters the virus particle and viral endonucleases degrade the bacterial chromosome. It hijacks the host and replicates, transcribes and translates all viral components for the assembly of new viruses.
The fourth stage is maturation in which new virions are created followed by the final stage which is released. Fully developed viruses burst out of the host cell in a process called lysis and the progeny viruses are liberated into the environment to infect new cells.
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What is the Lysogenic Cycle?
In the lysogenic cycle, the phage genome enters the host cell through attachment and penetration. A good example of a phage with this type of life cycle is the lambda phage.
During the lysogenic cycle, instead of killing the host, the phage genome which is called a prophage integrates itself to the bacterial chromosome and becomes part of the host. A bacterial host with a prophage is called a lysogen and then the entire process in which a bacterium is infected by a temperate phage is called lysogeny.
As the bacterium replicates its chromosome, the phage’s DNA is also replicated and is passed on to the new daughter cells during bacterial reproduction. The presence of the phage may alter the phenotype of the bacterium since the phage virus can bring in extra genes (e.g., toxin genes that can increase bacterial virulence). This process of change in the host phenotype is called lysogenic conversion or phage conversion. There are some bacteria, Vibrio cholerae and Clostridium botulinum, which are less virulent in the absence of the prophage.
During the lysogenic cycle, the prophage will persist in the host chromosome until induction, which leads to the excision of the viral genome from the host chromosome. After induction takes place the temperate phage can proceed through a lytic cycle and then again undergo lysogeny in a newly infected cell.
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Lytic and Lysogenic Cycle Difference
There are various differences between the lytic and lysogenic cycles of bacteriophage. The below article discusses some of them
Q1: What are the Similarities Between the Lytic Cycle and the Lysogenic Cycle?
A: The lytic and the lysogenic cycle also have many similarities. These are:
Both are mechanisms of viral reproduction.
They take place within the host cell.
The cycles produce thousands of copies of the original virus.
Both lytic and lysogenic can moderate the DNA replication and the protein synthesis of the host cell.
Q2: What is a Latent Virus?
A: Not all animal viruses replicate themselves using the lytic cycle. Some viruses are capable of hiding themselves or remain dormant inside the cell in a process called latency. These viruses are called latent viruses and may cause latent infections. The viruses which are capable of latency may initially cause an acute infection before becoming dormant.
Latent viruses can remain dormant by existing as circular viral genome molecules outside of the host cell chromosome. Others become proviruses or prophages by integrating with the host genome. During dormancy, latent viruses do not cause any symptoms of disease which makes them difficult to detect. A patient may be unaware that he or she is carrying the virus unless a viral diagnostic test is carried out.
Example: Varicella zoster (the virus that causes chickenpox)
Q3: Does the Lytic Cycle Kill the Viral Host Cell?
A: Some bacteriophages can only reproduce via a lytic cycle, in which they burst and kill their host cells. Other pages like the lambda phage can alternate between a lytic and a lysogenic cycle, in which they don't kill the host cell and are instead copied along with the host DNA each time the cell undergoes division.
Q4: How is the Lysogenic Cycle Triggered?
A: In the lysogenic cycle, the bacteriophage DNA is incorporated into the host cell genome, where it is passed on to subsequent generations. Various environmental stress factors such as starvation or exposure to toxic chemicals may cause the prophage to excise and enter the lytic cycle.