[Biology Class Notes] on Difference Between Lytic and Lysogenic Cycle Pdf

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. 

For reproduction and metabolic functions, all viruses rely on cells. Viruses do not encode the enzymes required for viral replication on their own. A virus, on the other hand, may seize cellular machinery to manufacture new viral particles within a host cell. Bacteriophages can only multiply in the cytoplasm since prokaryotic organisms lack a nucleus and organelles. Most DNA viruses can reproduce inside the nucleus in eukaryotic cells, with the exception of big DNA viruses like poxviruses, which may replicate in the cytoplasm. Infectious RNA viruses frequently multiply in the cytoplasm of animal cells.

Because comparable mechanisms have been found for viruses, which can induce rapid cell death or create a latent or persistent infection, the life cycle of bacteriophages has been a helpful model for understanding how viruses influence the cells they infect. Virulent phages usually cause cell lysis, which results in cell death. Temperate phages, on the other hand, may integrate into a host chromosome and proliferate alongside the cell genome until they are triggered to produce new viruses or offspring viruses. 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 the virus is given below. 

What is the Lytic Cycle?

The lytic cycle starts with a virulent phage 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 bacteriophages.

Some of them are discussed below.

Lytic vs Lysogenic Cycle

 

In this article, we have learnt about both the lytic and lysogenic cycles and also the differences between them.