What Are the Main Steps in the Lytic and Lysogenic Cycles?
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.
(Image will be uploaded soon)
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.
(Image will be uploaded soon)
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.
FAQs on Lytic vs Lysogenic Cycle: Complete Comparison for Students
1. What is the main difference between the lytic and lysogenic cycles of a virus?
The primary difference lies in the immediate outcome for the host cell. In the lytic cycle, the virus actively replicates and assembles new virions, leading to the rapid lysis (bursting) and death of the host cell. In contrast, the lysogenic cycle involves the integration of the viral DNA into the host's chromosome, where it remains dormant as a prophage and is replicated along with the host cell, without causing immediate harm.
2. What are the key stages of the lytic cycle?
The lytic cycle is a multi-step process that results in the destruction of the host cell. The main stages are:
Attachment: The virus binds to specific receptors on the surface of the host cell.
Penetration: The virus injects its genetic material (DNA or RNA) into the host cell's cytoplasm.
Biosynthesis: The viral genome hijacks the host's cellular machinery to replicate its own genetic material and synthesise viral proteins.
Maturation: Newly synthesised viral components are assembled into complete, new virus particles (virions).
Lysis: The host cell is broken open, releasing hundreds of new virions to infect neighbouring cells.
3. What happens to the viral DNA during the lysogenic cycle?
During the lysogenic cycle, the viral DNA does not immediately start replicating. Instead, it integrates itself directly into the host cell's chromosome. In this integrated state, the viral DNA is referred to as a prophage (in bacteriophages) or a provirus. It remains latent, posing no immediate threat to the cell. As the host cell divides, this integrated viral DNA is passively replicated and passed on to all daughter cells.
4. What environmental factors can trigger a virus to switch from the lysogenic to the lytic cycle?
The switch from the dormant lysogenic state to the active lytic cycle is often triggered by environmental stressors that signal the host cell is damaged or unlikely to survive. This induction prompts the virus to escape. Key triggers include:
Exposure to UV radiation or ionising radiation.
The presence of certain toxic chemicals or mutagens.
Nutrient deprivation or starvation conditions for the host cell.
These factors can cause the prophage to excise itself from the host chromosome and begin the lytic replication process.
5. How does the concept of a 'provirus' or 'prophage' relate to the lysogenic cycle?
A prophage (in bacteria) or provirus (in eukaryotes) is the central feature of the lysogenic cycle. It is the term for the viral genetic material once it has been integrated into the host's chromosomal DNA. This state allows the virus to remain dormant and hidden, effectively becoming part of the host's genome. The existence of the virus as a prophage is what enables it to be passively replicated and spread to subsequent generations of host cells without producing new virions or causing disease symptoms.
6. Why is the lysogenic cycle often considered a more 'stealthy' survival strategy for a virus?
The lysogenic cycle is considered a stealth strategy because it allows the virus to multiply without killing its host. This provides a significant long-term advantage by:
Ensuring Host Survival: By keeping the host cell alive, the virus ensures its own genetic material is propagated every time the host cell divides.
Evading Immune Detection: While dormant as a prophage, the virus does not produce viral proteins that would trigger an immune response from the host.
Persistence: It allows the virus to persist in a host population during unfavourable conditions, waiting for an opportune moment to enter the lytic cycle and spread aggressively.
7. Can a single type of virus, like the bacteriophage lambda, exhibit both lytic and lysogenic cycles?
Yes, many viruses, known as temperate phages, can switch between both cycles. The classic example is the bacteriophage lambda, which infects E. coli bacteria. This ability to choose a pathway is a key survival mechanism. The 'decision' is often based on the environmental conditions and the health of the host cell population. If host cells are abundant and healthy, the lytic cycle is favoured for rapid multiplication. If host cells are scarce or stressed, the lysogenic cycle is favoured for long-term persistence.
