Lytic Cycle – Definition, Steps, Importance, Examples

What is Lytic Cycle?

• The lytic cycle, denoted scientifically as /ˈlɪtɪk/ LIT-ik, represents one of the dual pathways of viral reproduction, specifically in relation to bacterial viruses, commonly termed bacteriophages. The counterpart to this cycle is the lysogenic cycle.
• A distinguishing feature of the lytic cycle is the eventual rupture and consequent destruction of the infected cell, including its membrane. Bacteriophages that exclusively employ the lytic cycle for reproduction are categorized as virulent phages, setting them apart from the temperate phages.
• Within the confines of the lytic cycle, the viral DNA is present as an independent molecule, freely suspended within the bacterial cell. This DNA undergoes replication independent of the host bacterial DNA.
• In contrast, during the lysogenic cycle, the viral DNA integrates itself into the host DNA. This integration versus independence of viral DNA serves as the primary distinction between the lytic and lysogenic cycles. Nevertheless, in both scenarios, the virus or phage capitalizes on the host’s DNA machinery for replication.
• The term “lytic” is derived from the process of “lysis,” which transpires when a virus, post-infecting a cell and replicating new virions, ruptures the cell membrane, leading to the release of the newly formed virions. These virions are then free to invade and infect additional cells.
• It is noteworthy that many bacteriophages, which are viruses that target bacteria, can alternate between the lytic and lysogenic cycles. Upon the virus introducing its genetic material into the host bacteria, this genetic information can embark on either the lytic or lysogenic pathway.
• In the latter, the bacteriophage’s DNA remains largely inactive. Yet, as the host bacteria undergoes division, the viral DNA is concurrently replicated. This allows the virus to perpetuate its existence within the host.
• As long as the host bacteria thrives, the viral DNA can remain in this dormant state. However, under specific conditions, a transition may occur, prompting the virus to initiate the lytic cycle.
• During the lytic phase, the host cell’s machinery is co-opted to express the viral DNA or RNA. Essentially, the viral genetic material harnesses the host cell’s proteins to facilitate its own replication and the synthesis of viral proteins.
• These newly formed proteins, along with copies of the viral DNA, amalgamate to form new virions. The host cell, now subjugated by the viral entity, eventually succumbs to the increasing internal pressure exerted by the accumulating virions, leading to its lysis and the consequent release of the virions.
• These virions then seek out new cells to infect and perpetuate the lytic cycle. However, if the environmental conditions are conducive and the host cell is actively dividing, the virus might revert to the lysogenic phase. Nevertheless, for maximal proliferation and to invade a broader cell population, the virus will predominantly favor the lytic cycle, generating vast quantities of virions in a relatively condensed timeframe.

Definition of Lytic Cycle

The lytic cycle is a phase of viral reproduction wherein a virus infects a host cell, utilizes the cell’s machinery to replicate, and subsequently causes the cell to burst (lyse), releasing new virions to infect other cells.

Bacteriophage(virus)

• A bacteriophage, commonly referred to as a phage or bacterial virus, is a specialized virus that targets and infects bacterial cells. The term “bacteriophage” is derived from the Greek words meaning “bacteria” and “to devour,” aptly describing its role in the microbial world.
• Historically, the discovery of bacteriophages can be attributed to two pioneering researchers: Frederick W. Twort from Great Britain in 1915 and Felix d’Herelle from France in 1917. Their groundbreaking work laid the foundation for subsequent studies on these unique viruses and their interactions with bacterial hosts.
• Structurally, a bacteriophage is characterized by its nucleic acid core, which can be either DNA or RNA, encapsulated within a protective protein coat. This proteinaceous structure ensures the stability and integrity of the viral genetic material. The morphological features of a typical bacteriophage include a distinct head, which houses the nucleic acid, and a tail apparatus. The tail is further segmented into various components such as the collar, sheath, baseplate, and long tail fibers. These components play crucial roles in the bacteriophage’s life cycle, particularly in recognizing and attaching to specific bacterial hosts.
• Upon encountering a suitable bacterial cell, the bacteriophage adheres to it using its tail fibers. Following attachment, the phage introduces its genetic material into the bacterial cell, initiating a series of events that can lead to the replication of the phage and eventual destruction of the host bacterium.
• In essence, bacteriophages represent a fascinating intersection of microbiology and virology, offering insights into the dynamic interactions between viruses and their bacterial hosts. Their study has not only expanded our understanding of microbial ecology but also holds promise for therapeutic applications, especially in an era of increasing antibiotic resistance.

Life cycle of bacteriophage

• The life cycle of a bacteriophage, a virus that specifically infects bacterial cells, is a complex and multifaceted process. Upon encountering a suitable bacterial host, the bacteriophage initiates its life cycle by attaching to the bacterial cell, a crucial step that facilitates the subsequent stages of infection.
• There are primarily two well-defined life cycles that a bacteriophage can follow: the lytic cycle and the lysogenic cycle. In the lytic cycle, the bacteriophage commandeers the host cell’s machinery to synthesize phage components.
• Once these components are assembled into new phage particles, the host cell is lysed, or ruptured, leading to the release of the newly formed phage particles. These particles are then free to infect other bacterial cells, perpetuating the cycle.
• Conversely, in the lysogenic cycle, the phage introduces its nucleic acid into the host cell, where it integrates with the bacterial genome without causing immediate harm. This integrated phage DNA, termed a prophage, can remain dormant within the host genome for extended periods. However, under certain conditions, the prophage can be activated, leading to the initiation of the lytic cycle and the eventual destruction of the host cell.
• In addition to these primary life cycles, bacteriophages can also exhibit alternative life cycles, such as pseudolysogeny and chronic infection. Pseudolysogeny is a temporary state that arises when a bacteriophage encounters a host cell under suboptimal growth conditions.
• This state allows the phage genome to be preserved until the host’s environmental conditions become favorable again, ensuring the phage’s survival. On the other hand, during chronic infection, the bacteriophage continuously produces new phage particles over extended durations without causing overt harm or lysis to the host cell.
• In summary, the life cycle of a bacteriophage is characterized by its adaptability and versatility, allowing it to navigate the challenges of the microbial world. Whether through the lytic or lysogenic cycle, or alternative pathways like pseudolysogeny and chronic infection, bacteriophages have evolved intricate strategies to ensure their replication and survival within their bacterial hosts.

Steps of the Lytic Cycle

The lytic cycle of a bacteriophage delineates the sequential steps that lead to the infection of a bacterial host and the subsequent production of progeny virions. This cycle is characterized by the following stages:

1. Attachment or Adsorption: The initiation of the lytic cycle begins when the bacteriophage identifies and attaches to its specific bacterial host. This attachment is facilitated by viral adhesins, primarily located on the phage’s tail, which recognize and bind to specific molecules on the bacterial surface, such as lipopolysaccharides, oligopolysaccharides, pili, and flagellar proteins. This interaction is highly specific, determining the host range of the phage. The initial contact is reversible, but it becomes irreversible when the phage’s tail pins firmly anchor to the bacterial surface.
2. Penetration: Following attachment, the bacteriophage prepares to introduce its genetic material into the bacterial cell. A hole is formed in the viral capsid, and the phage’s tubular tail penetrates the bacterial outer membrane and peptidoglycan layer. This penetration is often facilitated by a phage-encoded lysozyme. Subsequently, the tail fuses with the bacterial inner membrane, allowing the phage’s nucleic acid to be injected into the bacterial cytoplasm, reminiscent of a syringe’s action. This step requires energy, typically derived from ATP. The viral protein coat remains outside the bacterial cell, termed a “ghost.”
3. Biosynthesis of Phage Components: Once inside, the phage takes control of the host’s cellular machinery. Host functions, including DNA replication and protein synthesis, are rapidly inhibited. Early viral gene products, including nucleases, degrade the host DNA and modify host RNA polymerases to favor phage production. Subsequent gene products, synthesized from delayed early genes, facilitate the production of phage-specific nucleic acids and proteins. Late genes direct the synthesis of structural components, such as the phage head, tail, and associated fibers.
4. Maturation or Assembly: As the components are synthesized, they are assembled to form mature phage particles. The assembly process begins with the formation of the phage head, which encapsulates the viral nucleic acid. This is followed by the attachment of the tail and, finally, the tail fibers, culminating in the formation of a complete virion.
5. Lysis and Release: Approximately 25 minutes post-infection, the bacterial cell is filled with newly assembled phage particles. A phage-encoded protein creates lesions in the bacterial cell membrane, while another protein, endolysin, weakens the cell wall. This results in the lysis of the bacterial cell, releasing the progeny phage particles. These virions are then free to infect new bacterial hosts, perpetuating the lytic cycle.

Importance of Lytic Cycle

The lytic cycle is a fundamental aspect of viral biology, particularly in the context of bacteriophages, which are viruses that infect bacteria. The importance of the lytic cycle can be understood from multiple perspectives, both in terms of basic biology and potential applications:

1. Viral Reproduction: The primary objective of any virus is to reproduce and propagate. The lytic cycle ensures rapid replication of the virus, leading to the production of numerous progeny virions that can go on to infect other cells.
2. Ecological Balance: Bacteriophages play a crucial role in regulating bacterial populations in various ecosystems. By lysing bacterial cells, phages help control bacterial overgrowth, thereby maintaining ecological balance.
3. Genetic Transfer: While the lytic cycle primarily results in the destruction of the host cell, the process can occasionally lead to genetic exchanges. Fragments of bacterial DNA can be inadvertently packaged into phage particles, which can then be transferred to other bacteria during subsequent infections, a phenomenon known as transduction.
4. Research Tool: The lytic cycle has been instrumental in advancing molecular biology research. Studies on the lytic cycle have provided insights into DNA replication, transcription, and protein synthesis, serving as a model system for understanding these fundamental biological processes.
5. Therapeutic Potential: Given the specificity of bacteriophages in targeting certain bacterial strains, there’s growing interest in using phages as alternatives to antibiotics, especially in the face of rising antibiotic resistance. The lytic cycle’s ability to destroy bacterial cells can be harnessed for therapeutic purposes, a strategy known as phage therapy.
6. Biotechnological Applications: Bacteriophages and their lytic enzymes have potential applications in biotechnology. For instance, phage lytic enzymes can be used in food safety to target and eliminate specific bacterial pathogens.
7. Evolutionary Implications: The interactions between bacteriophages and their bacterial hosts during the lytic cycle drive evolutionary dynamics. Bacteria develop defense mechanisms to evade phage infection, and in response, phages evolve counter-strategies to overcome these defenses. This evolutionary “arms race” has implications for microbial diversity and evolution.

Examples of the lytic cycle

Examples of the lytic cycle can be observed in various bacteriophages (viruses that infect bacteria) and some animal viruses. Here are some specific examples:

1. T4 Bacteriophage Infecting E. coli: The T4 bacteriophage attaches to an E. coli bacterium, injects its DNA, takes over the bacterial machinery to produce new phage particles, and eventually lyses the bacterium to release the progeny phages.
2. Lambda Bacteriophage (under certain conditions): While the lambda bacteriophage typically undergoes the lysogenic cycle in E. coli, it can be induced to enter the lytic cycle, leading to the production of new phages and lysis of the bacterial cell.
3. Phi X174 Bacteriophage Infecting E. coli: This bacteriophage follows the lytic cycle to reproduce within E. coli cells.
4. Herpes Simplex Virus (HSV): Although HSV can establish latent infections (akin to the lysogenic cycle), during its lytic phase, it actively replicates in epithelial cells, leading to cell lysis and the formation of cold sores or genital lesions.
5. Influenza Virus: After infecting host respiratory cells, the influenza virus replicates and produces new virions that bud off from the host cell, eventually leading to cell death.
6. Rabies Virus: Upon infecting nerve cells, the rabies virus replicates and leads to cell lysis, contributing to the severe neurological symptoms associated with the disease.

Quiz

What is the first step of the lytic cycle?
a) Penetration
b) Biosynthesis
c) Attachment
d) Release

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Which of the following stages involves the virus injecting its genetic material into the host cell?
a) Maturation
b) Attachment
c) Penetration
d) Lysis

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During which stage of the lytic cycle are new viral components synthesized?
a) Attachment
b) Maturation
c) Biosynthesis
d) Release

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What happens during the final stage of the lytic cycle?
a) Virus attaches to the host cell
b) Viral DNA integrates into host DNA
c) New virions are assembled
d) Host cell bursts and releases new virions

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Which cycle results in the immediate destruction of the host cell?
a) Lysogenic cycle
b) Lytic cycle
c) Replication cycle
d) Integration cycle

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Which enzyme is often used by bacteriophages to penetrate the bacterial cell wall during the lytic cycle?
a) DNA polymerase
b) RNA polymerase
c) Lysozyme
d) Ligase

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Which of the following is NOT a stage of the lytic cycle?
a) Integration
b) Attachment
c) Penetration
d) Biosynthesis

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In which stage of the lytic cycle are the viral components assembled into complete virions?
a) Attachment
b) Penetration
c) Biosynthesis
d) Maturation

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What is the primary objective of a virus during the lytic cycle?
a) To integrate its DNA into the host’s genome
b) To remain dormant within the host cell
c) To replicate and produce new virions
d) To promote the growth of the host cell

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Which of the following viruses is known to undergo the lytic cycle?
a) HIV
b) Lambda bacteriophage (under certain conditions)
c) Epstein-Barr virus during latency
d) Human papillomavirus during integration

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FAQ

References

1. https://biologydictionary.net/lytic-cycle/
2. https://thebiotechnotes.com/2019/05/01/bacteriophage-reproduction-lytic-cycle/
3. https://www.geeksforgeeks.org/lytic-cycle/
4. http://www.biologyaspoetry.com/terms/lytic_cycle.html
5. https://courses.lumenlearning.com/suny-microbiology/chapter/the-viral-life-cycle/
6. https://aklectures.com/lecture/bacterial-envelope-and-viruses/viruses-lytic-cycle-and-lysogenic-cycle
7. https://collegedunia.com/exams/difference-between-lytic-and-lysogenic-cycle-bacteriophage-life-cycle-biology-articleid-2466