4.1 Overview of lytic and lysogenic cycles
3 min read•august 1, 2024
Viruses have two main ways to replicate: lytic and lysogenic cycles. The destroys host cells quickly, releasing new viruses. The lets viruses hide in host cells, sometimes for years. These strategies help viruses survive and spread.
Understanding these cycles is key to grasping how viruses work. Lytic cycles cause quick infections, while lysogenic cycles lead to long-term persistence. Viruses can switch between these methods, adapting to different conditions and shaping their evolution alongside hosts.
Lytic vs Lysogenic Replication
Fundamental Differences
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- Lytic cycles destroy host cells immediately while lysogenic cycles allow viruses to remain dormant within host cells
- Viral genes express immediately upon infection in lytic cycles whereas most viral genes repress in lysogenic cycles
- Lytic cycles produce new virions rapidly while lysogenic cycles integrate viral DNA into host genomes for long-term persistence
- Acute infections typically associate with lytic cycles while chronic or latent infections often link to lysogenic cycles
- Lytic cycles become irreversible once initiated whereas lysogenic cycles can switch to lytic under certain conditions (environmental stressors)
Comparative Outcomes
- Host cell destruction occurs in lytic cycles releasing numerous new viral particles capable of infecting other cells
- Host cell survival and continued division happens in lysogenic cycles with viral genome as part of host chromosome
- Rapid viral proliferation and spread results from lytic cycles potentially causing acute symptoms in host organisms (high fever)
- Long-term viral persistence within host populations enables through lysogenic cycles often without causing immediate symptoms
- Lysogenic conversion confers beneficial traits to host cells in some lysogenic cycles (increased antibiotic resistance in bacteria)
Stages of Viral Replication
Lytic Cycle Stages
- binds viral surface proteins to specific receptors on host cells (influenza virus binding to sialic acid receptors)
- introduces viral genetic material into host cells (endocytosis or membrane fusion)
- replicates viral genetic material and synthesizes viral proteins (using host cell machinery)
- assembles new viral particles (capsid formation and genome packaging)
- allows newly formed virions to exit host cells often through cell lysis (bursting of bacterial cells in infections)
Lysogenic Cycle Stages
- Attachment and penetration occur similarly to lytic cycle (adsorption of bacteriophage to bacterial cell wall)
- incorporates viral DNA into host cell's genome (formation of in bacterial chromosomes)
- Replication of viral genome happens passively along with host cell division ( to daughter cells)
- triggers switch from lysogenic to lytic cycle through environmental factors (UV radiation exposure)
Outcomes of Viral Replication
Host Cell Impacts
- Lytic cycle destroys host cells releasing numerous new viral particles (cell death in influenza infections)
- Lysogenic cycle allows host cell survival and continued division (Epstein-Barr virus in B lymphocytes)
- Rapid viral proliferation from lytic cycles potentially causes acute symptoms (sudden onset of flu-like symptoms)
- Long-term viral persistence enabled by lysogenic cycles often without immediate symptoms (latent herpesvirus infections)
- Lysogenic conversion can confer beneficial traits to host cells (increased virulence in Corynebacterium diphtheriae)
Viral Population Dynamics
- Lytic cycles result in rapid viral proliferation and spread (exponential growth of bacteriophage populations)
- Lysogenic cycles enable long-term viral persistence within host populations (maintenance of HIV reservoirs)
- Switch from lysogenic to lytic cycle triggered by various factors (stress-induced reactivation of herpes simplex virus)
- Balance between lytic and lysogenic strategies influences virus-host coevolution (temperate phage interactions with bacterial hosts)
Evolutionary Significance of Viral Replication Strategies
Adaptive Flexibility
- Switching between lytic and lysogenic cycles provides viruses with adaptive flexibility in different environmental conditions
- Lysogeny allows viruses to persist during low host density or unfavorable transmission conditions (bacteriophage survival in nutrient-poor environments)
- Lytic replication enables rapid viral population growth in favorable conditions maximizing transmission opportunities (outbreak situations)
- Altering replication strategy based on multiplicity of infection optimizes viral survival and spread (bacteriophage decision-making in high vs. low host density)
Ecological and Evolutionary Impacts
- Lysogenic cycle facilitates horizontal gene transfer between bacterial species contributing to bacterial evolution and adaptation (acquisition of antibiotic resistance genes)
- Existence of both strategies allows viruses to exploit different ecological niches and host types (broad host range of some bacteriophages)
- Balance between lytic and lysogenic strategies influences dynamics of viral epidemics (seasonal patterns of influenza outbreaks)
- Virus-host coevolution shaped by interplay between replication strategies (arms race between bacterial CRISPR systems and phage anti-CRISPR mechanisms)
Key Terms to Review (21)
Attachment: Attachment refers to the initial binding of a virus to a host cell, a crucial first step in the viral infection process. This process is facilitated by specific interactions between viral proteins and host cell receptors, which determine the virus's ability to infect and replicate within the host.
Bacteriophage: A bacteriophage is a type of virus that specifically infects and replicates within bacteria. These viruses are characterized by their unique structure, typically consisting of a protein coat encasing genetic material, and their ability to hijack bacterial cellular machinery to reproduce. Bacteriophages can play a significant role in understanding viral characteristics, the mechanisms of viral life cycles, and the potential for using these viruses in medical treatments.
Biosynthesis: Biosynthesis refers to the process by which living organisms produce complex molecules from simpler ones, often involving the synthesis of proteins and nucleic acids. This process is critical for the propagation of viruses, as they rely on the host's cellular machinery to replicate their genomes and synthesize viral proteins, enabling the virus to assemble and spread. Understanding biosynthesis is essential to grasp how viruses interact with host cells during both lytic and lysogenic cycles.
DNA Virus: A DNA virus is a type of virus that has its genetic material composed of deoxyribonucleic acid (DNA), which can be single-stranded or double-stranded. These viruses rely on the host cell's machinery to replicate and produce new virions. DNA viruses have specific characteristics that distinguish them from RNA viruses, including their stability and mechanisms of replication, which can impact their life cycles and interactions with the host.
Dormancy: Dormancy refers to a state in which a virus remains inactive or latent within a host cell, avoiding replication and evading the host's immune response. This period of inactivity can last for an extended duration, allowing the virus to persist within the host without causing disease, and may reactivate under certain conditions, leading to viral replication and potential symptoms. Understanding dormancy is crucial in the context of viral life cycles, particularly in how viruses can establish long-term relationships with their hosts.
Horizontal Transmission: Horizontal transmission refers to the spread of viruses from one individual to another within the same generation, typically through direct contact, airborne particles, or vectors. This type of transmission is crucial for understanding how viruses propagate among populations, impacting their lifecycle and infection dynamics, particularly during the lytic and lysogenic cycles, the spread of animal viruses, and various patterns of viral disease transmission.
Host cell lysis: Host cell lysis refers to the process where a virus or other pathogen causes the destruction of a host cell, releasing new viral particles into the environment. This process is a key event in the lytic cycle of viral replication, where the virus takes over the host's cellular machinery to reproduce itself, ultimately leading to the death of the host cell. The resulting release of viral particles allows for further infection of neighboring cells.
Immediate replication: Immediate replication refers to the process by which certain viruses, particularly those that utilize the lytic cycle, quickly replicate their genetic material and produce new viral particles immediately after infecting a host cell. This rapid replication is a hallmark of the lytic cycle, where the virus takes over the host's cellular machinery to produce many copies of itself, leading to cell lysis and the release of new virions. Understanding this concept is crucial to distinguishing it from other viral life strategies, such as the lysogenic cycle, where replication may be delayed.
Induction: Induction refers to the process through which a virus is triggered to switch from a latent or lysogenic state to an active lytic state. This phenomenon can be prompted by various factors such as environmental stress, DNA damage, or specific signals from the host cell. Understanding induction is crucial as it highlights the dynamic relationship between viruses and their hosts, showing how viruses can adapt and respond to changes in their environment.
Integration: Integration is the process by which viral DNA or RNA is incorporated into the host cell's genome, allowing the virus to persist within the host and utilize the host's cellular machinery for replication and gene expression. This mechanism is crucial for certain viruses, particularly retroviruses, as it facilitates the long-term survival of the viral genome within the host and influences how the virus can reactivate or cause disease.
Lysogenic cycle: The lysogenic cycle is a method of viral reproduction in which the viral genome integrates into the host cell's DNA, allowing the virus to replicate along with the host cell without immediately causing cell death. This cycle enables the virus to persist in a dormant state, becoming a part of the host's genetic material and can later switch to the lytic cycle, where it actively produces new viruses and destroys the host cell.
Lytic Cycle: The lytic cycle is a viral replication process in which a virus infects a host cell, hijacks the cell's machinery to produce new viral particles, and ultimately leads to the destruction of the host cell. This cycle results in the release of newly formed virions, which can go on to infect additional cells, making it a crucial aspect of viral propagation.
Maturation: Maturation is the final stage of viral replication where newly formed viral particles undergo structural modifications and become infectious. This process involves the assembly of viral proteins and genomic material into complete virions, which are then released from the host cell. Understanding maturation is crucial as it directly influences the efficiency of viral propagation and the potential for infection in new host cells.
Penetration: Penetration refers to the process by which a virus enters a host cell after the initial attachment. This step is crucial for viral infection and can involve various mechanisms, including direct fusion with the host cell membrane or endocytosis. Understanding penetration is key to grasping how viruses exploit host cellular machinery to replicate and propagate.
Prophage: A prophage is a form of a bacteriophage, which is a virus that infects bacteria, that has integrated its viral DNA into the bacterial genome. This integration allows the viral DNA to be replicated along with the bacterial DNA during cell division, meaning the virus can remain dormant within the host cell for extended periods. When environmental conditions are favorable or stressors are present, the prophage can reactivate, leading to the production of new virus particles and the lytic cycle.
Release: Release refers to the final stage in the viral life cycle where newly formed viral particles exit the host cell to infect other cells. This process is crucial as it determines how effectively a virus can spread and propagate its genetic material, impacting the overall infection process and the host's immune response.
Replication: Replication refers to the process by which viruses reproduce and make copies of their genetic material inside a host cell. This process is crucial for viral survival and propagation, as it enables the virus to hijack the host's cellular machinery to create new viral particles, allowing for further infection and spread.
Retrovirus: A retrovirus is a type of RNA virus that uses reverse transcription to convert its RNA genome into DNA after entering a host cell. This unique process allows retroviruses to integrate their genetic material into the host's genome, enabling them to replicate and persist within the host. Retroviruses are known for their association with various diseases, including certain types of cancer and acquired immunodeficiency syndrome (AIDS).
RNA Virus: An RNA virus is a type of virus that uses ribonucleic acid (RNA) as its genetic material, which can either be single-stranded or double-stranded. These viruses rely on host cells to replicate and often have high mutation rates, leading to rapid evolution. This unique characteristic plays a critical role in their interaction with host organisms and their replication cycles, influencing how they can cause infections and diseases.
Vertical Transmission: Vertical transmission refers to the transfer of pathogens from a parent organism to its offspring, typically occurring during pregnancy, childbirth, or breastfeeding. This mode of transmission is crucial in understanding how certain viruses can perpetuate within populations, as it allows for the direct passage of viral genetic material from one generation to the next.
Viral latency: Viral latency is a state in which a virus remains inactive within a host's cells after initial infection, allowing it to evade the immune response and potentially reactivate later. This characteristic enables viruses to persist in the host for long periods, influencing disease progression and transmission dynamics.