13.2 Viral Structure, Replication, and Life Cycles
3 min read•august 7, 2024
Viruses are fascinating microorganisms that blur the line between living and non-living. They're incredibly diverse, with structures ranging from simple protein shells to complex enveloped particles. Understanding their structure is key to grasping how they infect and replicate.
Viral replication cycles are a crucial part of their life cycle. Whether lytic or lysogenic, these processes showcase how viruses hijack host cells to reproduce. This topic ties into the broader chapter by highlighting the unique characteristics of viruses compared to other microbes.
Viral Structure
Components of a Virus
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- consists of protein subunits called capsomeres that protect the (DNA or RNA)
- surrounds the capsid in some viruses, composed of a lipid bilayer with embedded glycoproteins that help the virus attach to and enter host cells (influenza virus)
- Viral genome contains the genetic material (DNA or RNA) that carries the information needed for viral replication
Variations in Viral Structure
- Viruses can be enveloped or non-enveloped, with enveloped viruses having an additional lipid bilayer surrounding the capsid (HIV)
- Capsid shape varies among viruses, with common shapes including icosahedral (poliovirus), helical (tobacco mosaic virus), and complex ( T4)
- Some viruses have additional structures, such as the matrix protein layer between the envelope and capsid in influenza viruses or the tail fibers and base plate in bacteriophages (T4 phage)
Viral Replication Cycles
Lytic Cycle
- is a type of viral replication that results in the destruction of the host cell
- Virus attaches to and enters the host cell, releasing its genetic material into the cytoplasm
- Viral genes are expressed, leading to the synthesis of viral proteins and replication of the viral genome
- New viruses are assembled and released from the host cell through lysis, causing cell death (bacteriophage T4 in E. coli)
Lysogenic Cycle
- is a type of viral replication in which the viral genome integrates into the host cell's genome, becoming a provirus
- Provirus is replicated along with the host cell's DNA during cell division, allowing the virus to persist in a dormant state
- Environmental factors or cellular signals can trigger the provirus to enter the lytic cycle, leading to the production and release of new viruses (lambda phage in E. coli)
Viral Assembly and Release
- Viral assembly involves the packaging of the replicated viral genome into new capsids
- Assembly can occur in the nucleus (herpesviruses) or cytoplasm (picornaviruses) of the host cell, depending on the virus
- Enveloped viruses acquire their envelope by budding through the host cell membrane, incorporating host cell lipids and viral glycoproteins (influenza virus)
- Release of new viruses occurs through lysis (non-enveloped viruses) or budding (enveloped viruses), with budding allowing the host cell to survive
Types of Viruses
Retroviruses
- Retroviruses are RNA viruses that use the enzyme reverse transcriptase to convert their RNA genome into DNA upon entering the host cell (HIV)
- Retroviral DNA integrates into the host cell's genome, becoming a provirus that can be transcribed to produce new viral RNA and proteins
- Retroviruses are known for their ability to establish chronic infections and cause diseases such as AIDS (HIV) and some types of cancer (human T-cell leukemia virus)
Bacteriophages
- Bacteriophages, or phages, are viruses that infect bacteria (T4 phage, lambda phage)
- Phages can undergo both lytic and lysogenic cycles, depending on the specific virus and host conditions
- Lytic phages (T4) quickly replicate and lyse the bacterial cell, while temperate phages (lambda) can integrate their genome into the bacterial chromosome as a prophage
- Phages have been used in research to study gene regulation, DNA replication, and other cellular processes, as well as in phage therapy to treat bacterial infections
Key Terms to Review (18)
Acute infection: An acute infection is a rapid onset infection that usually lasts for a short duration and is often severe in nature. These infections typically occur when a pathogen, such as a virus or bacteria, enters the body, multiplies quickly, and causes noticeable symptoms. Understanding acute infections is crucial for grasping how viruses replicate and spread during their life cycles.
Attachment: In the context of viruses, attachment refers to the initial step in the viral infection process where a virus binds to a specific host cell. This binding is crucial as it determines the virus's ability to enter the cell and initiate replication. The interaction between viral surface proteins and specific receptors on the host cell is essential for this process, ultimately influencing the virus's ability to infect and propagate within the host organism.
Bacteriophage: A bacteriophage, or phage, is a type of virus that specifically infects bacteria. These viruses consist of genetic material encased in a protein coat and can hijack the bacterial machinery to replicate themselves. Understanding bacteriophages is crucial for grasping viral structure, replication processes, and their unique life cycles, especially since they can lead to the destruction of bacterial cells or the incorporation of viral genes into bacterial genomes.
Capsid: A capsid is the protein shell that encases and protects the genetic material of a virus. This structure is crucial for the virus's ability to infect host cells, as it facilitates attachment and entry into the host. Capsids can have different shapes, which play a role in how the virus interacts with its environment and host organisms.
Chronic infection: A chronic infection is a long-lasting infection that persists over an extended period, often with ongoing symptoms or recurrence after periods of remission. This type of infection can be caused by various pathogens, including viruses, which can remain in the host and continue to replicate without causing immediate or severe illness. Understanding chronic infections is crucial for comprehending how certain viruses can evade the immune system and lead to long-term health issues.
Electron microscopy: Electron microscopy is a powerful imaging technique that uses electron beams to create high-resolution images of specimens at a molecular or atomic level. This method allows scientists to visualize structures that are too small to be seen with traditional light microscopy, making it especially valuable for studying the intricate details of viruses and their interactions during replication and life cycles.
Endocytosis: Endocytosis is a cellular process in which substances are brought into the cell by engulfing them with the cell membrane, forming vesicles that transport materials into the cytoplasm. This process is crucial for nutrient uptake, cell signaling, and immune responses, linking to various cellular functions and interactions.
Envelope: In virology, an envelope is a lipid membrane that surrounds some types of viruses, derived from the host cell's membrane during the viral replication process. This envelope plays a crucial role in protecting the viral genetic material and facilitates the entry of the virus into host cells by merging with their membranes. The presence or absence of an envelope significantly influences a virus's stability, transmission methods, and immune evasion strategies.
Fusion: Fusion is the process by which two or more entities combine to form a single, more complex entity. In the context of viruses, fusion refers specifically to the merging of the viral envelope with the host cell membrane, allowing the viral genetic material to enter the host cell. This process is crucial for viral entry and subsequently initiates the viral replication cycle, leading to the infection of the host organism.
Host range: Host range refers to the variety of host organisms that a particular virus can infect and replicate within. This concept is crucial in understanding how viruses interact with different biological systems, as it highlights the specificity of viral infection and the potential for cross-species transmission. The host range can vary significantly among viruses, influencing their epidemiology, pathogenicity, and the strategies needed for prevention and treatment.
Lysogenic cycle: The lysogenic cycle is a method of viral replication in which a virus integrates its genetic material into the host cell's DNA, allowing it to be replicated along with the host's genome without immediately destroying the host. This cycle can remain dormant for extended periods, with the viral DNA, called a prophage, being passed on to daughter cells during cell division. The lysogenic cycle is distinct from the lytic cycle, where the virus replicates and causes immediate cell death.
Lytic cycle: The lytic cycle is a viral replication process where a virus infects a host cell, takes over its machinery, and ultimately causes the host cell to burst, releasing new virus particles. This cycle leads to the rapid production of viruses and results in cell death, which can have significant implications for the health of the host organism and the spread of the virus.
Penetration: Penetration refers to the process by which a virus enters a host cell, allowing it to initiate infection and replication. This critical step is essential for viral life cycles, as it determines how effectively a virus can hijack cellular machinery to reproduce and spread within the host. Penetration can occur through various mechanisms, including direct fusion with the host cell membrane or via endocytosis, depending on the virus's structure and type.
Retrovirus: A retrovirus is a type of virus that uses reverse transcription to convert its RNA genome into DNA after infecting a host cell. This unique ability allows the virus to integrate its genetic material into the host's genome, leading to a life cycle that can result in persistent infections. Retroviruses are known for their role in various diseases, particularly HIV/AIDS, which highlights their impact on human health.
Reverse transcription: Reverse transcription is the process by which RNA is converted back into DNA, typically using the enzyme reverse transcriptase. This process is crucial for retroviruses, allowing them to integrate their genetic material into the host's genome, effectively altering the host cell's biology. It plays a pivotal role in viral replication and contributes to the understanding of how viruses can hijack cellular machinery.
Viral genome: A viral genome is the complete genetic material of a virus, consisting of either DNA or RNA that encodes the information necessary for the virus to replicate and produce new virus particles. This genetic material is crucial as it dictates the virus's structure, replication strategy, and how it interacts with host cells. Understanding the viral genome is essential for studying viral structure, replication mechanisms, and the various life cycles that viruses undergo to propagate.
Viral plaque assay: A viral plaque assay is a laboratory technique used to quantify the number of viral particles in a sample by observing the formation of plaques, which are areas of cell lysis caused by viral infection. This method relies on the ability of a virus to infect and kill host cells, resulting in visible clear zones or plaques on a layer of cells in culture. The size and number of plaques provide crucial information about the virus's infectivity and titer.
Viral specificity: Viral specificity refers to the preference of a virus to infect specific types of cells or organisms, determined by the interactions between viral surface proteins and host cell receptors. This specificity is crucial because it dictates how a virus enters a host and what type of diseases it can cause. Understanding viral specificity is important for developing vaccines and treatments, as different viruses can target distinct cellular pathways and processes.