Viruses, Viroids, and Bacteriophage Lifecycles

General Characteristics of Viruses

Viruses are unique infectious agents that differ fundamentally from cellular life forms. They are obligate intracellular parasites, meaning they require a host cell to replicate and cannot carry out metabolic processes independently.

• Obligate intracellular parasites: Viruses infect host cells and often cause harm, leading to disease in susceptible organisms.

• Simplicity compared to cells: Viruses lack cellular structures such as cytoplasmic membranes, cytosol, and organelles.

• Genetic material: Viruses contain either DNA or RNA, but never both.

• No metabolic activity: Viruses do not possess ribosomes or enzymes for metabolism; they rely entirely on host cell machinery for replication.

• Dependence on host: All viral replication and synthesis of viral components occur using the host cell's enzymes and metabolic pathways.

Structure of Viruses

Viruses exist in two main states: the extracellular state (virion) and the intracellular state.

• Virion (Extracellular State):

  • Nucleic acid core: Contains viral genetic material (DNA or RNA).

  • Capsid: Protein coat that surrounds and protects the nucleic acid.

  • Envelope: Some viruses possess a lipid envelope surrounding the capsid, derived from the host cell membrane during viral release.

  • Recognition elements: Glycoproteins or other molecules embedded in the capsid or envelope facilitate attachment to host cells.

  • Accessory proteins: Some virions carry enzymes such as lysozyme (in bacteriophages) or viral polymerases.

• Intracellular State:

  • Capsid and envelope are removed; the virus exists as nucleic acid within the host cell.

Classification of Viruses

Viruses are classified based on several structural and functional criteria.

• Capsid shape: The capsid can be helical, icosahedral, or complex in structure.

• Type of nucleic acid: Viruses may have single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), single-stranded RNA (ssRNA), or double-stranded RNA (dsRNA).

• Envelope presence: Viruses are categorized as enveloped or naked (non-enveloped).

• Host range:

  • Species specific: Infect only certain species.

  • Strain specific: Some bacteriophages infect only specific bacterial strains.

  • Cell specific: Animal viruses may target specific cell types.

Examples: Influenza virus (enveloped, ssRNA), Adenovirus (naked, dsDNA), Bacteriophage T4 (complex capsid, dsDNA).

Viral Genomes

Viral genomes exhibit greater diversity than those of cellular organisms.

• Forms: Linear or circular, single or segmented molecules.

• Types: ssDNA, dsDNA, ssRNA (positive-sense or negative-sense), dsRNA.

• Size: Generally much smaller than cellular genomes.

• Unique features: Some genome types (e.g., ssDNA, dsRNA) are rare or absent in cellular life.

Viral Envelope

The viral envelope is a lipid bilayer derived from the host cell membrane, present in some viruses.

• Composition: Phospholipid bilayer with embedded proteins (glycoproteins encoded by the virus and host proteins).

• Function: Facilitates recognition and attachment to host cells; provides some protection from the immune system.

• Stability: Enveloped viruses are less stable outside the host and more susceptible to detergents and desiccation than naked viruses.

Host Specificity

Viruses exhibit specificity for their hosts, determined by interactions between viral attachment molecules and host cell surface proteins.

• Species specificity: Infect only certain species.

• Strain specificity: Some bacteriophages infect only specific bacterial strains.

• Cell type specificity: Animal viruses may target specific cell types within a host.

• Generalists: Some viruses can infect multiple species or cell types.

Bacteriophage Lifecycles

Bacteriophages (viruses that infect bacteria) can replicate via two main cycles: the lytic cycle and the lysogenic cycle.

Lytic Cycle

The lytic cycle results in the rapid production of new virions and destruction of the host cell.

1. Attachment (Adsorption): Tail fibers recognize and bind to specific receptors on the bacterial cell surface.

2. Penetration: The tail core is inserted into the cell, and viral nucleic acid is injected into the cytoplasm. Lysozyme degrades the bacterial cell wall to facilitate entry.

3. Synthesis: Host DNA is degraded, and the host's protein synthesis machinery is redirected to produce viral components.

4. Assembly (Maturation): Viral nucleic acids and proteins are assembled into complete virions.

5. Release: Host cell lyses (bursts), releasing new virions. Lysozyme and internal pressure contribute to cell lysis.

Burst size: The number of virions released per lysed cell.

Lysogenic Cycle

The lysogenic cycle allows the viral genome to integrate into the host genome and be replicated along with it, without immediate destruction of the host cell.

1. Attachment and Penetration: Similar to the lytic cycle, but host DNA is not degraded.

2. Integration: Viral genetic material integrates into the host chromosome, becoming a prophage.

3. Replication: The host cell divides normally, replicating the prophage DNA along with its own genome. All daughter cells carry the prophage.

4. Lysogenic conversion: The prophage may carry genes that alter the phenotype of the host bacterium (e.g., toxin production).

5. Induction: Environmental triggers can cause the prophage to excise from the host genome and enter the lytic cycle.

Comparison of Lytic and Lysogenic Cycles

Example: Bacteriophage lambda can undergo both lytic and lysogenic cycles in Escherichia coli.

Additional info: Lysogenic conversion is responsible for the acquisition of new traits in bacteria, such as toxin production in Corynebacterium diphtheriae and Vibrio cholerae.