Viruses

Viral Life Cycle Overview

  • The viral life cycle is fundamental to viral replication, irrespective of the virus type or host.
  • There are common steps all viruses must undertake: attachment to the host cell, entry into the host cell, uncoating, synthesis of viral components, assembly of new virions, and exiting the host cell.

Key Steps in Viral Life Cycle

1. Attachment or Absorption
  • Adherence to Host Cell: The initial binding of the virus to the host is referred to as attachment or absorption. This is facilitated through viral receptors binding to specific cellular receptors on the host cell.
  • Types of Viruses:
    • Enveloped Viruses: Utilize proteins in their viral envelope for receptor attachment.
    • Naked Viruses: Use capsid proteins for adherence.
  • Receptor Specificity: The receptors are not specifically for viruses but are utilized by the virus for attachment and entry. Virus binding is often determined by tissue tropism, where a virus may prefer certain tissues due to receptor distribution.
    • Example: Poliovirus binds to receptors in the nasopharynx, spinal cord, and gut. Mild infections can lead to a respiratory illness, while severe infections can lead to paralysis due to spinal cord receptor binding.
  • Species Hosting: Viruses such as rabies infect multiple species by utilizing common receptors found across different organisms.
  • Multiple Receptors: Some viruses, like HIV, can use multiple receptors, ensuring a higher likelihood of successful host cell invasion.
  • Lipid Rafts: Many viral receptors are found in lipid rafts in the host cell membrane, enhancing viral entry efficiency.
2. Entry into Host Cell
  • Mechanisms of Entry:
    • Animal Cells: Entry tends to be easier due to the absence of cell walls, facilitating entry via:
    • Endocytosis: The virus gets engulfed by the host cell membrane.
    • Membrane Fusion: Direct integration of virus membrane with host membrane.
    • Plant/Fungal/Bacterial Cells: These cells possess rigid cell walls, necessitating:
    • Injection Method: Viruses inject their genomic material directly into the cell.
  • Genomic Cargo Entry:
    • The entering genetic material can be just the genome or the entire nucleocapsid, requiring subsequent uncoating.
  • Uncoating: The process of removing the capsid to expose the viral genome can occur via:
    • Fusion of Viral Envelope: Only for enveloped viruses.
    • Endocytosis: Applicable to both enveloped and nonenveloped viruses.
    • Direct Injection: Commonly seen in plant viruses.
  • Endocytosis Process: Involves changes in pHpH that allow viral nucleocapsids to escape from the endosome and uncoat.
3. Synthesis of Viral Components
  • Gene Expression: After uncoating, the viral genome can be transcribed and replicated to produce viral components:
    • Viral Gene Expression: Use of host cellular machinery for transcription and translation.
  • Replication Variability: Depends on genome type:
    • Double-Stranded DNA/RNA: Direct use of host machinery.
    • Single-Stranded RNA Viruses:
    • Retroviruses: Convert RNA to DNA via reverse transcriptase, then back to RNA, creating higher mutation rates due to errors in replication.
    • Negative-Stranded RNA Viruses: Use RNA-dependent RNA polymerase to synthesize complementary [+] RNA first, leading to additional mutation opportunities.
  • Stages of Synthesis:
    • Early Stage: Enzymes to commandeer host machinery.
    • Middle Stage: Production of structural proteins necessary for new virions.
    • Late Stage: Assembly of viral particles from the synthesized components.
4. Assembly of New Virions
  • Assembly Line Model: Viruses typically use a systematic approach to assemble their components, which may involve co-assembly of different parts concurrently:
    • Bacteriophage Example: The T4 bacteriophage exhibits a complex symmetry in its structure and assembles by concurrently creating the head, tail, and sheath features before combining them at the end of the assembly line.
  • General Assembly Principles:
    • Viral components can self-assemble or rely on host cell chaperones.
    • Assembly can occur in various cellular compartments: cytoplasm, nucleus, or at host membranes.
    • Genome packaging is a precise process, ensuring the correct genetic material is enclosed within the capsid.
5. Exiting the Host Cell
  • Release Mechanisms: Viruses must exit the host cell to infect new cells. Release mechanisms vary based on virus type:
    • Lysis:
    • Primarily used by non-enveloped viruses.
    • Involves the rupture of the host cell membrane, leading to cell death and the release of progeny virions.
    • Budding:
    • Characteristic of enveloped viruses.
    • The nucleocapsid acquires its lipid envelope as it exits the host cell, often by passing through the plasma membrane or internal membranes (e.g., ER, Golgi).
    • This process can be gradual and may not immediately kill the host cell, allowing for prolonged viral shedding.
    • Role of Viral Proteins: Viral matrix proteins often guide the budding process by interacting with both the nucleocapsid and the host cell membrane.
    • Cell-to-Cell Spread: Some viruses can spread directly from one cell to an adjacent cell without fully exiting into the extracellular space, often facilitated by direct cell contact or syncytia formation.