Virus Structure and Composition

Virus Structure and Composition

• Viruses are acellular entities, meaning they are not composed of cells.
• They are obligate intracellular parasites. This means they cannot replicate outside a host cell.
• Viral structure varies greatly depending on the specific virus but generally includes:
  • Nucleic acid core: Either DNA or RNA, which carries the genetic information.
  • Protein coat (capsid): Protects the nucleic acid and aids in attachment to the host cell. The capsid is often made up of identical protein subunits called capsomeres.
  • Envelope (some viruses): An outer lipid membrane derived from the host cell membrane, which surrounds the capsid. Viral proteins are embedded in this envelope and are important for binding to and entry into host cells.

Viral Replication

• Viral replication requires a host cell to function. The virus must be able to attach to the host cell, enter the cell, replicate its genetic material, synthesize viral proteins, assemble new virions, and exit the cell to infect new cells.
• Different mechanisms exist for various virus types for viral entry, replication and exit.
• Steps in viral replication:
  • Attachment: The virus attaches to specific receptors on the host cell surface. This recognition is crucial for host specificity.
  • Penetration: The virus enters the host cell through endocytosis or membrane fusion.
  • Uncoating: Viral nucleic acid is released from the capsid.
  • Replication: Viral genetic material is replicated, usually using the host cell's machinery. Synthesis of viral proteins occurs.
  • Assembly: New viral particles are assembled using the viral genetic material and proteins.
  • Release: New viruses exit the cell by lysis (breaking open the host cell) or budding (a gentler process where the virus pushes through the host cell membrane).

Viral Proteins: Diversity and Functions

• Various viral proteins regulate different aspects of the virus life cycle.
• Viral proteins are involved in:
  • Binding to host cells (important for host specificity)
  • Entry into host cells (membrane fusion, endocytosis)
  • Uncoating of viral genetic material
  • Replication of viral genetic material
  • Viral assembly
  • Exit from host cells (lysis, budding)
  • Immune evasion (modulation of immune responses)
• Viral proteins can also interact with and modulate cellular processes such as cell signaling and apoptosis.

Viral Classification

• Viruses are classified based on their structure, genetic material, replication strategy, and host range.
• Several classification schemes exist.

Viral Pathogenesis

• Viral pathogenesis describes the steps of how a virus causes disease.
• Viral pathogenesis can result in several outcomes depending on factors involved like the specific cell types targeted by a virus, virus type, host susceptibility, immune responses.
• Outcomes can range from acute infections to chronic infections and some can even lead to cancer.

Viral Interactions with the Host Immune System

• The host immune system plays a crucial role in combating viral infections.
• Viruses often employ various strategies to evade the host immune system and persist.