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Understanding Viruses: Structure and Evolution

May 13, 2025

HL IB Biology: Viruses

Key Topics

  • Virus Structure
  • Replication in Viruses
  • Origin & Evolution of Viruses

Virus Structure

General Characteristics

  • Non-cellular infectious particles; not considered alive.
  • Do not fit within the three-domain classification due to lack of cellular features.
  • Simple structure and smaller than prokaryotic cells (20-300 nm).
  • Visible only with an electron microscope.
  • Acellular with no metabolism.

Common Structural Features

  • Size: Small and fixed.
  • Nucleic Acid Core: DNA or RNA; can be single or double-stranded, linear or circular.
  • Capsid: Protein coat with attachment proteins for binding to host cells.
  • Cytoplasm: Absent.
  • Enzymes: Few or absent.
  • Lipid Envelope: Present in some viruses, aiding in cell recognition.
  • Parasitic Nature: Reproduce only by infecting living cells, using their ribosomes.

Structural Diversity

  • Genetic Material: RNA or DNA, single or double-stranded.
  • Envelope: Present or absent.
  • Shape: Threadlike, polyhedral, spherical.
  • Host Specificity: Determined by attachment proteins; e.g., HIV targets white blood cells, hepatitis targets liver cells.

Examples

  • Bacteriophage Lambda: Infects E. coli, double-stranded DNA, uses tail for infection.
  • Coronaviruses: Respiratory viruses, single-stranded RNA, spherical with glycoproteins.
  • HIV: Retrovirus with RNA and reverse transcriptase; transmitted via body fluids.

Replication in Viruses

General Process

  • Non-living; do not undergo cell division.
  • Lytic Cycle: Rapid viral replication and host cell lysis.
    • Attachment, DNA injection, biosynthesis, assembly, lysis.
  • Lysogenic Cycle: Viral DNA incorporates into host genome, remains dormant until triggered.
    • Viral DNA integrated, latency period, triggered by environmental changes.

Origin & Evolution of Viruses

Origin Theories

  • Escape Theory: Origin from genetic elements that became mobile.
  • Regressive Theory: Origin from cellular organisms or parasitic cells.
  • Virus-First Theory: Viruses predate cellular hosts.
  • Viruses likely have diverse origins; convergent evolution may explain common features.

Viral Evolution

  • Rapid evolution due to high mutation rates, large populations, short generation times.
  • Antigenic Drift: Gradual genetic changes, leads to immune evasion (e.g., HIV).
  • Antigenic Shift: Major genetic changes, creating new virus forms (e.g., Influenza).
  • Disease Treatment
    • Vaccines updated annually for antigenic drift.
    • Antigenic shift requires isolation of individuals to prevent spread.