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.