Viruses and Cell Theory

Definition of Life

  • The 7 fundamentals of life:
    • Metabolism: chemical reactions that release energy
    • Offspring: ability to reproduce sexually or asexually
    • Response: ability to react to environmental changes
    • Growth: irreversible increase in size
    • Homeostasis: maintaining internal conditions
    • Excretion: removal of waste from metabolism
    • Nutrition: acquiring food for energy and material for growth

Cell Theory

  • Three principles:
    1. All living things are composed of one or more cells.
    2. The cell is the smallest unit of life.
    3. Cells arise only from pre-existing cells.

Viruses

  • Viruses: Not considered alive due to dependence on host cells for reproduction and lack of cellular structure (no cytoplasm, organelles).
    • Infect every kingdom of life.
    • Host: Organism that carries the virus, can vary in specificity (some infect only one type, others multiple).

Size of Viruses

  • Extremely small: 20-200 nm (nanometers), discovered in the 1940s.

Viral Structure

  • Composed of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid).
  • Some have an additional viral envelope derived from the host cell.

Viral Shapes

  • Icosahedron: 20-sided shape (e.g., polio).
  • Membranous envelopes: Surrounding core (e.g., influenza).
  • Rod-shaped: Central spiral shape (e.g., tuberculosis).
  • Bacteriophages: Have complex structures to infect bacteria.

Host Interaction

  • Viruses are obligate parasites; they require a host for reproduction.
  • Viral replication stages:
    1. Attachment
    2. Entry
    3. Replication
    4. Assembly
    5. Release

Viral Replication Cycles

  • Lytic Cycle: Rapid replication leading to host cell death (e.g., Ebola).
  • Lysogenic Cycle: Viral DNA integrates into host chromosomes and can remain dormant, later reactivating (e.g., HIV, herpes).

Virus Transmission

  • Methods:
    • Direct contact (e.g., HIV, measles)
    • Airborne (e.g., influenza)
    • Fecal matter (e.g., hantavirus)
    • Animal bites (e.g., rabies)

Vaccines

  • Made from inactive or weakened viruses to train the immune system.
  • Promotes antibody production for future protection.
    • Herd immunity: Community protection via widespread vaccination.

Viral Mutations and Pandemics

  • Antibodies may fail to recognize mutated viruses due to shape changes in their proteins.
  • Pandemic: Occurs when a new virus appears with high mortality due to lack of immunity.
  • Example: New strains of influenza can emerge from other species, causing pandemics (e.g., COVID-19).