Notes on Viruses in IB Biology (HL)

Virus Structure (HL)

Structural Features of Viruses

  • Definition: Viruses are non-cellular infectious particles that are not classified as organisms because they lack characteristics of life.
      - Non-living: They do not possess cellular structures and cannot metabolize.
      - Classification: Viruses do not fit into the three-domain classification system of life.
  • Size:
      - Much smaller than prokaryotic cells, with diameters ranging from 20 nm to 300 nm.
      - Can only be visualized using an electron microscope.
Common Structural Features of All Viruses
  • Small Size: Generally consist of few molecules, hence do not form large structures.
  • Fixed Size: Viruses do not grow in size.
  • Nucleic Acid Core:
      - Composed of either DNA or RNA.
      - Nucleic acids can be single-stranded or double-stranded.
      - Genetic material can exist in linear or circular forms.
  • Protein Coat (Capsid):
      - Protects the viral nucleic acid.
      - Composed of protein subunits; contains attachment proteins on the surface that facilitate binding and entry into host cells.
  • Absence of Cellular Structures:
      - Lack cytoplasm and have very few, or no, enzymatic functions.
  • Lipid Envelope:
      - Some viruses possess an additional lipid layer formed from the host cell membrane.
      - Involves membrane-phospholipids that assist with cell recognition.
  • Parasitic Nature:
      - Viruses can only reproduce by infecting living host cells, utilizing the host's cellular machinery (like ribosomes) to synthesize new virus particles.
      - They do not respire and derive energy needed for replication from the host cell.
General Virus Structure Diagram
  • (Insert diagram illustrating typical virus structure)

Structural Diversity of Viruses

  • Variety in Structure and Shape: Although they are simple, viruses exhibit a wide diversity in structure and shape.
      - Genetic material may be RNA or DNA (double or single stranded).
      - Enveloped versus non-enveloped forms.
      - Shapes include threadlike, polyhedral, and spherical forms.
  • Specificity in Infection: Different viruses target specific types of host cells, dictated by their attachment proteins.
      - Example: HIV infects white blood cells while Hepatitis viruses target liver cells.
Examples of Diverse Virus Structures
  • Bacteriophage Lambda:
      - A bacterial virus infecting E. coli.
      - Contains double-stranded DNA within a capsid head.
      - Utilizes a tail and fibrils for attachment and DNA injection into the host cell.
      - Tail structure helps penetrate the bacterial cell wall.
  • Coronaviruses:
      - Cause respiratory diseases in mammals and birds.
      - Transmitted via respiratory fluids.
      - Structure includes single-stranded RNA, spherical shape, and an envelope.
      - Examples: SARS-CoV-2, MERS, SARS.
  • HIV (Human Immunodeficiency Virus):
      - Transmitted through intimate contact or exchange of body fluids (e.g., sexual intercourse, blood donation).
      - Structure includes:
        - Two RNA strands.
        - Proteins (e.g., reverse transcriptase is an enzyme that converts viral RNA into DNA).
        - A protein capsid.
        - Viral envelope consisting of a lipid bilayer and glycoproteins for attachment.
Virus Structure Diagrams
  • (Insert diagrams illustrating Bacteriophage Lambda, Coronaviruses, and HIV)

Replication in Viruses (HL)

The Lytic Cycle

  • Non-living Nature: Viruses do not replicate through cell division but instead through host infection.
  • Parasitic Reproduction: All viruses must infect living host cells to reproduce.
Steps of Viral Replication
  1. Attachment: Virus attaches to specific receptor sites on the host cell membrane using its attachment proteins.
  2. Injection: Virus injects its nucleic acid into the cytoplasm of the host cell.
  3. Utilization of Host Machinery: The virus commandeers the host's cellular machinery for protein synthesis, leading to the production of new viral proteins. This is referred to as biosynthesis.
  4. Assembly: New virus particles are assembled from the synthesized proteins and nucleic acids, maturing into virions.
  5. Release: The host cell undergoes lysis, releasing the newly formed virions into the environment to infect other cells.
Lytic Cycle Diagram
  • (Insert diagram depicting the lytic cycle)

The Lysogenic Cycle

  • Distinction from Lytic Cycle: Unlike the lytic pathway, new virus particles are not immediately released following infection.
      - Viral nucleic acid integrates with host DNA, remaining dormant.
      - This latency allows the host cell to continue normal functions including division, producing daughter cells that also contain viral DNA.
  • Latency: Occurs until a trigger (e.g., environmental factors like UV exposure or chemicals) prompts the viral DNA to become active and enter the lytic pathway.
Steps of the Lysogenic Cycle
  • Integration: Viral nucleic acid combines with the host cell's DNA.
  • Repressor Gene: A viral gene codes for a repressor protein that inhibits transcription of the viral genome.
  • Activation: External triggers can activate the virus, prompting a shift to the lytic cycle.
Lysogenic Cycle Diagram
  • (Insert diagram illustrating the lysogenic cycle)

Origin & Evolution of Viruses (HL)

Origin of Viruses

  • Timeline: Viruses are believed to have existed for approximately 3.5 billion years, evolving alongside other species through a process called coevolution.
  • Human Genome Evidence: Around 8% of the human genome consists of endogenous retroviruses (ERVs), remnants from ancient viral infections.
  • Debate in Origin Theories: Due to the absence of fossil evidence, the exact origin of viruses remains debated, and three prominent theories have emerged:
      - Escape Theory: Proposes that viruses originated from genetic elements (DNA/RNA) that gained the ability to move between cells and acquired an outer boundary, forming a virus.
      - Regressive/Reduction Theory: Suggests that viruses are remnants of once-living cellular organisms that became parasitic, shedding unnecessary cellular structures over time.
      - Virus-First Theory: Posits that viruses predated cellular life, implying that simpler organisms (viruses) evolved before more complex ones.
Theories of Virus Origin Diagram
  • (Insert diagram summarizing virus origin theories)

Common Features of Viruses

  • Convergent Evolution: Shared characteristics among viruses indicate potential convergent evolution. Common features include:
      - Capsid protein outer shell with no cytoplasm.
      - Genetic material (DNA or RNA) that utilizes a common genetic code.
      - Parasitic nature requiring a host cell for replication.

Evolution in Viruses

Viral Evolution Characteristics

  • Rapid Evolution: Some viruses undergo rapid evolutionary changes due to high mutation rates largely attributable to their RNA genetic material, along with large population sizes and short generation times.
  • Examples: The evolution of influenza and HIV demonstrates this rapid change.
Mechanisms of Genetic Change
  • Antigenic Drift:
      - Refers to minor, gradual changes in the viral genome leading to variations in surface proteins.
      - Over time, the immune system may no longer recognize the virus (notable in HIV).
  • Antigenic Shift:
      - Involves significant, rapid changes due to genetic material recombination when multiple virus types infect the same cell.
      - Results in new viruses that the host immune system fails to recognize (prominent in influenza).
Implications for Vaccine Development
  • Vaccination Adjustments: For viruses with antigenic drift, vaccines are updated regularly to maintain effectiveness.
      - Adjustments are successful in response to small changes in the virus.
  • Challenges:
      - For rapidly evolving viruses like HIV, creating successful vaccines is challenging due to high mutation rates.
      - Antigenic shift complicates vaccine development because changes are often unpredictable.
  • Adjunct Strategies: Isolating infected individuals may be necessary to curtail the spread of fast-evolving viruses.
Summary on HIV Vaccine Development
  • Although HIV mutates quickly, no effective vaccine has been achieved due to this rapid evolving nature.
  • Strategies for dealing with viruses that undergo antigenic shift may include isolating infected individuals to manage spread effectively.
Concluding Notes
  • Understanding viral structure, replication, and evolution is crucial for developing effective treatments and preventive measures against viral diseases. Health initiatives and resources should target the dynamic nature of viral pathogens.