Lecture Notes on Cell Theory and Viruses

Cell Theory

• Defines criteria for living organisms.
• States the basic unit of life is a cell.
• All living organisms consist of one or more cells.
• Cells arise from pre-existing cells, enabling replication and reproduction.

Viruses

• Nature:

  • Small agents that infect cells (animal, plant, and other types).
  • Do not satisfy cell theory as they cannot replicate/reproduce independently.
  • Classified as non-living.

• Structure:

  • Contain nucleic acids (either RNA or DNA, never both).
  • Encased in a protein covering called a capsid.
  • Some have an additional lipid envelope, derived from other living cells, aiding in attachment to host cells.

• Reproduction:

  • Cannot reproduce independently due to lack of cellular machinery like ribosomes.
  • Infect host cells and utilize host organelles to reproduce.

Bacteriophages

• Viruses that specifically infect bacterial cells.
• Structure includes nucleic acids within a protein capsid (head), protein midsection, and protein tail.
• Attach to bacteria via tail sections to inject nucleic acids into host cell cytoplasm.

Viral Reproductive Cycles

Lytic Cycle

• Followed by bacteriophages:
  • The virus injects its nucleic acid into the bacterial cell.
  • Uses host cell machinery to synthesize new viruses.
  • Host cell eventually bursts (lyses), releasing new viruses (virions).
  • Latent period: Time between infection and lysis.

Lysogenic Cycle

• Viral DNA/RNA integrates into host genome.
  • Viruses like HIV use reverse transcriptase to convert RNA to DNA before integration.
• Infected cells live without immediate virus production.
  • Can remain dormant until triggered by environmental factors (e.g., UV radiation) to enter the lytic cycle.

Differences Between Cycles

• Lytic Cycle:

  • Immediate replication and release of viruses.
  • Results in host cell death.

• Lysogenic Cycle:

  • Integration with host genome allows viruses to remain dormant.
  • Potential to switch to lytic cycle under stress.

• Implications:

  • Lytic cycle is less advantageous as it kills host cells quickly.
  • Lysogenic cycle allows extended survival of the virus within the host, with the potential for later lytic activity.