Lecture Notes: Understanding Viruses

Introduction to Viruses

• Viruses are not considered alive.
  • Smaller and simpler than single-celled organisms like bacteria.
  • Do not perform metabolism or reproduce independently.
• Considered biologically inert, in a gray area between molecules and living organisms.

Structure of Viruses

• Consist of genetic material in a protein casing.
• Lack membrane, organelles, or cellular structures.
• Types of structures:
  • Rod-shaped or helical (e.g., tobacco mosaic virus)
  • Icosahedral (e.g., adenovirus)
  • Membranous envelope with spikes (e.g., influenza virus)
  • Bacteriophage (rod-shaped and icosahedral, with fiber tails)
• Genetic material can be DNA or RNA, double or single-stranded.
• The protein shell is called a capsid, made of capsomeres.

Reproduction of Viruses

• Hijack host cell machinery to replicate.
• Specific recognition between virus and host cell surface receptors.
• Infection mechanisms:
  • Injection of genetic material (bacteriophages)
  • Whole virus entering cell (endocytosis)
• Viral DNA is transcribed and translated by host cell.

Viral Replication Cycles

• Lytic Cycle:
  • Host cell is destroyed after replication.
  • Cell bursts open (lyses) to release new viruses.
  • Can rapidly destroy bacterial populations.
• Lysogenic Cycle:
  • Viral DNA integrates into host genome as a prophage.
  • Host cell divides, passing on the prophage.
  • Environmental signals can trigger switch to the lytic cycle.

Other Virus Types

• Viruses with envelopes use endocytosis/exocytosis for entry/exit.
• Retroviruses:
  • Contain reverse transcriptase enzyme.
  • Transcribes RNA into DNA.
• Viroids:
  • Naked circular RNA molecules, disrupt plant regulatory systems.
• Prions:
  • Infectious protein particles, cause protein aggregation in brain cells.

Evolutionary Perspectives

• Bacteria evolve surface receptors to evade viral entry.
• Viruses mutate to adapt to new bacterial receptors.
• Constant evolutionary flux between bacteria and viruses.
• Origin of viruses debated, possibly arose after unicellular life.
• Some complex viruses resemble cellular organisms genetically.

Conclusion

• Discussion of virus-caused diseases and combat strategies reserved for pathology courses.
• Transition to studying biological structure of the simplest organisms.

These notes cover key points about the nature, structure, and replication of viruses, as well as their evolutionary interactions with bacteria. Further details on the pathology of viruses will be covered in future courses.