Lecture 1-2: Viruses and Prokaryotes

Viruses

Basics of Viruses

• Viruses are obligate intracellular parasites.
  • Obligate means they must infect host cells to replicate.
  • Intracellular means they invade host cells.
  • Parasites means they harm the host and rely on it for nutrients and energy.
• Viruses can't replicate autonomously; they require host cell machinery.

Viral Infection Cycles

• Lytic Cycle
  • Attachment: Virus attaches to host cell using specific protein-protein interactions.
  • Entry: Virus or its genetic material enters the host cell.
    • Methods: Penetration, vesicle uptake, or membrane fusion.
  • Synthesis (Replication): Viral proteins and genomes are made using host resources.
  • Assembly: Viral components come together to form new virus particles.
  • Egress (Release): New virions exit the host cell by lysis or budding.
• Lysogenic Cycle
  • Similar to lytic cycle but includes integration of viral genome into host DNA.
  • Integration: Viral DNA integrates into host genome, becoming a provirus.
  • Propagation: Viral DNA replicates with host cell division.
  • Induction: Stress triggers switch back to the lytic cycle.

Specificity and Transmission

• Viruses have specific host ranges due to protein interactions.
• Bacteriophage: A virus that infects bacteria.
• Plant and animal viruses have significant economic and health impacts but are usually host-specific.

Treatment and Prevention

• Antiviral drugs are limited and often ineffective due to host resource utilization.
• Vaccines are more effective than antiviral drugs for preventing infections.

Other Acellular Entities

Prions

• Misfolded proteins causing neurological diseases.
  • Examples: Mad cow disease, scrapie, kuru.
  • Not caused by toxins, viruses, or living organisms.
  • Prions convert normal proteins to misfolded versions, leading to disease.

Viroids

• Infectious RNA molecules without a capsid.
• Only known to infect plants.

Introduction to Prokaryotes

Definition and Characteristics

• Prokaryotes: Group including bacteria and archaea.
  • Paraphyletic group: Not a true clade.
  • Defined by lack of a nucleus.
  • Ubiquitous in all environments, including extreme ones.

History and Evolution

• Oldest life forms, originating near hydrothermal vents.
• Photosynthesis significantly altered Earth's atmosphere.

Prokaryotic Diversity

• Some are extremophiles, thriving in harsh environments.
• Bacteria: Often cultured for medical diagnostics but have specific growth requirements.
• Archaea: Mostly unculturable, less understood.

Prokaryotic Cell Structure

• Common morphologies: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral).
• Some have additional structures:
  • Pili: Used for attachment to surfaces.
  • Flagella: Used for movement.

This concludes the lecture on viruses and the introduction to prokaryotes. The next chapter will delve deeper into the characteristics and biology of prokaryotes.