Biology 132 Mini Lecture 11: Viruses

Introduction

• Transitioning from evolution and phylogenetics to studying organisms
• Importance of understanding phylogenetic trees and relationships
• Encouragement to review units one and two for foundational knowledge

Focus of the Lecture

• Living things that aren't organisms, specifically viruses
• Discuss three hypotheses on viral evolution
• General structure and replication of viruses
• Classification systems with emphasis on the Baltimore classification
• Diseases caused by viruses and their economic impacts
• Comparison of vaccination and antiviral drugs
• Encouragement to read the textbook for additional information

What is a Virus?

• Definition: Obligate intracellular parasite
  • Obligate: Must live within a host cell
  • Intracellular: Exist within cells
  • Parasite: Cannot replicate without a host
• Characteristics:
  • Very small (20-250 nanometers), visible with electron microscopes
  • Viron: A single virus particle
  • Not considered organisms due to their dependency on host cells

Evolution of Viruses

• Hypotheses:
  1. Regressive Hypothesis: Free-living cells that lost complexity and became parasites
  2. Progressive Hypothesis: RNA or DNA escaped from host cells
  3. Self-replicating Hypothesis: Originated from pools of replicons

Viral Structure and Classification

• Structure:
  • Nucleic acid core
  • Capsid: Protein coat
  • Envelope: Phospholipid membrane, not always present
• Complexity: Not linked to host complexity
  • Viruses can be complex (e.g., bacteriophages)
• Shapes:
  • Helical (e.g., plant viruses)
  • Icosahedral (e.g., polio, herpes)
  • Enveloped (e.g., HIV)
  • Head and tail (e.g., bacteriophages)
• Classification:
  • Traditionally: Enveloped or non-enveloped
  • Baltimore Classification: Based on morphology, genetics, and mRNA production

Viral Replication Cycle

• Key Steps:
  1. Permissive Binding: Host cell must have specific receptor
  2. Attachment: Virus finds and attaches to the receptor
  3. Entry: Through endocytosis or membrane fusion
  4. Replication and Assembly: Viral genome replication
  5. Egress: New virons exit via lysis or budding

Viral Diseases

• Types:
  • Acute: Short-term, intense symptoms (e.g., 24-hour bug)
  • Chronic: Long-term infections (e.g., long-term COVID)
  • Asymptomatic: No symptoms but can spread virus
  • Oncogenic: Can lead to cancer (e.g., HPV)

Prevention and Treatment

• Vaccination:
  • Types: Live, killed, or molecular subunit vaccines
  • Effectiveness: Varies, often targets previous strains
  • Mechanism: Prepares immune system to recognize virus
• Antiviral Drugs:
  • Manage symptoms, control replication
  • Not curative
  • Example: HIV drugs allowing long-term survival

Non-Virus Disease-Causing Agents

• Prions:
  • Cause transmissible spongiform encephalopathies
  • Examples: Chronic wasting disease, mad cow disease
  • Characteristics: No DNA, cause protein misfolding, always fatal

Conclusion

• Encouragement to further explore viruses in the textbook
• Next topic: Prokaryotes in the upcoming lecture