Viruses

Chapter 2: Viruses

Learning Objectives

  • Identify and explain the basic structure and components of viruses.
  • Explain the major hypotheses about the origin of viruses.
  • Differentiate between various types of viral infections.
  • Describe the principle of viral vaccines.
  • Explain the different types of viral vaccines and antiviral drugs.
  • Create and explain a model of a virus.

Timeline

  • Viruses
    • Review the previous week’s material
    • Lecture on Viruses
  • Lab Activities
    • Activity 1: Learning Checkpoint
    • Activity 2: Handwashing Activity with GloGerm
    • Activity 3: Create Your Own Virus Model
  • Wrap-Up
    • Memory Recall: Students will independently complete a worksheet to assess their understanding of the day’s topics. This is not graded, but serves as a self-study tool.
    • Plicker questions
    • In-Class quiz

Background Information

Introduction to Viruses

  • Definition: Viruses are acellular, parasitic entities, not classified within any kingdom.
  • Unique Characteristics:
    • Lack cellular structure like living organisms.
    • Contain genetic material (either DNA or RNA) enclosed in a protein coat.
    • Do not possess machinery for metabolic processes or reproduction, requiring a host cell for replication and survival.

Origins of Viruses

Major Hypotheses About the Origin of Viruses

  1. Progressive (Escape) Hypothesis
    • Suggests that viruses originated from RNA and DNA molecules that escaped from a host cell with the ability to infect other cells.
  2. Regressive (Devolution) Hypothesis
    • Proposes that viruses may have evolved from free-living cells or intracellular parasites that gradually lost their cellular components.
  3. Virus-First Hypothesis
    • Suggests that viruses may have been the first self-replicating entities before the first cells.

Basic Structure and Components of Viruses

General Structure of Viruses

  • Viruses are extremely small, typically much smaller than bacteria.
  • Basic structural components include:
    • Genetic Material: Consists of nucleic acid (either DNA or RNA, but not both), carrying genetic information.
    • Capsid: A protein shell protecting the genetic material. Composed of multiple copies of one or more types of proteins.
    • Capsid Shapes:
    • Icosahedral (20-sided)
    • Helical (rod-shaped)
    • Complex (irregular shapes, like bacteriophages)
    • Envelope (in some viruses): An additional outer lipid layer derived from the host cell’s membrane, aiding evasion of host defenses.
    • Spike Proteins: Protruding proteins crucial for attachment and entry into host cells.

Classification of Viruses by Capsid Structure

  • Categories:
    • Naked Icosahedral: Simple, 20-sided capsid without an envelope (e.g., Hepatitis A virus).
    • Enveloped Icosahedral: Capsid surrounded by a lipid envelope (e.g., HIV-1).
    • Enveloped Helical: Helical-shaped capsid with an envelope (e.g., Influenza virus).
    • Naked Helical: Helical capsid without an envelope (e.g., Tobacco mosaic virus).
    • Complex: Intricate structures combining icosahedral and helical elements (e.g., T4 bacteriophage).

Virus Size

  • Viruses are much smaller than bacteria and human cells:
    • The Ebola virus is approximately 1/3 the size of the E. coli bacterium.
    • The smallest viruses (e.g., Poliovirus) are over 100 times smaller than E. coli.
    • Comparison highlights how even the largest viruses are dwarfed by human cells, which are about 10 times larger than E. coli.

Viral Infections

Stages of Viral Infections

The typical process of viral infection involves several key steps:

  1. Attachment: The virus binds to a specific receptor on the host cell's surface.
  2. Penetration:
    • Bacteriophages inject their genetic material directly.
    • Plant and animal viruses enter via endocytosis, where the entire virus is engulfed by the cell.
  3. Replication: Viral genome is replicated, and proteins synthesized using the host's machinery.
  4. Assembly: New viral particles are assembled from replicated materials.
  5. Release: New virions are released from the host cell, often resulting in the cell's destruction.

Patterns of Viral Infections

Viral infections can exhibit varied patterns based on the interaction with the host:

  • Acute Infection: Rapidly worsening symptoms for a short duration (e.g., Influenza).
  • Long-term Chronic Infections: Persistent infections with fluctuating symptoms (e.g., Hepatitis B).
  • Asymptomatic Infections: Viral replication occurs without noticeable symptoms but can be transmitted (e.g., certain cases of HIV).
  • Viral Latency: Dormant state within a host's cells, presenting no symptoms for potentially years, but capable of reactivation (e.g., herpes or HIV).
  • Lysogeny: Certain viruses, like bacteriophages, integrate their DNA into the host's genome without destroying the host, replicating as the host divides.

Animal Viruses

  • Cause a spectrum of diseases from mild colds to severe illnesses (e.g., meningitis).
  • Antiviral drugs and vaccines can be effective, however, some viruses (like HIV) can evade the immune system and develop drug resistance.

Principles of Viral Vaccines

Main Principle

  • Vaccines stimulate the immune system to recognize and combat a pathogen without causing disease.

Types of Vaccines and Their Differences

  1. Inactivated Vaccines:
    • Contains killed virus.
    • Typically requires multiple doses.
    • Example: Polio (IPV), Hepatitis A.
  2. Live Attenuated Vaccines:
    • Weakened form of the live virus.
    • Produces a stronger, longer-lasting immune response with fewer doses.
    • Examples: Measles, Mumps, Rubella (MMR), Chickenpox.
  3. mRNA Vaccines:
    • Use mRNA to instruct cells to make specific viral protein, triggering immunity.
    • Contains no live virus; a newer technology used in COVID-19 vaccines (e.g., Pfizer-BioNTech and Moderna).
  4. Subunit/Conjugate Vaccines:
    • Use specific parts of the virus (protein, sugar, or capsid) to trigger immunity.
    • May require booster shots.
    • Examples: Hepatitis B, HPV.
  5. Toxoid Vaccines:
    • Use modified toxins from pathogens to induce immunity.
    • Effective against diseases where the toxin, not the pathogen itself, poses the primary threat.

Key Differences in Vaccine Types

  • Composition: Varies from whole killed or weakened viruses to specific parts or instructions for viral protein production.
  • Immune Response: Live attenuated vaccines often produce the strongest responses; others may need boosters.
  • Safety Profile: Inactivated and subunit vaccines deemed safer for immunocompromised individuals.
  • Production: mRNA vaccines can be produced more quickly than traditional vaccines.
  • Stability: Live viruses require careful storage and handling; other types may be more stable.

Antiviral Drugs

  • Mechanism of Action: Antiviral drugs inhibit viral proteins while sparing host cell integrity.
  • Targeted proteins must be encoded by viral genes and absent in healthy host cells.

Viral DNA Integration and Paleovirology

  • Some viruses can integrate their DNA into the host's DNA, allowing transmission to future generations if it occurs in germ cells (sperm or egg cells).
  • Ancient viral DNA sequences serve as 'molecular fossils' useful in studying the age and evolution of certain viruses, a field called paleovirology.

Lab Activities

Activity 1: Learning Checkpoint

  • Objective: Watch a video and answer questions about genomics, virus mechanisms, host interactions, mutations, and transmission across generations.

Activity 2: Handwashing Activity with GloGerm

  1. GloGerm Application: Apply GloGerm to hands as lotion.
  2. UV Visualization: Observe under UV light for visible 'germs'.
  3. Handwashing: Perform washing and recheck under UV to identify residue.
  4. Reflection: Emphasized importance of hygiene in preventing contamination in various professional fields (medicine, research, food production).

Activity 3: Create Your Own Virus Model

  1. Group Work: Form small groups to model a virus using modeling clay (representing half for internal observation).
  2. Common Components: Include essential viral structures and genomic representation.
  3. Presentation: Explain features and hypothetical characteristics to class.

Evaluation Criteria

  • Common components of all viruses: 0.5 points
  • Visible genomic structure: 0.5 points
  • Creativity in design: 0.5 points
  • Creativity in viral information: 0.5 points

Memory Recall Worksheet for Viruses

  1. Are viruses living organisms?
    • Justify in 2-3 sentences.
  2. Match Definitions: Each virus origin hypothesis to descriptions provided.
    • a. Progressive hypothesis: Origin from escaped RNA/DNA
    • b. Regressive hypothesis: Evolved from free-living cells
    • c. Virus - First hypothesis: First self-replicating entities.
  3. Draw and Label: Typical virus diagram with major components.
  4. Describe Steps: List and explain the steps in viral infection.
  5. Different Types of Vaccines: Describe three types.
  6. Antiviral Drug Development Challenges: Why are antiviral drugs difficult to develop?
  7. Host Cell Interaction: Describe how the host cell is compelled to produce new viral particles.
  8. Transmission to Next Generation: Can viruses be passed down? How?