Ch19

Chapter 19: Viruses

19.1 Viral Structure

• Viruses are minimalist pathogens that require a living host to replicate.

  • Key Steps for Viral Replication:

    1. Invade host (or a part of the host)

    2. Use host resources to replicate

    3. Spread to new host

    4. Evade immune responses

• Viruses are significantly smaller than host cells, generally around 20-400 nm, while ribosomes are approximately 25 nm in size.

• Infected hosts provide essential resources for the virus’s metabolism and synthesis.

• Viruses are not considered "alive"; their effects on the host can range from minimal to fatal.

• Hosts can include bacteria, archaea, or eukaryotes.

• Host Specificity of Viruses:

  • No plant viruses infect animals.

  • Broad specificity: Influenza can infect birds, pigs, and humans.

  • Narrow specificity: Smallpox infects only humans; Tobacco Mosaic Virus infects plants; Coronaviruses infect human cells.

19.3 Viral Disease

• Specific focus on SARS-CoV-2 (COVID-19) and how it impacts human health.

• Infection often leads to severe respiratory illness in humans.

• Understanding the mechanism of how the virus enters and affects human cells is crucial for developing treatments.

Virus Basics

• Viruses are composed of:

  • Genome: Either DNA or RNA that encodes viral proteins.

  • Protein Capsid: Serves as a protective container, displaying many geometric shapes (e.g., HPV).

  • Outer Envelope (optional): A lipid membrane associated with some viruses, such as all coronaviruses.

Viral Genome

• Viral genomes are typically small, ranging from 3,000 to 300,000 base pairs.

• Coronaviruses, like SARS-CoV-2, have a genome of approximately 30,000 bases.

• Categories of Viral Genomes:

  • DNA Viruses: Can be single-stranded or double-stranded.

  • RNA Viruses: Can be double-stranded, single-stranded, sense, or antisense.

  • RNA viruses often use specialized enzymes for replication, including Reverse Transcriptase (converts RNA to DNA) and RNA-dependent RNA Polymerase (replicates RNA), crucial for therapeutic drug targeting.

Human-Virus Interaction

Virus Entry

• Methods of virus entry into the body include:

  • HPV: Enters through abrasions in the epithelium.

  • Influenza and SARS-CoV-2: Inhaled through droplets into the airways.

Virus Binding and Entry into Cells

• Viral proteins (either from the envelope or capsid) bind to specific receptors on the host cell.

  • For SARS-CoV-2, the spike glycoprotein specifically binds to the ACE2 receptor on lung cells, facilitating infection.

Viral Replication within Host

• After entering the cell, the virus uses the host's cellular machinery to synthesize its proteins and replicate its genome.

• The viral RNA is translated into approximately 28 proteins, including the crucial RNA-dependent RNA Polymerase for viral genome replication.

Our Immune System

Adaptive Immunity Overview

• B-cells produce antibodies specific to non-self antigens, including those presented by viruses.

• Each B-cell can generate one specific antibody, having around 10 million different antibody genes.

• When an antibody binds to a viral antigen, it can neutralize the threat by either marking it for destruction or directly targeting infected cells.

Memory and Vaccination

• After recovering from infections, such as with SARS-CoV-2, B-cells generate memory cells that survive for years, improving the response to future encounters with the same antigen.

• Vaccination: A method to induce immune memory by introducing antigens that prompt a robust immune response. Approved vaccines for SARS-CoV-2 include those containing inactivated virus or mRNA encoding viral proteins.

• Potential challenges include managing weak or excessive immune responses and tracking how long memory cells persist.

Core Concepts + Learning Objectives

1. Viral Structure:

   • Describe features of viruses, including genome and capsid.

   • Define RNA-dependent RNA polymerase and Reverse Transcriptase.

2. Viral Pathogenicity:

   • Outline how SARS-CoV-2 operates within its host, causing disease.

3. Immune Response to Viruses:

   • Explain how the immune system interacts with viruses and how vaccinations help in mitigating viral infections.