Module 4 Ch. 6.1-6.4, 6.8, 24.1

6.1 Overview of Viruses

Early Searches for the Tiniest Microbes

  • Invention of Light Microscope

    • Important for linking many microorganisms to diseases by late 1800s.

    • Robert Koch identified bacteria causing tuberculosis, cholera, anthrax, etc.

  • Challenge of Other Diseases

    • Diseases like smallpox and polio transmitted person to person could not identify bacterial causes.

  • Loeffler and Frosch's Experiment (1898)

    • Found that infectious fluid from host organisms (foot and mouth disease in cattle) was still infectious after passing through porcelain filters designed to trap bacteria.

    • Concluded that a submicroscopic particle (filterable agent) was responsible for the disease, marking a key milestone in virology.

  • Invention of Electron Microscope

    • Allowed visualization of viruses 40 years later.

The Position of Viruses in the Biological Spectrum

  • Unique Group of Biological Entities

    • Infect every type of cell (bacteria, protists, fungi, plants, animals).

    • Credit for knowledge of animal viruses largely comes from experiments with bacterial and plant viruses.

  • History & Evolution

    • Viruses have existed for billions of years, outnumbering cells on earth significantly.

    • Virome in the human body outnumbers human cells at least 10 to 1.

    • Important role in the evolution of Bacteria, Archaea, and Eukarya.

  • Characteristics of Viruses

    • Obligate intracellular parasites unable to multiply without invading host cells.

    • Unique structure and behavior raise debates on whether they are truly alive.

  • Viral Classification

    • Viruses defined as infectious particles, described as active or inactive, rather than alive or dead.

  • Table 6.1 Properties of Viruses

    • Obligate intracellular parasites

    • Ultramicroscopic size (20 nm to 750 nm in diameter)

    • Not cellular, compact structure

    • Inactive outside host, active inside

    • Basic structure: protein shell (capsid) surrounding nucleic acid core (DNA or RNA)

    • Nucleic acid can be:

    • Double-stranded DNA

    • Single-stranded DNA

    • Single-stranded RNA

    • Double-stranded RNA

    • Surface molecules for specific host attachment

    • Multiply by controlling host cell's genetic material

    • Lack enzymes for metabolic processes and protein synthesis.

6.2 The General Structure of Viruses

General Structure and Size Range

  • Ultramicroscopic

    • Most viruses are

Viral Components: Capsids, Nucleic Acids, and Envelopes

  • Capsid: Protective outer shell of viruses made of capsomers (identical protein subunits).

  • Types of capsids:

    • Helical Capsids: Rod-shaped capsomers forming hollow discs into a continuous helix.

    • Icosahedral Capsids: Symmetrical polygon with 20 sides and 12 evenly spaced corners.

  • Nucleocapsid: Capsid and nucleic acid combination.

  • Envelope: Presence in some viruses derived from host membranes, may contain viral proteins and spikes necessary for host cell attachment.

  • Functions:

    • Protect nucleic acid

    • Assist in viral entry into host cells and immunogenic response.

Types of Viruses

  • Complex Viruses: Larger structures lacking typical capsids, including poxviruses and bacteriophages.

Nucleic Acids: Core of a Virus

  • Genome Composition

    • Viruses carry either DNA or RNA, not both.

    • Viral genomes smaller than cellular genomes (e.g. HIV: 9 genes, pandoravirus: 2,500 genes).

  • Types of Viral Genomes:

    • DNA: single-stranded (ss) or double-stranded (ds).

    • RNA: often ss but can be segmented.

Other Virus Characteristics

  • Enzymes for replication (e.g., polymerases and reverse transcriptase).

  • Lack metabolic enzyme genes, rely on host.

6.3 How Viruses Are Classified and Named

Classification Scheme

  • Viruses classified separately from domains of life.

  • Experiments lead to informal categories: animal, plant, bacterial, enveloped, naked, DNA, RNA, etc.

  • International Committee on Taxonomy of Viruses: 59 orders, 189 families, 2,224 genera.

  • Nomenclature:

    • Families italicized with suffix -viridae; genera italicized with -virus.

    • Issues with broad species definition among viruses due to their variability.

Characteristics for Classification

  • Based on structure, chemical composition, genetic makeup, host range, etc.

  • Nomenclature derived from appearances (e.g., rhabdoviruses), anatomical/geographical origins, or disease effects.

6.4 Modes of Viral Multiplication

Relationship with Host

  • Viruses need host cells for multiplication, also spreading infection.

  • Viral Life Cycle: Critical steps: adsorption, penetration, synthesis, assembly, and release.

  • Host Range: Specific attachment to cell receptors limits host range. Examples include hepatitis B and rabies viruses.

Penetration

  • Two main ways for animal viruses to enter a host cell: fusion and endocytosis.

  • Fusion: For enveloped viruses, the viral envelope merges with host membrane.

  • Endocytosis: Both enveloped and naked viruses engulfed in vesicles.

Synthesis and Assembly

  • Virus controls host's metabolism via its nucleic acids after entry into the cytoplasm.

  • RNA viruses usually replicate in the cytoplasm.

Release of Viruses

  • Nonenveloped viruses released by cell lysis; enveloped viruses by budding or exocytosis.

Cytopathic Effects

  • Caused by viral infections can alter or damage host cells.

  • Persistent viral infections may lead to long-term symptoms or inactivity.

  • Oncogenic viruses can lead to cancerous transformations.

6.8 Prions and Other Nonviral Infectious Particles

Prions

  • Definition: Proteinaceous infectious particles without nucleic acids.

  • Associated with transmissible spongiform encephalopathies (TSEs), leading to nerve tissue deterioration and characteristic spongelike appearance.

  • Examples: scrapie in sheep, bovine spongiform encephalopathy, variant CJD in humans.

Viroids

  • Virus-like agents composed solely of RNA, without a protein coat.

  • Pathogenic to several economically important plants (e.g., tomatoes, potatoes).

24.1 Viral Diseases in Humans

  • Common Viral Diseases:

    • Influenza: Highly contagious and seasonal virus affecting respiratory system.

    • COVID-19: Caused by the SARS-CoV-2 virus, leading to a global pandemic.

  • Viral Transmission Methods:

    • Respiratory droplets, sexual contact, blood transfusions, and vector-borne routes (e.g., mosquitoes).

  • Impact on Public Health:

    • Vaccination programs are crucial to control viral outbreaks (e.g., measles, polio).

    • Surveillance and rapid response strategies are essential for managing emerging viral threats.

  • Viral Pathogenesis:

    • Mechanisms include the ability to evade immune responses, replicate quickly, and cause cell death or dysfunction.

  • Treatment Approaches:

    • Antiviral medications (e.g., acyclovir for herpes, oseltamivir for influenza) and vaccines are key components in managing viral diseases.

  • Challenges in Treatment:

    • Emerging resistance to antiviral drugs and the need for broad-spectrum antivirals that can target multiple viruses.