Viruses are fascinating microorganisms that have shaped life on Earth. From their discovery in the late 19th century to modern detection methods, our understanding of these tiny pathogens has grown immensely. Their unique structures and replication strategies set them apart from other microbes.

Viral evolution remains a topic of debate, with hypotheses ranging from viruses predating cellular life to emerging from genetic elements. Their rapid mutation rates and ability to jump between species drive their evolution. Classification systems have evolved to reflect our growing knowledge of viral genetics and replication.

Virus Discovery and Evolution

Milestones in virus discovery

• Early discoveries
  • 1892: Dmitri Ivanovsky discovered tobacco mosaic virus (TMV) could pass through filters that blocked bacteria
  • 1898: Martinus Beijerinck coined term "virus" recognizing them as distinct from other microorganisms like bacteria
• Advances in virus detection and characterization
  • 1935: Wendell Stanley crystallized TMV demonstrating viruses are particulate not liquid
  • 1939: Electron microscopy allowed direct visualization of virus particles for first time
  • 1949: Renato Dulbecco developed plaque assay method for quantifying number of infectious virus particles
• Modern techniques
  • Polymerase chain reaction (PCR) amplifies small amounts of viral genetic material for detection
  • Next-generation sequencing enables rapid sequencing of entire viral genomes for identification and comparison

Hypotheses of viral evolution

• Virus-first hypothesis
  • Proposes viruses evolved before cellular life originated
  • Suggests viruses may have contributed to origin of first cells by providing genetic material
• Reduction hypothesis (regressive evolution)
  • Posits viruses evolved from small parasitic cells that lost genes over time becoming dependent on host cells
  • Views viruses as degenerate remnants of cellular life no longer capable of independent replication
• Escape hypothesis (progressive evolution)
  • Suggests viruses originated from mobile genetic elements like plasmids or transposons that escaped from cells
  • Proposes these elements gained ability to form protein capsids to protect and transmit their genes to other cells
  • Horizontal gene transfer may have played a role in this process

Viral Evolution Mechanisms

• High mutation rates contribute to rapid viral evolution
• Quasispecies formation allows viruses to adapt quickly to new environments
• Zoonosis enables viruses to jump between different host species, driving evolution
• Viral genomes can incorporate host genes, leading to new functions

Virus Structure and Classification

Structure and shapes of viruses

• Basic components
  • Genetic material consisting of DNA or RNA that carries information to replicate virus
  • Protein capsid shell that encases and protects viral genome
  • Some viruses have lipid envelope derived from host cell membrane surrounding capsid
• Capsid symmetry and shapes
  • Helical symmetry produces cylindrical or rod-shaped viruses (TMV)
  • Icosahedral symmetry creates spherical or polyhedral viruses (adenovirus)
  • Complex or irregular shapes seen in poxviruses and bacteriophages with separate head and tail structures

Evolution of virus classification

• Early classification systems
  • Based on observable properties like host range (animal, plant, bacteria), symptoms produced, and morphology
  • Limited by lack of understanding of viral genetics and replication mechanisms
• Baltimore classification system (1971)
  • Categorizes viruses based on type of genetic material (DNA or RNA) and replication strategy
  • Defines seven classes: I (double-stranded DNA), II (single-stranded DNA), III (double-stranded RNA), IV (positive-sense single-stranded RNA), V (negative-sense single-stranded RNA), VI (positive-sense single-stranded RNA with DNA intermediate), VII (double-stranded DNA with RNA intermediate)
  • Reflects fundamental differences in how viruses replicate and interact with host cells at molecular level

Types of viral morphology

• Enveloped vs. non-enveloped viruses
  • Enveloped viruses have lipid bilayer surrounding capsid acquired when budding from host cell
    • More sensitive to environmental conditions like drying out or exposure to detergents
    • Enter host cells by fusing envelope with cell membrane
  • Non-enveloped viruses have no lipid envelope, only protein capsid and genetic material
    • More stable and resistant to environmental factors
    • Enter cells by receptor-mediated endocytosis or penetrating cell membrane
• Capsid symmetry and host interactions
  • Helical capsids common in plant viruses, may help movement between cells through pores (plasmodesmata)
  • Icosahedral capsids efficiently package viral genomes, widespread in animal and bacterial viruses
  • Complex capsids contain specialized structures for attaching to and entering specific host cells (bacteriophage tail fibers bind bacterial receptors)
  • Viral tropism influences which tissues or cell types a virus can infect

Application of classification methods

• Sequence-based classification
  • Compare genome sequence of new virus to known viruses using bioinformatics software
  • Assign new viruses to existing taxonomic groups based on degree of genetic similarity
• Structural classification
  • Determine capsid symmetry and morphology of new viruses using electron microscopy or X-ray crystallography
  • Compare structural features to characteristics of established virus families and genera
• Replication strategy and host range
  • Identify type of genetic material and replication strategy of new virus, classify using Baltimore system
  • Determine range of hosts and cell types virus can infect
  • Use replication and host information together with genetic and structural data to assign virus to appropriate taxon
• Viral replication cycle analysis helps determine classification and understand virus-host interactions