Tobacco Mosaic Virus and Electron Microscopy

Historical Context of Viral Diseases

• Tulip variegation, a viral disease, was recognized in ancient times and depicted in 17th-century Dutch paintings.
• Monochrome tulips were less valued compared to variegated ones, indicating a historical appreciation for viral traits in plants.
• Dutch breeders experimented with grafting to produce unique tulip varieties by combining healthy and infected plants.

Foundations of Virology

• Tobacco mosaic disease was one of the first viral diseases studied extensively.
• In 1886, Adolf Mayer established that the mosaic symptoms in tobacco resulted from injecting infected plant juice into healthy plants.
• Mayer initially hypothesized that the pathogen was bacterial and noted the loss of infectivity upon boiling.

Key Discoveries in Virology

• D.I. Ivanovsky's research in 1892 identified viruses as new infectious agents, detailing properties such as infectivity and filterability.
• Ivanovsky described intracellular inclusions known as X-bodies, still relevant for diagnostics today.
• Martin Beyerinck’s 1898 research confirmed Ivanovsky's findings, gaining acceptance for the existence of viruses.

Advancements in Virus Research

• The introduction of electron microscopy in the 1930s allowed for in-depth analysis of virion structure.
• Stanley obtained crystalline particles of the tobacco mosaic virus in 1935, enhancing understanding of viral morphology.
• The ability of purified viral nucleic acids to induce infections was demonstrated, cementing DNA and RNA's importance in virology.

Emergence of Virology as a Science

• The discovery of bacteriophages led to the establishment of virology as an independent field of study.
• Notable virologists contributed to the field, including Mathews, Hull, Jones, Maramorosch, Ryzhkov, and Sukhov among others.

Phytovirology in Kazakhstan

• Academician H.J. Zhumatov is recognized as the founder of virology in Kazakhstan.
• Prominent Kazakh scientists made significant contributions, including N.B. Akhmatulina and U. Abenova in general virology.
• Research addressed viral diseases affecting potatoes, cereals, and industrial crops in Kazakhstan.

Molecular Biology and Viral Research

• Advances in molecular biology led to decoding the genomes of various viruses, facilitating the development of genetically engineered vaccines.
• Effective diagnostic test systems for phytoviruses were developed, improving management strategies.

Characteristics and Attributes of Viruses

• Viruses are obligate intracellular parasites, replicating only within host cells and lack cellular structure.
• They possess either RNA or DNA, are dependent on host cellular mechanisms for reproduction, and do not grow on artificial media.
• Viral infectivity is attributed to nucleic acid, with protein envelopes providing protection.

Variability and Adaptation in Viruses

• Viruses display both genotypic (hereditary changes) and phenotypic (observable traits) variability, adapting to environmental conditions through their hosts.
• Different strains of viruses exhibit varied pathogenic effects depending on host types, illustrating the complexity of virus-host interactions.

Evolution of Viruses

• Each virus has a specific ecological niche with a defined host range, adapting dynamically to changes in hosts.
• Natural selection drives the evolution of viruses, reinforcing their characteristics as living organisms when in host cells, despite showing non-living properties outside.

Structure and Morphology of Virions

• Phytopathogenic viruses exhibit various morphologies: rod-shaped, filamentous, isometric (close to spherical), bacilliform, and bullet-shaped.
• Virions consist mainly of nucleic acid encased in a protein shell called a capsid, which dictates the virus's shape.
• Capsids vary in structure: rod-shaped and filamentous viruses have spiral protein shells, while isometric viruses possess icosahedral shells.
• Size of virions varies considerably:
  • Isometric viruses: 17-75 nm in diameter.
  • Rod-shaped, filamentous, and bacilliform viruses: lengths of several tens to 2000 nm, with diameters of 3-10 nm (filamentous and rod-shaped) and 18-75 nm (bacilliform).

Particles with Spiral Symmetry

• Tobacco mosaic virus (TMV) is a well-studied example, measuring 300 nm in length with a central hollow channel of 4 nm diameter.
• The RNA of TMV is embedded within a protein envelope, with protein subunits stacked in a helical arrangement.
• TMV structure reveals:
  • 2130 protein subunits and 6340 nucleotides of single-stranded RNA.
  • Molecular mass: TMV particle (39.4 x 10^6), viral RNA (2.05 x 10^6).
• Other bacilliform viruses exhibit a similar structure but may differ in helix dimensions.

Particles with Icosahedral Symmetry

• Isometric virions appear polyhedral under electron microscopy, comprised of protein subunits (capsomeres) typically arranged as pentamers or hexamers.
• Icosahedron structure: 12 vertices, 20 faces, and 30 edges.
• Cucumber mosaic virus (CMV) serves as an example, containing 180 subunits in 32 capsomeres and a diameter of 30 nm.

Bacilliform and Bullet-Shaped Particles

• Bacilliform and bullet-shaped viruses exhibit elongated forms rounded at both ends or at one end, respectively.
• Alfalfa mosaic virus is bacilliform, with lengths varying from 28 to 58 nm and a thickness of 18 nm.
• Complex structures may include viruses like potato yellow dwarfism virus, with multilayered envelopes.

Chemical Composition of Viruses

• Viruses consist mostly of proteins, with complex viruses containing lipids up to 20% of their weight.
• Water constitutes 10-50% of viral particle weight, not included in the analysis of nucleic acid and dry weight components.
• Viruses may contain metal ions and polyamines (about 1%).
• Nucleic acids carry genetic information, existing as single-stranded or double-stranded RNA/DNA, and may consist of one or multiple molecules.
• Nucleic acid molecular weights vary significantly, typically averaging around 2 x 10^6 for most phytoviruses.

Protein Composition

• Proteins in phytoviruses share amino acid composition with other organisms but have higher serine and threonine content.
• Molecular mass of protein subunits varies from 10 to 18 thousand, with sizes ranging from 150 to over 600 amino acid residues.

Explore the significant discoveries related to the Tobacco Mosaic Virus (TMV), particularly focusing on the advancements made through electron microscopy in the 1930s. This quiz covers important studies conducted by researchers like Stanley, Girer, and Schram, which enhanced our understanding of viral structures and their nucleic acids.